US20140140091A1 - Waveguide illumination system - Google Patents

Abstract

An illumination system employing a waveguide. Light received from an edge or an end of a waveguide is propagated in response to transmission and total internal reflection. Light deflecting elements distributed along the propagation path of light continuously change the out-of-plane propagation angle of light rays and cause decoupling of portions of the propagated light from the core of the waveguide at different distances from the light input edge or end. Light escapes from the waveguide into an intermediate layer at low out-of-plane angles and is further redirected by light extraction features out of the system. In one embodiment, the illumination system is configured to emit collimated light. In one embodiment, the illumination system includes shallow surface relief features. In one embodiment, the light deflecting elements include forward-scattering particles distributed throughout the volume of the waveguide. Additional collimating and non-collimating illumination units and methods are also disclosed.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS
• This application claims priority from U.S. provisional application Ser. No. 61/563,018 filed on Nov. 22, 2011, incorporated herein by reference in its entirety, and from U.S. provisional application Ser. No. 61/648,236 filed on May 17, 2012, incorporated herein by reference in its entirety.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
• Not Applicable
INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC
• Not Applicable
NOTICE OF MATERIAL SUBJECT TO COPYRIGHT PROTECTION
• A portion of the material in this patent document is subject to copyright protection under the copyright laws of the United States and of other countries. The owner of the copyright rights has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the United States Patent and Trademark Office publicly available file or records, but otherwise reserves all copyright rights whatsoever. The copyright owner does not hereby waive any of its rights to have this patent document maintained in secrecy, including without limitation its rights pursuant to 37 C.F.R. §1.14.
BACKGROUND OF THE INVENTION
• 1. Field of the Invention
• The present invention relates to light emitting waveguides such light pipes, optical fibers or planar waveguides provided with a series of light-deflecting features distributed along the optical path and configured to create a controlled illumination pattern emitted from a major waveguide's surface. This invention also relates to optical illuminators and light distribution systems employing such waveguides, for example, panel luminaires, side-emitting optical fibers, edge-lit LED front lights and backlights, lighting panels, LCD display backlights, daylighting luminaires, diffusers, computer screens, advertising displays, road signs, and the like, as well as to a method for redistributing light from various-type light sources.
• 2. Description of Background Art
• Conventionally, light emitting devices employing a waveguide include a series of optical features distributed along the light propagation path in the waveguide and configured to extract light from the waveguide in a perpendicular direction. The optical features are conventionally formed by small cuts, notches or grooves in the waveguide surface which extract light by means of reflection, refraction and/or scattering.
• FIG. 1 depicts an example of the prior-art planar slab waveguide which employs an array of surface microstructures formed in a major surface of the waveguide.FIG. 2 depicts a prior-art planar slab waveguide having an array of linear V-grooves formed in the waveguide's surface.FIG. 3 depicts an example of the conventional side-emitting optical fiber which includes a series of notches formed in the fiber's surface along its longitudinal axis.
• FIG. 4 generalizes the optical operation of the prior-art devices shown inFIG. 1,FIG. 2 andFIG. 3 and depicts a cross-section of the waveguide and an individual light extracting feature formed in the waveguide's surface. The light extracting feature commonly has the cross-sectional shape of a prismatic cavity and employs a reflective face inclined at a considerable slope angle to the surface of the waveguide, usually about 45° or so. Since the light is confined within the waveguide by means of a total internal reflection (TIR) from waveguide's longitudinal walls, this slope angle generally needs to be sufficiently high in order to overcome TIR and allow the reflected light to exit from the light emitting surface opposing the light extracting feature. As illustratedFIG. 4, at least some light rays may be reflected from the reflective face of the light extracting feature and then exit from the opposing surface of the waveguide. Accordingly, when a large array of such light extracting feature is distributed along a major surface of the waveguide, the waveguide can emit luminous flux from the opposing surface in a broad angular range.
• The reflective face usually has an optically transmissive surface and provides reflection by means of TIR when the incidence angles are greater than the TIR angle with respect to a normal to the reflective face. However, since the light rays propagating in the waveguide have essentially random angular distribution within the acceptance angle of the waveguide, at least a portion of the rays can strike the reflective face at angles being lower than the TIR angle. In this case, TIR will not occur and the respective rays may exit from the waveguide through the unwanted face (seeFIG. 4), resulting in light loss and reduced system efficiency. The reflective faces can be selectively mirrored to eliminate this light spillage. However, doing so will introduce reflection losses at the mirrored surface compared to the lossless TIR and will also add fabrication steps, such as fabricating a mask, applying the mask to the waveguide surface with precision alignment, vacuum metallization, etc.
• In conventional edge-lit waveguide illumination devices, the light extracted from the waveguide by the sloped reflective faces generally has a high angular dispersion from the waveguide's surface normal. Particularly, the divergence of light emitted by prior-art waveguide-based devices often approximates that of a lambertian source with a full 180° angular spread. The lack of beam directionality hampers the utility of conventional waveguide illumination systems in the applications requiring at least some degree of light collimation.
• Furthermore, the sloped reflective surfaces of light extracting features refract light propagating along the line of sight perpendicular to the waveguide's surface. This makes the conventional devices ill suited for the front light applications in which an edge-lit lighting panel is positioned in front of a viewable screen or image print. Each of the light extracting features alters the light path from the viewer to the print and bends the light towards other portions of the print compared to the neighboring smooth areas of the front light panel. As a result, the visual appearance and resolution of the print may deteriorate. Considering than at least some light propagating in the waveguide may also escape from the waveguide toward the viewer, the print contrast may also be affected.
• Also, when the front and rear surfaces of the planar waveguide generally have the same optical properties, e.g., being characterized by the same stepped drop in the refractive index, the light ray which obtains a non-TIR propagation angle within the waveguide may escape from either surface. Various mechanisms may contribute to such light leakage. The rays which propagate at less-than-TIR angles (with respect to surface normal) may include high-incidence-angle portions of the initial light beam injected at the waveguide's edge, light scattered by impurities in the waveguide, stray light from light extraction elements, stray light resulting from natural divergence or leakage from the waveguide, light which propagation angles are altered by the non-parallelism of waveguide walls, light reflected from the opposing wall by means of a Fresnel reflection, etc. Since such light has about equal chance to escape through either front or rear surface, at least a substantial portion of it will exit from the unwanted side of the waveguide resulting in energy loss and considerable glare. The prior-art lighting panels employing light extraction features based on light scattering rather than on reflection typically introduce even more unwanted glare and light spillage due to the uncontrolled nature of light scattering mechanism. Additionally, such lighting panels are usually characterized by a relatively high level of opacity and either substantially opaque or can be translucent at best, but not fully transparent, which inhibits the basic light guiding function of the panel. Yet further, when the conventional planar waveguide employing light extracting features (such as surface microstructure or scattering elements) is lit from an edge, at least a portion of light extracted by these features is emitted towards the viewer which substantially degrades the contrast and visibility of the bodies or images disposed behind the waveguide. This prevents using these panels for front lights where the perceptible quality of the background to be lit is important.
• Besides sometimes being characterized by reduced optical qualities or light spillage, the conventional systems employing relatively deep cuts, notches or grooves may also be affected by at least some loss of structural strength and rigidity compared to a smooth-surface panel having no such microstructures.
• It is therefore an object of this invention to provide an improved waveguide illumination system providing an efficient light extraction with a minimum light loss and without using excessively deep (relatively to the transversal size) microstructures in the waveguide's surface. It is another object of this invention to eliminate or at least substantially reduce the light spillage through the unwanted side of the waveguide. It is yet another object of this invention to provide an improved waveguide illumination system which can be configured for enhanced light collimation and controlled directionality of the emitted beam. It is yet another object of this invention to provide a waveguide illumination system capable of distributing light from a compact source over a large area and emitting the distributed light from said area in the form of a collimated beam with a prescribed angular spread or pattern. It is yet another object of this invention to provide an improved waveguide illumination system which can effectively used as a backlight or a front light panel that will not substantially alter the light paths and apparent image fidelity for the viewer. Other objects and advantages of this invention will be apparent to those skilled in the art from the following disclosure.
BRIEF SUMMARY OF THE INVENTION
• Accordingly, the present invention is directed to waveguide illumination systems which may be employed to emit directional light beams or uniformly illuminate a designated area with a very low light loss. More particularly, at least some embodiments of this invention are directed to planar light-emitting waveguides and at least some embodiments are directed to cylindrical side-emitting waveguides and optical fibers. This invention is also directed to directional (collimating) illumination systems which employ light-emitting waveguides, such as lighting luminaires, backlights, front lights and the like.
• The present invention solves a number of light distribution and illumination problems within a compact optical system which is not hindered by the limitations of conventional waveguides employing various kinds of light extraction features used to decouple light from the waveguide mode.
• An advantage of the present system is to provide controlled light extraction through a designated surface of the waveguide while minimizing light loss and controlling the angular distribution of the extracted light. Light is extracted from the waveguide into an intermediate layer by means of incremental deflections from the prevailing propagation direction after which it is further redirected out of the waveguide. A two-stage light extraction process enables the directionality of the emitted light and minimizes light spillage into non-functional directions.
• In at least one embodiment, the invention features a multi-layer optical structure having a waveguide layer, an intermediate buffer layer and a light extraction layer. Various implementation of the invention include a planar configuration of the waveguide and a cylindrical configuration of the waveguide.
• The buffer layer has a lower refractive index than the waveguide layer and preserves the waveguiding function of the waveguide layer at least for a range of incidence angles. The waveguide includes light deflecting elements distributed along the intended path of light propagation configured to incrementally deflect light rays by relatively small angles upon each interaction. According to an aspect of the present invention, the differential between the refractive indices at the opposing surfaces or sides of the waveguide and the smallness of the deflection angles ensure light extraction into the buffer layer while generally preventing light escape through the surface portion which is not disposed in optical contact with buffer layer. The light extraction layer further extracts and redirects light out of the illumination system.
• In at least one embodiment, the light deflecting elements include relatively low-profile surface relief features deflecting light by means of a total internal reflection (TIR). Each surface relief feature may have at least one facet which forms a relatively low dihedral angle with a surface plane. According to an aspect, the dihedral angle may be sufficiently low to prevent premature light leakage from waveguide through the respective facet. According to another aspect, each interaction of light with the facet results in light deflection from its original propagation path by means of TIR and bends further away from a prevailing plane or axis of the waveguide. According to a further aspect, this process may continue until TIR is suppressed at least at one surface of the waveguide and light exits from the waveguide and may be further redirected by the light extraction layer. In at least one implementation, the surface relief features include low-profile prismatic surface relief features.
• In at least one implementation, the surface relief features include shallow surface undulations or corrugations.
• In at least one implementation, the dihedral angle of the facets varies across the surface as a function of a distance from a light input area of the waveguide. In at least one implementation, the waveguide includes two symmetrically disposed segments each having a light input edge or end and each provided with an array of light-deflecting surface relief features.
• In at least one implementation, the waveguide includes a linear array of surface relief features extending parallel to a reference line. In at least one implementation, the waveguide includes a two-dimensional array of discrete surface relief features.
• In at least one implementation, the waveguide illumination system is configured for a generally unimpeded transversal light passage through its body. In at least one implementation, the waveguide illumination system is substantially transparent at least along a direction normal to its waveguiding surface.
• In at least one embodiment, the light deflecting elements include an internal corrugated boundary between two optically transmissive materials having different refractive indices.
• In at least one embodiment, the light deflecting elements include light scattering particles distributed throughout the body of the waveguide and configured to continuously change the light propagation direction by means of forward scattering.
• In at least one embodiment, the light deflecting elements include an internal corrugated boundary between two optically transmissive materials having different refractive indices.
• In at least one embodiment, the waveguide illumination system of this invention includes at least one light source configured to input light into the waveguide. In at least one embodiment, the light source is optically coupled to a light input edge or a light input end of the waveguide. Various implementations of the light source include light emitting diodes (LEDs), LED arrays, fluorescent lamps, incandescent lamps, cold-cathode or compact fluorescent lamps, halogen, mercury-vapor, sodium-vapor, metal halide, electroluminescent lamps or sources, lasers, etc. In at least one embodiment, the light source may include various light-collimating or beam shaping elements.
• In at least one implementation, the light extraction layer includes a light turning film or structure. In further implementations, the light turning film or structure may include one or more microstructured surfaces, one or more optically transmissive layers, one or more inter-layer corrugated boundaries or a reflective layer. In at least one implementation, the light turning film or structure includes a plurality of reflective or refractive facets inclined at an angle to the layer's surface. The reflective or refractive surfaces may be formed by prismatic grooves, notches, undercuts or other type of surface modification. Alternative implementations of the light extraction layer include a screen comprising a scattering layer or image print.
• In at least one embodiment, the waveguide illumination system of this invention is incorporated into a daylighting system. According to an aspect, the waveguide illumination system is configured and used as a hybrid luminaire combining natural and artificial illumination. Such waveguide illumination system has a layered panel structure which transmits sunlight delivered from a skylight in a transversal direction with respect to the prevailing plane of the panel and distributes and emits light received from an array of LEDs coupled to an edge of the waveguide.
• In at least one embodiment, the waveguide illumination system of this invention is configured as an edge-lit front light for an image screen which provides high optical transparency and image fidelity while efficiently illuminating the underlying image.
• In at least one embodiment, the waveguide illumination system of this invention is configured as an edge-lit backlight.
• In at least one embodiment, the waveguide illumination system of this invention is incorporated into a lighting luminaire with improved beam directionality. According to an aspect, the edge-lit waveguide panel distributes and emits light from a broad-area surface of the panel in the form of a collimated beam.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)
• The invention will be more fully understood by reference to the following drawings which are for illustrative purposes only:
• FIG. 1 is a schematic perspective view of a conventional light emitting waveguide employing surface microstructures for light extraction.
• FIG. 2 is a schematic perspective view of a conventional light emitting waveguide employing V-shaped grooves for light extraction.
• FIG. 3 is a schematic perspective view of a conventional side-emitting optical fiber employing cuts or notches formed in the fiber surface for light extraction.
• FIG. 4 is a schematic cross-sectional view of a conventional light-emitting waveguide portion, showing the principles of light extraction.
• FIG. 5 is a schematic perspective view of a waveguide illumination system, according to at least one embodiment of the present invention.
• FIG. 6 is a schematic cross-sectional view and raytracing of a waveguide illumination system portion, according to at least one embodiment of the present invention.
• FIG. 7 is a schematic cross-sectional view and raytracing of a waveguide illumination system portion, showing light reflection by a surface relief feature, according to at least one embodiment of the present invention.
• FIG. 8 is a schematic cross-sectional view and raytracing of a waveguide illumination system portion, showing light interaction with an opposing surface of a waveguide, according to at least one embodiment of the present invention.
• FIG. 9 is a schematic cross-sectional view of and raytracing of a waveguide illumination system portion, showing the operation of a light turning film attached to a buffer layer, according to at least one embodiment of the present invention.
• FIG. 10 is a schematic view of a waveguide illumination system portion, illustrating the principles of light collimation, according to at least one embodiment of the present invention.
• FIG. 11 is a schematic view of a waveguide illumination system, showing a buffer layer having a corrugated boundary with a light extraction layer, according to at least one embodiment of the present invention.
• FIG. 12 is a schematic cross-sectional view and raytracing of an illumination system portion, showing surface microstructure formed in a light extraction layer, according to at least one embodiment of the present invention.
• FIG. 13 is a schematic cross-sectional view of and raytracing of a waveguide illumination system portion, showing a light scattering layer attached to a buffer layer, according to at least one embodiment of the present invention.
• FIG. 14 is a schematic cross-sectional view of a waveguide illumination system portion in a front light configuration, showing an exemplary light path between a light scattering layer and a viewer, according to at least one embodiment of the present invention.
• FIG. 15 is a schematic cross-sectional view of a waveguide illumination system portion, showing a translucent light scattering layer, according to at least one embodiment of the present invention.
• FIG. 16 is a schematic cross-sectional view of a waveguide illumination system portion, showing a smooth corrugated surface of a waveguide, according to at least one embodiment of the present invention.
• FIG. 17 is a schematic cross-sectional view of a waveguide illumination system portion, showing a cladding layer attached to a corrugated surface of a waveguide, according to at least one embodiment of the present invention.
• FIG. 18 is a schematic view of a waveguide illumination system having a cylindrical configuration, according to at least one embodiment of the present invention.
• FIG. 19 is a schematic view and raytracing of a waveguide illumination system, showing multiple reflections from surface relief features, according to at least one embodiment of the present invention.
• FIG. 20 is a schematic view and raytracing of a waveguide illumination portion, showing a light collimator associated with a light input edge of a waveguide, according to at least one embodiment of the present invention.
• FIG. 21 is a schematic view of a waveguide illumination system including a wedge-shaped waveguide and a light turning film, according to at least one embodiment of the present invention.
• FIG. 22 is a schematic view of a waveguide illumination system including a wedge-shaped waveguide and a light scattering layer, according to at least one embodiment of the present invention.
• FIG. 23 is a schematic view of a waveguide illumination system, showing a light extraction layer having a mirrored surface, according to at least one embodiment of the present invention.
• FIG. 24 is a schematic view of a waveguide illumination system, illustrating a light collimation function of the system, according to at least one embodiment of the present invention.
• FIG. 25 is a schematic view of a waveguide illumination system used in conjunction with a daylighting device, according to at least one embodiment of the present invention.
• FIG. 26 is a schematic view of a waveguide illumination system portion, showing a buffer layer attached to a corrugated surface of a waveguide, according to at least one embodiment of the present invention.
• FIG. 27A throughFIG. 27I show various configurations of surface relief features, according to at least some embodiments of the present invention.
• FIG. 28 is a schematic elevation view of a waveguide illumination system, showing a linear arrangement of parallel surface relief features formed in a waveguide surface, according to at least one embodiment of the present invention.
• FIG. 29 is a schematic elevation view of a waveguide illumination system, showing a two dimensional array of surface relief features, according to at least one embodiment of the present invention.
• FIG. 30 is a schematic view of a waveguide illumination system portion, showing a buffer layer and a back-scattering light extraction layer attached to a waveguide surface comprising surface relief features, according to at least one embodiment of the present invention.
• FIG. 31 is a schematic view of a waveguide illumination system portion, showing a light diffusing layer, according to at least one embodiment of the present invention.
• FIG. 32 is a schematic view of a waveguide illumination system portion, showing light redirecting prismatic grooves within a light diffusing layer, according to at least one embodiment of the present invention.
• FIG. 33 is a schematic view of a waveguide illumination system portion, showing light redirecting slits within a light diffusing layer, according to at least one embodiment of the present invention.
• FIG. 34 is a schematic view of a waveguide illumination system, showing surface relief features having a variable slope, according to at least one embodiment of the present invention.
• FIG. 35 is a schematic view of a waveguide illumination system, showing two opposing arrays of surface relief features and two light sources coupled to opposing edges or ends of a waveguide, according to at least one embodiment of the present invention.
• FIG. 36 is a schematic exploded view of a front light implementation of a waveguide illumination system, according to at least one embodiment of the present invention.
• FIG. 37 is a schematic cross-sectional view and raytracing of a waveguide illumination system, showing light scattering features within a waveguide, according to at least one embodiment of the present invention.
• FIG. 38 is a schematic cross-sectional view and raytracing of a waveguide illumination system, showing light scattering features within a waveguide and a buffer layer positioned between the waveguide and a light extraction layer, according to at least one embodiment of the present invention.
• FIG. 39 is a schematic cross-sectional view and raytracing of a waveguide illumination system in a two-sided backlight implementation, according to at least one embodiment of the present invention.
• FIG. 40 is a schematic cross-sectional view and raytracing of an illumination system portion illustrating the light extracting operation of scattering particles in conjunction with a low-index layer, according to at least one embodiment of the present invention.
• FIG. 41 is a schematic cross-sectional view and raytracing of an illumination system portion illustrating the light extracting operation of forward-scattering particles in conjunction with a low-index layer, according to at least one embodiment of the present invention.
• FIG. 42 is a schematic cross-sectional view and raytracing of a waveguide further illustrating a forward-scattering operation of light scattering features, according to at least one embodiment of the present invention.
• FIG. 43 is a schematic view of a scattering pattern characterizing an exemplary forward-scattering particle, according to at least some embodiments of the present invention.
• FIG. 44 is a schematic view of a scattering pattern characterizing another exemplary forward-scattering particle, according to at least some embodiments of the present invention.
• FIG. 45 is a schematic view of an illumination system in an axisymmetrical configuration, showing a light source in the center, according to at least one embodiment of the present invention.
• FIG. 46 is a schematic view of an illumination system in an alternative axisymmetrical configuration, showing a plurality of light sources in a central ring area, according to at least one embodiment of the present invention.
• FIG. 47 is a schematic cross-sectional view and raytracing of an illumination system portion, showing a reflective layer adjacent to a waveguide and a right-angle turning film adjacent to the same waveguide, according to at least one embodiment of the present invention.
• FIG. 48 is a schematic elevation view of a waveguide illumination system, showing an annular arrangement of parallel surface relief features formed in a waveguide surface, according to at least one embodiment of the present invention.
• FIG. 49 is a schematic cross-sectional view and raytracing of a waveguide portion, showing a corrugated boundary between two transmissive materials having different refractive indices, according to at least one embodiment of the present invention.
• FIG. 50 is a schematic perspective view of a directional lighting fixture employing a collimating illumination system, according to at least one embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
• Referring more specifically to the drawings, for illustrative purposes the present invention is embodied in the apparatus generally shown in the preceding figures. It will be appreciated that the apparatus may vary as to configuration and as to details of the parts without departing from the basic concepts as disclosed herein. Furthermore, elements represented in one embodiment as taught herein are applicable without limitation to other embodiments taught herein, and in combination with those embodiments and what is known in the art.
• A wide range of applications exist for the present invention in relation to the collection of electromagnetic radiant energy, such as light, in a broad spectrum or any suitable spectral bands or domains. Therefore, for the sake of simplicity of expression, without limiting generality of this invention, the term “light” will be used herein although the general terms “electromagnetic energy”, “electromagnetic radiation”, “radiant energy” or exemplary terms like “visible light”, “infrared light”, or “ultraviolet light” would also be appropriate.
• The present invention seeks to provide waveguide illumination systems capable of progressively extracting at least a substantial portion of light propagating in a waveguide and emitting the extracted light from a waveguide's side wall towards one or more predetermined directions in a controlled manner and without substantial changing of the waveguide surface smoothness and/or continuity, such as introducing high-aspect-ratio cuts, notches or grooves.
• According to the present invention, there is provided an illumination system employing a waveguide. The waveguide is configured to guide light toward a predetermined direction by means of a Total Internal Reflection (TIR) from its opposing walls having substantially smooth surfaces and defining a waveguide core. The waveguide core should preferably be made from a material having good broadband optical clarity and transmission. When the illumination system is designed to operate in a specific spectral range, the material should be highly transmissive at least in that spectral range. The waveguide core may be manufactured from glass or a suitable polymeric material including but not limited to optical quality PMMA (acrylic), silicone, polycarbonate, PET (polyethylene terephthalate), polystyrene, polyolefin, polyesters, APET, PETG, or PVC, as well as any optically clear resin which is obtainable by polymerization and curing of various compositions. The waveguide may be formed by a single layer of the appropriate optically transmissive material or it may also include any number of additional layers made from the same or different materials having sufficient optical clarity for light guiding purposes.
• In one exemplary case of an edge-lit lighting waveguide panel, the waveguide may be configured to receive light from a light source on one edge and guide the light toward an opposing terminal end or edge. In another exemplary case of a planar waveguide having an axisymmetrical or free-form configuration, light may me input through an opening in the central area of the waveguide and subsequently propagate radially from the input area towards the outer edge. In a further exemplary case of an optical fiber or a cylindrical-configuration light pipe, the waveguide may be configured to guide light from a first terminal end to an opposing second terminal end. Once light is input into the waveguide and its propagation angles permit for TIR to occur at waveguide's one or more major surfaces, it becomes trapped within the core boundaries and can propagate considerable distances until it is extracted, absorbed or it reaches the opposing edge, outer edge or terminal end of the waveguide.
• The present invention is generally directed to edge-lit planar waveguides emitting from a broad-area surface and to end-lit, side-emitting cylindrical waveguides. Accordingly, when the waveguide has a planar configuration with parallel walls, each of the opposing major surfaces of the waveguide as well as the body of waveguide may be characterized by a plane which may be referred to as a prevailing plane of the respective element. Likewise, the illumination system based on the waveguide may also have a well defined planar shape and may thus also be characterized by a prevailing plane. It will be appreciated that, in case of the parallelism of the opposing broad-area waveguide surfaces, the prevailing planes of the surfaces and the prevailing plane of the waveguide will be generally parallel to each other. Thus, when a particular plane or a reference line makes an angle with respect to one of those prevailing planes, it will also make the same angle with the other parallel prevailing planes. Particularly, when the term like “out-of-plane angle” is used to describe the angular relationship a light ray or reference geometry object such as plane or axis with one of the above reference planes, such term may also be generally applied to the other parallel reference planes without limitations.
• A cylindrical waveguide may be characterized by a prevailing axis, such as a longitudinal axis of the cylindrical body forming the waveguide. In the context of the present inventions, as well as for the purpose of illustrating the operation of the presently preferred embodiments, a side-emitting waveguide having a cylindrical configuration may also be characterized by a prevailing plane. However, unlike the case of a planar waveguide having well defined planes based on geometrical dimensions, the prevailing plane of a cylindrical waveguide may be defined as a plane which is extending parallel to the prevailing axis and which is also generally separating the emitting and non-emitting sides of the waveguide.
• The waveguide of the illumination system includes a light emitting region associated with light deflecting elements distributed throughout the waveguide's body or throughout at least one of its major surface along the intended light propagation path. The function of the light deflecting elements is to cause a continuous change of the out-of-plane angle of a ray with the distance which the ray has propagated along the waveguide. The light deflecting elements distributed along the optical path gradually deflect light from the original propagation direction in an incremental manner and eventually communicate such light a greater angle with respect to a surface normal than the critical TIR angle thus causing the light to exit from the waveguide at different locations along the extent of the waveguide. It is preferred that the deflecting elements are substantially non-absorbing so that the repetitive interaction of light rays with such elements does not cause perceptible ray attenuation along the propagation path. Suitable lossless or near-lossless mechanism which may be employed for deflecting light rays include but are not limited to TIR and/or refraction at a boundary between two dielectric materials having different refractive indices.
• The distance which a light ray may travel within the waveguide before it is extracted by overcoming TIR at the waveguide surface largely depends on the initial propagation angle. Generally, rays having greater initial out-of-plane (in case of a planar waveguide) or also out-of-axis (in case of a cylindrical waveguide) angle will travel shorter longitudinal distances than rays initially propagating at more oblique angles with respect to the waveguide's prevailing plane or axis, as they require less interactions with the light deflecting elements to overcome TIR. However, it should be understood that the actual light path of each ray as well as its distance traveled within the waveguide may depend on other factors as well, particularly in view of the random character of light propagation within the waveguide and random ray interactions with deflecting elements.
• In at least some embodiments of the present invention, the illumination system includes a buffer layer or cladding layer attached to a major waveguide surface with a good optical contact. The buffer layer has a lower refractive index than the waveguide but higher than that of the outside medium. The buffer layer provides a differential in refractive index drop at the opposing major surfaces or sides of the waveguide and suppresses TIR at least for some uttermost incidence angles compared to the opposing boundary contacting with low-index medium (such as air). This causes the controlled leakage of the deflected out-of-plane rays primarily through the designated major surface of the waveguide contacting with the buffer layer rather than through both opposing surfaces of the waveguide. Suitable materials for the buffer layer can advantageously be selected for low-n fluoropolymers or resins, such as, for example, FEP, ETFE (both having a refractive index of 1.34-1.35), PFTE AF 1600 (n≈1.31), PFTE AF 2400 (n≈1.29), certain silicones, and the like.
• According to at least some embodiments, the light deflecting elements include shallow surface relief features formed in the waveguide surface. The surface relief features are configured to slightly alter the angular distribution of light upon each interaction and cause small portions of light to overcome TIR and leak out of the waveguide's core into the buffer layer while the main portion of can remain trapped within the waveguide.
• The surface relief features may be formed by shallow (low aspect ratio) recesses or depressions in the light guiding surface of the waveguide. Each surface relief feature may comprise a reflective face inclined to the surface plane at a sufficiently low dihedral angle αnd configured to reflect light by means of TIR. The TIR surface of the reflective face should generally face the light input edge or end of the waveguide so that it can be illuminated by a light source attached to that edge or end. The dihedral angle or slope of each reflective face with respect to the waveguide surface should be substantially less than 45° and may ordinarily be less than 20° and, more preferably, less than 10°. According to at least one embodiment, the dihedral angle may take an angular value between 1° and 3°. According to at least one embodiment, the dihedral angle may be between 0.1° and 1°.
• The upper practical limit for the dihedral angle of the reflective face may be selected from various considerations depending on the intended application of the illumination system. According to some embodiments employing the buffer layer, each reflective face should preferably be configured to reflect, by means of TIR, substantially the entire light beam impinging onto the reflective face back into the waveguide. This means that the dihedral angle of the reflective face should be smaller than a predetermined value defined by the differential between the refractive indices of the waveguide and the outside medium so as not to cause considerable light leakage through the reflective faces of respective surface relief features. According to some embodiments configured for emitting collimated light from a major surface of the waveguide, the dihedral angle of reflective faces may be selected based on the desired degree of collimation.
• The lower practical limit for the dihedral angle of the reflective faces may be selected, for example, based on the desired rate of light extraction that would ensure that most light injected into the waveguide can be removed along the propagation path. Various other factors may also be considered, such as the spacing between individual light extracting elements, the refractive index of the waveguide the adjacent layers, and whether or not the uniformity of light emission from the waveguide's surface is needed.
• While each surface relief feature may have a very low aspect ratio (e.g., the ration between the feature's depth or height to its width or base at the surface, it should be understood that the light deflecting portions of each reflective should still be essentially non-parallel to the light guiding surface of the waveguide at least in a cross-section perpendicular to the prevailing direction of light propagation. Therefore, it will be appreciated that each of the reflective faces of respective light deflecting features will alter the light propagation angles upon each interaction with the guided light. Particularly, each interaction of light with the reflective face will broaden the angular distribution of light propagating in the waveguide and cause at least a portion of reflected light to obtain greater out-of-plane angles and thus smaller incidence angles with respect to the TIR surface(s) of the waveguide.
• For the purpose of this discussion, the term “incidence angle” of a light ray in relation to a surface generally refers to an angle that this ray makes with respect to a normal to that surface. It will be appreciated by those skilled in the art of optics that, when referring to light or other waves passing through a boundary formed between two different refractive media, such as air and glass, for example, the ratio of the sines of the angles of incidence and of refraction is a constant that depends on the ratio of refractive indices of the media (the Snell's law of refraction). The following relationship can describe light bending property of an interface between two refractive media: n_Isin φ_I=n_Rsin φ_R, where n_Iis the refractive index of the material where the light is incident from, n_Ris the refractive index of the material where the light refract to, and φ_Iand φ_Rare the angle of incidence and the angle of refraction, respectively. It will be further appreciated that such optical interface can also be characterized by the angle of a Total Internal Reflection (TIR) which is the value of φ_Ifor which φ_Requals 90°. Accordingly, for a surface characterized by a stepped drop in refractive index along the ray propagation path, the incidence angle may be less than, equal to, or greater than the TIR angle at the given surface.
• A TIR angle φ_TIRcan be found from the following expression:
• φ_TIR=(n_R/n_I·sin 90°)=arcsin(n_R/n_I). In an exemplary case of the interface between acrylic with the reflective index n_Iof about 1.49 and air with n_Rof about 1, φ_TIRis approximately equal to 42°.
• Since each reflective face of the surface relief features broadens the angular distribution of light propagating in the waveguide, at least some uttermost out-of-plane rays may obtain incidence angles with respect to the boundary between the waveguide core and the buffer layer which are less than the TIR angles for the respective boundary. Therefore, these rays will refract into the buffer layer and thus exit from the waveguide core.
• It will be understood that, due to the nature of incremental deflection of light rays by relatively small angles using the light deflecting features, the rays escaping from the waveguide due to less-than-TIR incidence angles will generally have a relatively narrow angular spread. Additionally, the rays emerging from the major surface of the waveguide and refracting into a smaller-index medium at near-TIR incidence angles will have relatively low angles with respect to the waveguide surface and relatively high refraction angles. The angular spread of the emerging rays can be controlled by the appropriate configuration of the light deflecting means and their distribution density along the prevailing light path in the waveguide. Particularly, by way of example and not limitation, the dihedral angle of reflective faces may be advantageously selected to result in the refracted rays emerging at grazing angles with respect to the waveguide surface (corresponding to refraction angles close to 90°) and having a fairly narrow angular spread. Accordingly, the subsequent surface relief featured distributed along the waveguide will respectively deplete the remaining light from the waveguide and eventually extract it at grazing angles with respect to the surface plane. As the maximum deviation angle from the plane of the waveguide can be limited to a sufficiently low value, the resulting light beam may have a high degree of collimation while propagating nearly parallel to the longitudinal axis of the waveguide.
• The shallow surface relief features may be formed by a variety of suitable means and may comprise any surface irregularities, undulations or corrugations that slightly alter the reflection properties of the surface and cause light reflection at different angles compared to a perfectly flat or straight surface. When the surface relief features have a linear or cylindrical geometry, the longitudinal axis of each such feature should generally extend perpendicular to the prevailing direction of light propagation on the waveguide.
• The surface relief features may be fabricated together with the waveguide where the waveguide core may be cast or molded using a negative replica of the surface relief features. Alternatively, the waveguide may be fabricated first and the surface relief features may be formed in it by any suitable method for structural surface modification. Suitable methods may include laser ablation, chemical etching, embossing, grinding, polishing, molding, extrusion, material expansion or contraction, bending, etc. The surface relief features may also be formed in an external layer of optically transmissive material which may be then attached to a major surface of the main waveguide's body with a good optical contact.
• Suitable surface relief features may also be formed by corrugations or bends of the waveguide as a whole, as well as by any other means causing portions of the surface to reflect light at a greater out-of-plane angle compared to an ideally-flat, smooth surface.
• According to at least some embodiments of the present invention, the light deflecting elements are formed by light scattering features distributed throughout the body of the waveguide at least in its light emitting region. Particularly, the light scattering features can be formed by small dielectric particles or by any other form of optical irregularities in the otherwise homogenous body of the waveguide. By way of example, the light scattering features may be formed by very small proportions of imbedded, finely divided, spherical or aspherical particles made from a transparent plastic or glass material which refractive index differs from that of the waveguide by a predetermined amount.
• The light scattering particles can be homogenously distributed in the volume of the waveguide's body. Each particle can have a plain structure or a core-shell structure such as, for example that of known core-shell particles obtainable by emulsion polymerization. The light scattering features are configured to deflect light propagating in the waveguide from the original propagation path by means of forward scattering and communicate greater angles to said light with respect to a normal to the waveguide surface. Each light scattering feature should be designed to limit the scattering angle within a relatively narrow cone. Thus, a series of light scattering features on the optical path may thus provide function of incremental light deflection somewhat similar to the function of shallow surface microstructures explained above. Single or multiple interactions of light propagating by means of TIR in the waveguide with such light scattering features will result in the extraction of relatively small portions of light from the waveguide core at different locations across the waveguide's surface. The structure and optical properties of the light scattering features should be preferably selected to result in light out coupling from the waveguide's core at oblique angles with respect to the surface. When the buffer layer is employed, light should primarily leak out of the waveguide into this layer rather than exit from the opposing surface of the waveguide.
• According to at least some embodiments of the present invention, the illumination system may be further provided with a light extraction or light distribution layer adjacent to a major surface of the waveguide. The main function of the light extraction or distribution layer is to further direct or distribute the light extracted by the light deflecting elements. When the buffer or cladding layer is employed, the light extraction or distribution layer may be externally attached to the buffer layer with a good optical contact or made an integral part of the buffer layer.
• By way of example and not limitation, the light extraction layer may comprise a light turning film or structure. The light turning film may redirect the collimated light beam emerging from the waveguide towards a perpendicular of the waveguide surface thus providing a useful source of directional light emitted along the entire length of the light emitting region of the waveguide. At least some types of light turning or light redirecting films that may be incorporated into the light extraction layer of the present invention are disclosed in co-pending, co-owned application Ser. No. 13/662,311 which is incorporated by reference in its entirety herein.
• In a non-limiting example, the light turning film may be of a refractive type configured for turning the collimated light emerging from the waveguide by approximately 90° away from the waveguide surface. In an alternative non-limiting example, the light turning film may be of a reflective type and may direct light through the waveguide and also perpendicular to its surface.
• According to at least some embodiments, the light extraction layer may include a screen comprising a scattering layer or image print. The image print may be printed or otherwise deposited directly on the external surface of the buffer layer. Alternatively, the image print may be provided on a transparent substrate which can be attached to the buffer layer. The screen may reflect and/or scatter light propagating in the buffer layer toward an observer located at a distance from the waveguide. According to different variations of this invention, the light scattering screen may be configured to provide forward scattering, back scattering or any combination of the two. Particularly, the screen may be made opaque with back-scattering properties only, in which case the illumination system may be used as a front light. Alternatively, the screen may be made at least partially transmissive with the forward scattering function, in which case the illumination system may be used as a backlight. Furthermore, the transmissive properties of the screen may be adjusted so that the screen can be lit by the light emerging from the waveguide and made visible from both sides, thus forming a two-sided illumination system. Useful examples of the light extraction or distribution layer may also include light scattering surfaces or films, phosphorescent or fluorescent films, light filtering films or layers, diffusers, and the like.
• When the light extraction layer includes a viewable screen and the waveguide is positioned between the screen and the viewer in a front-light configuration, the surface relief features may be made substantially smooth and shallow so that they will not substantially alter the smoothness and continuity of the waveguide surface and will not notably bend the path of light propagating at near-normal angles with respect to the waveguide surface.
• Various layers employed in the waveguide illumination system, may be attached to each other or to the respective surfaces using any suitable method providing a good optical contact. For example, any two layers may be simply laminated onto each other with no air gaps. Alternatively, any intermediate layers may be used such as optical adhesives or two-sided transparent adhesive films to promote optical and physical contact. The respective layers of the illumination system may also be attached to each other by chemical bonding, heat bonding, ultrasonic bonding, welding, etc.
• According to at least some embodiments of this invention, the waveguide illumination system can be made optically transmissive in a transversal direction and configured for a generally unimpeded transversal light passage through its body. In addition to that, the system may be configured to emit collimated light from a selected broad-area surface or its portion and limit light emission from the opposing surface. In other words, the waveguide illumination system may be configured to provide directional illumination from one side with the prescribed degree of collimation of the emitted beam while precluding or at least substantially reducing light escape from the opposing side and preserving the transversal optical transmissivity. In contrast to the prior art illumination systems, the waveguide illumination system of this invention may be configured to not require using any opaque layers to prevent light decoupling from an unwanted side or surface.
• According to at least some embodiments of this invention, the waveguide can be made both highly transmissive and transparent along the light path perpendicular to the waveguide's prevailing plane or axis. Particularly, the waveguide can be configured to have a very high visual transparency at least in the direction along a normal to its broad surface. In other words, in addition to having high light transmissivity, the waveguide of this invention may have the property of transmitting light without appreciable scattering along a normal viewing direction so that bodies or images lying beyond can be seen clearly. While the light deflecting elements of this invention are used to deviate light from the waveguiding light paths by means of multiple incremental deflections, they can still be configured to not appreciably alter the propagation angles of light propagating perpendicular to the prevailing plane or axis of the waveguide. This is generally in contrast to the prior art illumination systems employing waveguides with other types of light extraction microstructures or scattering features which cause deterioration of either one or both the transmissivity or transparency.
• In operation, the light deflecting elements alter the propagation angles of light with respect to the prevailing plane or longitudinal axis of the waveguide by means of continuous incremental deflections along the propagation path. Each deflection alters the propagation angle by a relatively small amount which allows most rays to propagate a considerable distance in a waveguide. Multiple interactions of light rays with the light deflecting elements continues until at least the uttermost out-of-plane rays obtain less-than-TIR incidence angles with respect to the waveguide surface and exit from the waveguide core at relatively low angles with respect to the prevailing plane of the waveguide. When the buffer layer is employed, it creates a differential in the refractive index drop at the opposing surfaces or longitudinal sides of the waveguide. In turn, it creates a preference for light rays to exit from the waveguide through the side or surface to which the buffer layer is attached. The light extraction layer intercepts the light emerging from the waveguide and directs it further at higher angles with respect to the prevailing plane of the waveguide, thus finally extracting light from the illumination system and directing it towards one or more predetermined directions.
• The present invention will now be described by way of example with reference to the accompanying drawings.
• FIG. 5 depicts an embodiment of awaveguide illumination system2 in accordance with the invention.System2 includes awaveguide4 exemplified by a rectangular, planar slab waveguide having generally smooth-surface walls configured to conduct light by means of TIR.Waveguide4 is configured to have a first major broad-area surface10 and an opposing major broad-area surface12 extending generally parallel to surface10.Waveguide4 also has four edges, one of the edged being designated as a light input edge and the opposing edge being designated as a terminal edge. Aswaveguide4 may be associated with additional layers attached to either of its major surfaces or edges, the main waveguide body defined by itsmajor surfaces10 and20 and by the four edges is also hereinafter referred to as a waveguide core.
• Alight source400 is provided on the light input edge so thatwaveguide4 can guide light from the light input edge towards the opposing terminal edge by mean of TIR which involves bouncing light from atleast surfaces10 and12. The prevailing direction of light propagation inwaveguide4 defines alongitudinal axis200 of the waveguide. The waveguide edges extending parallel to the longitudinal axis (the longitudinal edges) may also be made smooth and polished in order to be able to guide light by means of TIR.Waveguide4 preferably has a refractive index sufficiently greater than the refractive index of the outside medium to provide for the TIR light guiding properties in a predetermined acceptance angle.
• Light source400 may include any suitable single or multiple light sources of any known type. According to one embodiment,light source400 may include one or more light emitting diodes (LEDs). Multiple LEDs may be arranged in a linear strip or a two-dimensional array. When high-brightness LEDs are employed, as may be the case, for example, whensystem2 is employed in an overhead lighting panel, other type of wide-area luminaire, the LEDs may also be provided with a heat sink to remove excess heat generated by the LED chips. A power supply (driver) may also be provided which electrical characteristics may be matched with those of the LEDlight source400. Furthermore, a suitable support frame or housing may also be provided to holdwaveguide4 andsource400 with the associated components together and/or encase all or at least some parts ofsystem2.
• It is noted that light-emitting devices suitable forlight source400 are not limited to LEDs and may also include fluorescent lamps, incandescent lamps, cold-cathode or compact fluorescent lamps, halogen, mercury-vapor, sodium-vapor, metal halide, electroluminescent lamps or sources, lasers, etc. Each light source may have any suitable shape, including compact two-dimensional or elongated one-dimensional shapes.
• Light source400 may have integrated optics such as collimating or light-redistributing lenses, mirrors, lens arrays, mirror arrays, light diffusers, waveguides, optical fibers and the like. Whenlight source400 includes a series of compact light sources, such as LEDs, each LED may be provided with individual collimating optics or, alternatively, a single collimating optical element may be supplied to inject light from all of the LEDs into the edge ofwaveguide4. Numerous applications ofsystem2 exist wherewaveguide4 may have a planar slab configuration and wherelight source400 may be associated with a strip of high-power LEDs optically coupled to the light input edge of the waveguide.
• Surface10 ofwaveguide4 has at least one light emitting region comprising light deflecting elements exemplified by surface relief features8 formed insurface10. More particularly, surface relief features8 are represented by repetitive shallow depressions formed in a stepped arrangement insurface10. The depressions can be characterized by alternating peaks and valleys connected by smooth, sloped surface portions. Surface relief features8 preferably have a linear geometry with a common longitudinal axis extending generally perpendicular to thelongitudinal axis200 ofwaveguide4. The shallow depressions forming surface relief features8 slightly alter the structure ofsurface10 yet allowing the surface to remain generally smooth and planar. Sincesurface10 is optically transmissive, the shallow surface relief features8 formed in this surface generally preserve both longitudinal and transversal transmissivity ofwaveguide4. Furthermore,surface10 can be characterized by a first TIR angle φ_TIR1which can be found from the expression: φ_TIR1=arcsin(n₀/n₁), where n₀is the refractive index of the outside medium and n₁is the refractive index of the core ofwaveguide4. Ordinarily, the outside medium can be air with n₀≈1, in which case φ_TIR1≈ arcsin(1/n₁).
• Waveguide4 further comprises abuffer layer6 disposed in a good optical and physical contact withsurface12 at least in the light emitting region.
• Buffer layer6 has a refractive index lower than the refractive index ofwaveguide4 and may also have a function of a cladding layer for the core ofwaveguide4. Accordingly, the interface between the core ofwaveguide4 andbuffer layer6 may be characterized by a second TIR angle φ_TIR2which can be found from the following expression: φ_TIR2=arcsin(n₂/n₁), where n₂is the refractive index ofbuffer layer6. The system may also be characterized by a critical TIR angle φ_TIRCwhich is the greater of the first and second TIR angles φ_TIR1and φ_TIR2respectively. When features8 are sufficiently shallow and make very low dihedral angles with the plane ofsurface10, the critical TIR angle φ_TIRCgenerally defines a minimum incidence angle that the light propagating inwaveguide4 must have with respect a normal to the longitudinal walls of the waveguide's core in order to remain being guided through the core by means of TIR.
• Referring further to the embodiment illustrated inFIG. 5, n₀<n₂<n₁and, consequently, φ_TIR2>φ_TIR1and φ_TIRC=φ_TIR2. It will be appreciated, as long as the incidence angles of light rays ontosurfaces10 and12 remain above φ_TIRC, light can propagate within the core in a waveguide mode.
• As it will be described in detail below, surface relief features8 are configured to redirect light withinwaveguide4 so that predefined portions of light are eventually communicated incidence angles which are less than φ_TIR2but still substantially greater than φ_TIR1. Maintaining the incidence angle generally above φ_TIR1ensures thatsurface10 and its portions formed byfeatures8 will continue to be reflective by means of TIR. On the other hand, making the incidence angle less than φ_TIR2for a small portion of light causes the extraction of the respective portion from the waveguide core intolayer6. In other words, a major function of surface relief features8 is to cause a controlled leakage of light from the core ofwaveguide4 intobuffer layer6 throughsurface12 along the propagation path, yet preventing light escaping throughsurface10.
• While the light-bending properties of surface relief features8 of the above-illustrated embodiment are selected to be insufficient to extract light fromsystem2 without additional means, they nevertheless play an important role insystem2 operation. Surface relief features8 act cooperatively withlayer6 to preliminary extract light from the waveguide core into the buffer layer so that light can be further directed and distributed with improved efficiency using additional light extraction features. The refractive index ofbuffer layer6 and the properties of surface relief features8 can be configured to recover light from the core ofwaveguide4 throughsurface12 at any desirable rate along the propagation path and without causing light loss throughsurface10.
• Accordingly,system2 further includes alight extraction layer20 exemplified by a light turning film disposed in contact with anexternal surface14 ofbuffer layer6.Surface14 is opposing the broad surface ofbuffer layer6 that is contactingwaveguide4. The light turning film comprises two transparent layers having different refractive indices and separated by a corrugated boundary between the layers. The film is configured to extract light frombuffer layer6 and redirect it toward a designated direction which may be advantageously selected to be normal to the plane ofwaveguide4.
• An optional specularly reflective or diffusively reflective layer (not shown) may be provided and positioned adjacent to surface10 ofwaveguide4 to reflect any stray light that may escape throughsurface10 outside of the waveguide. The stray light may include, for example, rays that are scattered by impurities in the materials ofwaveguide4,layer6 orlayer20, as well as by imperfections of surface relief features8 or the corrugated boundary within the light turning film.
• FIG. 6 illustrates further details and operation ofsystem2 depicted inFIG. 5. Eachsurface relief feature8 comprises afirst face16 generally facinglight source400 and asecond face18 generally turned away fromsource400. Both faces16 and18 are ordinarily planar and each form non-zero dihedral angles with the prevailing plane ofsurface10. These dihedral angles may also be referred to as slope angles of the respective faces to surface10.
• The slope of each face16 is preferably selected to be fairly low and substantially lower than the slope of therespective face18 so that faces16 have considerably greater surface area than faces18. Accordingly, this translates into a considerably longer cross-sectional profile offace16 compared to the profile offace18. The slope of each face18 may be advantageously selected to ensure that the face is completely shaded fromsource400 by therespective face16. Particularly, the angle which face18 makes with a normal to the prevailing plane ofsurface10 is preferably smaller than φ_TIRC. This can ensure that light propagating in the waveguide mode within the core ofwaveguide4 will never strike any offaces18 and will interact only with faces16. Suitable angles forfaces18 may include normal or near-normal angle with respect tosurface10.
• In operation,source400 illuminates the light input edge ofwaveguide4 with a divergent beam which causes at least a substantial part of the beam to enter the waveguide core at angles permitting for TIR.Waveguide4 further guides light toward the opposing terminal edge by bouncing said light from the opposingparallel surfaces10 and12. Since surface relief features8 are sufficiently shallow and the slopes offaces16 are low, the change in the propagation angle with respect toaxis200 is also low. Therefore, most light reflected by eachface16 continues its propagation in the waveguide by means of TIR while incrementally obtaining a slightly broader angular distribution with respect toaxis200 at each interaction withsurface10 in the light emitting region.
• It will be appreciated that, provided that there is a sufficient optical path along the waveguide's longitudinal axis, the incremental deviation of a light ray fromaxis200 will eventually result in saidray reaching surface12 at an incidence angle which is less than φ_TIRC. This, in turn, will ultimately cause ray extraction intobuffer layer6. Obviously, light rays having relatively small out-of-plane angles will generally undergo morel bounces fromfaces16 before reaching sub-TIR angles and before being extracted from the core ofwaveguide4 than rays having larger out-of-plane angles. For example, aray72 strikes face16 of one of the surface relief features8 and is losslessly reflected by TIR back intowaveguide4. While the reflection fromface16 increases the out-of-plane angle ofray72, the incidence angle with respect to surface12 still remains greater than the TIR angle φ_TIR2at the interface between thewaveguide4 andbuffer layer6. Therefore,ray72 undergoes TIR fromsurface12 and continues to be guided by means of TIR. Accordingly, longer optical path and additional interactions with surface relief features8 will be needed to extractray72 intobuffer layer6.
• In contrast, light propagating inwaveguide4 at angles close to the critical TIR angle φ_TIRCcan be extracted intobuffer layer6 near the light input edge of the waveguide, as illustrated by the path of aray74.Ray74 is emanated by the samelight source400 but has a greater initial out-of-plane angle thanray72.Ray74 strikes face16 of an individualsurface relief feature8 at an incidence angle which is greater than φ_TIR1. Therefore,ray74 is reflected fromface16 by means of TIR and is directed toward opposingsurface12 at a greater out-of-plane angle than beforestriking feature8. When the slope offace16 is sufficient to result in an less than φ_TIR2incidence angle ofray74 ontosurface12, no TIR will occur atsurface12 andray74 will refract intobuffer layer6.
• Ray74 further propagates inbuffer layer6 towards opposingsurface14 where it enters the light turning film oflight extraction layer20.Layer20 is preferably configured to have a good optical contact withbuffer layer6. Although it may be simply laminated ontosurface14 with no air bubbles, an adhesion promoting layer may also be used, such as a layer of optical adhesive of double-sided adhesive tape or film, for example. The refractive index of the inner layer of the light turningfilm contacting surface14 should preferably be not less than the refractive index ofbuffer layer6. Likewise, the refractive index of the adhesion promoting layer, is any, should also be no less than the refractive index oflayer6. According to some embodiments, the above refractive indices may be matched to each other in order to substantially reduce or eliminate the Fresnel reflections.
• The outer layer of the light turning film may have a refractive index greater than its inner layer. The corrugated boundary between the inner and the outer layer acts as a prismatic array and redirects light at a different angle with respect to the surface or its normal. The redirection mechanism may involve refraction and/or TIR. Accordingly, the internal boundary corrugations of the light redirecting film may be configured to intercept rays propagating at near-grazing angles inbuffer layer6 and redirect them towards a normal to the plane ofwaveguide4, as illustrated inFIG. 6 referring toray74.
• FIG. 7 depicts, in a cross-section parallel to the light propagation path, a portion ofwaveguide4 including an individual prismatic depression insurface10 and illustrates the light redirecting operation ofsurface relief feature8 formed by said surface depression.
• For the purpose of clarity and explaining the principles of light redirection bysurface10, the individualsurface relief feature8 is shown surrounded by flat portions ofsurface10 which are parallel tolongitudinal axis200. However, it should be understood thatsystem2 may have any number of surface relief features8 which can be spaced apart, contacting each other, overlapping, or otherwise distributed with any prescribed density along the intended propagation path.
• Surface relief features8 may have a constant pitch or spacing. Alternatively, the spacing betweenadjacent features8 can be made variable along the propagation path. Particularly, it may be advantageous to provide some initial spacing between surface relief features8 near the light input edge and gradually increase the density of the features as the distance from the light input edge increases. This may help improve the uniformity of light emission fromsurface12 as the increasing density of surface relief features8 will compensate the depletion of light by the preceding features8.
• Referring further toFIG. 7, light propagates inwaveguide4 left to right within an angular cone having an angular aperture of ±β, where β is an out-of-plane propagation angle counted fromlongitudinal axis200. Anuttermost ray174 having the propagation angle of −β is shownstriking face16 ofsurface relief feature8 at apoint90.
• Face16 is inclined at a dihedral angle α, hereinafter also referred to as a slope angle α, with respect to the prevailing plane ofsurface10. Angle α is selected to be sufficiently low in order to preserve TIR atface16 and to not cause decoupling ofray174 throughsurface10.16 and result in TIR back to the core ofwaveguide4. In will be appreciated that when light propagates in a waveguide mode, an angle complementary to angle β should generally exceed φ_TIRC. Considering that, when low-n buffer layer6 is employed (not shown inFIG. 7), φ_TIRC=φ_TIR2and φ_TIR2>φ_TIR1, the acceptable range of angle α may be generally defined by the following relationship: 0<α<φ_TIR2−φ_TIR1.
• Accordingly, theuttermost ray174 having the propagation angle −β upon enteringpoint90 reflects fromface16 by means of TIR and obtains a new propagation angle of β+2α, as a matter of optics. Therefore, the individualsurface relief feature8 causes widening the light propagation cone in thecore waveguide4 by angle 2α and also causes temporary angular asymmetry of the cone by the same angle.
• FIG. 8 depicts an upper portion ofwaveguide4 and illustrates the interaction of light with opposingsurface12 ofwaveguide4 after its passingsurface relief feature8 ofFIG. 7. Due to the smallness of slope α and the resulting smallness of the deflection angle caused bysurface relief feature8, a substantial part of light continues to propagate in the core ofwaveguide4 by means of TIR. Particularly, all rays having the incidence angles with respect to surface12 less than β are reflected fromsurface12 by means of TIR. In other words, all light remaining in the angular range ±β, with respect tolongitudinal axis200, remains also confined in a waveguide mode.
• However, the uttermost rays from the broadened angular propagation cone may now have incidence angles which no longer exceed second TIR angle φ_TIR2. As further illustrated inFIG. 8, those rays may refract intobuffer layer6, generally at relatively low acute angles γ with respect tosurface12.
• In an illustrative example where 90°−β≈φ_TIR2, substantially all of the light having the propagation angles greater than β will exit from the core ofwaveguide4 intobuffer layer6. Thus, it will be appreciated that, after interacting withsurface12, the light beam propagating withinwaveguide4 will shed a relatively narrow cone of 2α intobuffer layer6 and again obtain the prior ±β angular range. In other words, the optical interface formed bysurface12 and separatingwaveguide4 from smaller-refractive-index buffer layer6 is “shaving off” rays having propagation angles in excess of the critical angles that still permit TIR atsurface12. The escaping cone 2α represents a fixed portion of the angular distribution of light inwaveguide4. Thus, the slope a offace16 determines the amount of light extracted by eachfeature18 intobuffer layer6 along the optical path. Accordingly, the rate of light extraction can be accurately controlled by varying the slope of TIR faces in the light extraction area for a givenwaveguide4 geometry and relative refractive indices of the waveguide core andbuffer layer6.
• It should be noted that a certain portion of light may undergo reflection fromsurface12 even when the incidence angle onto said surface is less than φ_TIR2, owing to the so-called Fresnel reflection from the optical interface between two layers having different refractive indices. Light reflected fromsurface12 by means of Fresnel reflection will thus return back intowaveguide4 and can be recycled.
• Whenwaveguide4 has planar slab geometry, the shallow depressions of insurface10 that form surface relief features8 may be made by sheet casting or extrusion from a suitable transparent polymer, such as acrylic, polystyrene or polycarbonate, for example. Alternatively, surface relief features8 may be formed in a flat sheet of glass or polymer by any suitable methods for micro-replication or material removal. For example, grinding, milling or fly-cutting may be used with subsequent polishing ofsurface10. In a more specific example, surface relief features8 may be formed using a sharp diamond-tipped bit or cutter in which case the sufficient surface finish may be obtained without the need of subsequent polishing. In an exemplary implementation, the diamond cutting tool having the appropriate shape and slope of the cutting surface can be dragged across the surface ofwaveguide4 leaving a shallow groove. In an alternative exemplary implementation, the diamond cutting tool can be used in a “fly-cutting” mode as it can be spun in a spindle at a high speed (preferably at speeds of 20000 to 100000 RPM), plunged to the appropriate depth into the surface ofwaveguide4 and moved across the waveguide's surface. The rotation axis the tool should be preferably inclined at an angle with respect to a normal to the prevailing plane ofwaveguide4 corresponding to the desired slope angle of theface16 to be formed.Features8 may be formed directly in the surface ofwaveguide4 or they can also be formed in a separate optically transparent film or plate which can be attached to surface10.
• The structure and operation oflight extraction layer20 including a light turning film is illustrated inFIG. 9 by way of example.
• The light turning film oflayer20 comprises afirst layer104 and asecond layer106 disposed in contact with each other and having a corrugated boundary between the two layers.Corrugations108 forming the boundary have a linear triangular configuration in a cross-section with peaks and valleys extending perpendicular to the plane of drawing.Corrugations108 also define a plurality of alternating interface facets making predetermined angles with the prevailing plane ofsystem2 so that the corrugated boundary betweenlayers104 and106 comprises a plurality offacets124 alternating withfacets126.Facets124 are characterized by a firstdihedral angle162 with respect to the prevailing plane ofsystem2 andfacets126 are characterized by a seconddihedral angle164 with respect to that plane. Planes parallel to the prevailing plane ofsystem2 are indicated byreference lines142 and148 inFIG. 9. The light turning film is configured so that a refractive index n₃oflayer106 contactingbuffer layer6 is smaller than a refractive index n₄oflayer104 facing outwardly fromlayer6.
• The light turning film oflayer20 is configured to accept light propagating at low angles alongsurface14 inbuffer layer6 and redirect said light at a greater angle with respect to the surface so as to result in light decoupling throughsurface110. Referring further toFIG. 9, anangle160 represents the low angle thatincident ray74 makes withsurface14 when it enterslayer20. In the illustrated embodiment, the light turning film may be configured to accept light emerging fromlayer6 at angles between 0° and 20° from surface14 (corresponding to 90° and 70° incidence angles with respect to a surface normal, respectively) and communicate said light a smaller angle with respect to the surface normal so that TIR can be overcome atsurface110. Whensystem2 is configured for light collimation,angle160 should preferably be within a predefined narrow range of angles from the prevailing plane ofwaveguide4, said range being primarily defined by the desired angular cone of the collimated light. Whileangle160 may represent very sharp, near-grazing angles with respect tosurface14, it should also be understood that this angle may take any other suitable angular values provided thatsystem2 has the same basic operation.
• Facets124 are configured to have a generally smaller dihedral angle with respect to the prevailing plane ofsystem2 thanfacets126. Furthermore, thedihedral angle162 offacets124 is preferably selected to be less than an angle which is complementary toangle160 in order to provide refraction towards a normal to that plane.
• Thedihedral angle164 of eachfacet126 is preferably made greater than a maximum designed value of the angle that light can make withsurface14 inlayer104 after refracting atfacet124. At the same time, thedihedral angle164 of eachfacet126 should preferably be selected so that thefacet126 can intercept light refracted by a precedingadjacent facet124 and reflect it by means of TIR.
• Referring yet further toFIG. 9,light ray74 deflected by the respective surface relief feature8 (not shown) and receiving a sub-TIR angle with respect to surface12 is extracted from the core of waveguide4 (not shown) and entersbuffer layer6.Ray74 further crosses surface14 and enterslayer106 oflayer20. The refractive index n₂oflayer6 can be matched to the refractive index n₃oflayer106 so thatray74 can make about the same refraction angle as the incidence angle with respect to a surface normal140.
• Ray74 enteringlayer106 at a sharp angle with respect to surface14 propagates inlayer106 until it strikesfacet124 of the corrugated boundary withlayer104. Depending on theangle162,ray74 may slightly bend toward a normal144 by means or refraction at the interface between the lowerrefractive index layer106 and the higher refractive index oflayer104, after which it may strike the nextadjacent facet126. The slope offacet126 defined by thedihedral angle164 is selected to result in TIR at the interface between high-index layer104 and low-index layer106. Upon TIR,facet126 communicates an additional bend angle toray74, this additional bend angle being twice the angle betweenray74 andfacet126. As a result,ray74 may exit fromlayer20 nearly perpendicular tosurface110. It should be understood that light turning film may also be configured to result in the emergence angles other than normal. However, it should also be understood that, ordinarily,ray74 will be communicated an exit angle with respect to waveguide's4 prevailing plane which is substantially greater thanangle160. According to at least some embodiments of this inventions, whensystem2 is used for light distribution and improved collimation, it may be preferred that the slopes of surface relief features8 and other parameters of waveguide and the respective outer layers are selected so that the light beam emitted from a major broad area surface ofsystem2 has a divergence which is at least less than the initial divergence of light emitted bysource400.
• FIG. 10 illustrates yet further details and operational aspects of embodiments ofsystem2 shown in the preceding drawing figures and depicts an exemplary case whensystem2 is configured for emitting collimated light from its broad-area surface.
• A fan of rays802 exemplifies the angular distribution of light initially propagating inwaveguide4 before interacting with surface relief features8. It will be appreciated that the maximum half-angle of fan of rays802 is defined by the acceptance angle of the core ofwaveguide4. The acceptance angle is primarily defined by the refractive indices ofwaveguide4 andbuffer layer6.
• As light passes the firstsurface relief feature8 along its propagation path, a portion of its rays obtains a greater out-of-plane angle thus widening the angular distribution of light, as exemplified by a fan ofrays804. In order to enable this widening of the angular distribution, face16 offeature8 is inclined at low slope angle α with respect to prevailingplane202 ofsurface10.
• The slope offace16 is sufficiently small and 90°−α<<φ_TIR1. This prevents refraction atface16 and light escape fromwaveguide4 throughsurface10. Therefore, the interaction of light propagating inwaveguide4 withface16 will result in TIR back into the waveguide. Moreover, the slope angle α is also low so that the increment in the angular distribution it produces is substantially less than the angular span of fan of rays802.
• Face16 has a substantially planar shape and smooth surface.
• Accordingly, lightrays striking face16 will obtain an increment in their out-of-plane angles which is twice the slope angle α. It will be appreciated that the range of directions represented by fan of rays802 is at least partially overlapping with the range of directions represented by fan ofrays804.
• As a result of TIR fromface16 of an thefirst feature18, at least some of the uttermost rays in fan ofrays804 may form incidence angles with respect to a normal to surface12 greater than second TIR angle φ_TIR2. Therefore, upon reachingsurface12, these uttermost rays will cross said surface and refract intobuffer layer6, as illustrated by a fan ofrays808. It will be understood that this escaping light represents a small portion of light guided throughwaveguide4 and is generally characterized by relatively low emergence angles in layer6 (or high refraction angles with respect to a surface normal). Furthermore, it will be understood that, when angle α is sufficiently low, the angular span of fan ofrays808 will also be relatively low. Moreover, the divergence of fan ofrays808 can be easily controlled by varying slope angle α of therespective face16. For example, when a high degree of light collimation is desired, the fan ofrays808 may be provided with a very low divergence by making angle α very low, accordingly.
• As the extracted light emerges from the core ofwaveguide4, it further crossesbuffer layer6 and eventually strikes itsouter surface14.Surface14 representing the optical interface betweenlayer6 and light extractinglayer20 is configured for an unimpeded light passage intolayer20. Particularly, theinner layer106 of the light turningfilm exemplifying layer20 may be provided with the refracting index approximately matching the refractive index oflayer6, in which case TIR and Fresnel reflections can be substantially suppressed.
• Referring further toFIG. 10, a fan ofrays810 represents light which is extracted fromwaveguide4 and which further propagates intolayer106. Obviously, when the refractive indices oflayers6 and106 are matched, fan ofrays810 can have essentially the same narrow angular span and low slope with respect to the prevailing plane ofsystem2 as fan ofrays808.
• Referring yet further toFIG. 10, thecorrugated boundary154 betweenlayer106 and104 comprises a plurality ofasymmetric corrugations108 defined by two different slopes of the opposing facets forming each of said corrugations. As explained in the above examples,corrugations108 redirect light propagating at low angles towards a normal to the prevailing plane ofsystem2, as illustrated by a fan ofrays812.
• It will be appreciated that fan ofrays812 may have a slightly different angular span than fan ofrays810 due to the at least one refraction occurring atboundary154. However, it will also be appreciated thatcorrugations108 may be designed to result in the angular span of fan ofrays812 still being sufficiently narrow.
• As light redirected by the light turning film oflayer20 exits fromsystem2 along a normal tosurface110, it remains confined within a relatively narrow angular cone, as illustrated by a fan ofrays814. When exiting fromsurface110, the out-of-normal rays may undergo some refraction further away from the surface normal since the refractive index of the outside medium is lower than that oflayer104. However, when the angular distribution of fan ofrays812 is sufficiently narrow, the angular distribution within the emergent fan ofrays814 will also be relatively narrow.
• Thus,system2 can emit highly collimated light from its frontal surface without employing traditional collimating elements such as lenses or mirrors. Accordingly, it can be shown that the process of light extraction and collimation can continue along the light propagation path inwaveguide4. As illustrated by a fan ofrays806 representing light propagating at greater-than-TIR angles and reflected fromsurface12,layer6 depletes light fromwaveguide4 in a controlled manner by “shaving-off” only a narrow cone of the uttermost rays. The rays escaping intobuffer6 obtain their sub-TIR angles with respect to surface12 due to TIR from features8. Sincefeatures8 are distributed along thelongitudinal axis200 ofwaveguide4, they will continue providing additional angular bias to the guided light and thus result in continuous light extraction fromsystem2 throughsurface110.
• In view of the above description, it will be appreciated that the collimating function ofsystem2 was achieved using simple, non-collimating and non-focusing elements for light-deflection, such as shallow surface microstructures having planar surfaces. The prior-art devices used for directing light into a relatively narrow emission cone ordinarily use various complex-shape collimating optical elements such as spherical or aspherical lenses, parabolic or spherical mirrors, as well as arrays of such optical elements in various combinations. In contrast, the above illustrated embodiments ofsystem2 use no such complex shapes of elements.
• Furthermore, the conventional devices employing lenses, mirrors or their arrays and commonly require precise positioning of the light emitting features with respect to the collimating elements. Particularly, each light emitting feature should typically be positioned along the optical axis and in focal area of the respective collimating element. Contrary to that, the layers or individual light deflecting or redirecting features ofsystem2 do not necessarily require any special positioning or alignment with respect to each other except the very basic alignment or positioning of the layers with respect to each other. Thus, the relatively simple and manufacturing-friendly structure ofsystem2 can be advantageously selected for a number of illumination applications requiring at least some degree of collimation, such as, for example, directional wide-area illuminators, LED panel luminaires for general or special lighting, spotlights, accent lights, flashlights, backlights with a limited emission angle, and the like.
• FIG. 11 illustrates an alternative light-collimating variation ofsystem2 in whichbuffer layer6 has corrugatedexternal surface14 and light extractinglayer20 is disposed in contact withlayer6 conforming to the relief ofsurface14. Additionally, light extractinglayer20 has a higher refractive index thanlayer6 so that the pair oflayers6 and20 forms a light turning structure similar to the light turning film described in the above examples. Accordingly,corrugations108 are now formed by the corrugated boundary betweenlayers6 and20.
• Eachcorrugation108 includesfacet124 configured for refracting light towardssurface110 andadjacent facet126 configured for reflecting light by means of TIR generally along a normal tosurface110. Thus,facets124 and126 may be configured to provide nearly 90° light bending by two stages: the first stage being the refraction atfacet124 and the second stage being TIR atfacet126. It will be appreciated that the slope offacets124 may be advantageously selected to intercept and bend substantially all of the light escaping fromwaveguide4 intobuffer layer6.
• Similarly to the above-described embodiments and examples, sincebuffer layer6 has a lower refractive index than the core ofwaveguide4, it provides the required asymmetry in refractive indices at the optical interfaces formed bysurfaces10 and12 so that light escapes fromwaveguide4 primarily throughlayer6 and the light-turning structure formed bylayers6 and20.
• Accordingly, each face16 of surface relief features8 formed insurface10 has slope angle α which is low enough to prevent light leakage throughsurface10 but is sufficient to eventually extract at least a substantial part of light propagating inwaveguide4. As explained above, this requirement may be generally satisfied by limiting angles α to less than φ_TIR2−φ_TIR1, where φ_TIR1and φ_TIR2are the TIR angles at thewaveguide4 boundaries formed bysurfaces10 and12, respectively. Furthermore, angle α may be further restricted to even smaller angles to minimize the fan-out angle of the light escaping intolayer6 and/or reducing or eliminating the unwanted light leakage resulting from Fresnel reflections atsurface12.
• It will be appreciated by those skilled in the art that the Fresnel reflection generally occurs at each light passage from one refractive medium into another if there is a difference in refractive indices between the media. Although the Fresnel reflections usually account for a small fraction of light energy refracting into the other medium, especially when the difference of refractive indices is relatively small, the relative amount of reflected light increases at high incidence angles. Particularly, Fresnel reflection increases when light travels from a higher refractive index medium into a lower refractive index medium at an angle of incidence closely approaching the TIR angle at the optical interface between the two media. Therefore, referring to the optical interface formed by the core ofwaveguide4 andbuffer layer6, some light may still reflect fromsurface12 back into the waveguide core even when the incidence angle is lower that the second TIR angle φ_TIR2.
• In order to minimize the chance for such rays to exit throughsurface10, angle α may be limited to a reduced allowable angular range of 0<α<<φ_TIR2−φ_TIR1. In this case, sub-TIR rays reflected fromsurface12 back towardssurface10 by means of Fresnel reflection will strike therespective face16 at an incidence angle which is still less than φ_TIR1. Accordingly, therespective face16 will reflect said rays towardssurface12 by means of TIR which will prevent premature light escaping fromwaveguide4 throughsurface10 and still result in light decoupling fromsurface12. Thus, the sufficiently low angles α may provide a sufficient cushion for light recycling inwaveguide4 and maintaining its intended operation even in the presence of unwanted reflections fromsurface12.
• Referring further toFIG. 11, the light exit portion ofsystem2 may be provided with a light diffusing layer. For example, theexternal surface110 oflayer20 may be patterned to provide diffusing properties. Alternatively, a light diffusing film or sheet may be attached tosurface110. Such diffusing layer may be used to soften the angular distribution of collimated light emitted bysystem2. It may also have a function of masking the intensity irregularities across the light-emitting surface. The surface features of the light diffusing layer may be configured to limit the diffusion angle to a desired beam spread and generally preserve the directionality of the emitted beam.
• FIG. 12 shows an embodiment ofsystem2 in whichlayer20 has a microstructured surface configured for extracting light fromsystem2 by means of refraction and/or TIR. The microstructured surface oflayer20 represents a linear prism array where each linear prism extends perpendicular toaxis200 and generally parallel to linear surface relief features8 ofwaveguide4.
• Similarly to the embodiment ofFIG. 6,light source400 provided on the waveguide's light input edge illuminates the edge and injects light intowaveguide4. The light injected into the waveguide propagates in a waveguide mode by bouncing from opposingsurface10 and12 of the waveguide. The repetitive pattern of surface relief features8 alongaxis200 ensures that most light rays undergo multiple interactions withfeatures8 along the propagation path. Surface relief features8 deflect light from the original propagation direction by incrementally communicating the respective light rays a greater out-of-plane angle at each interaction eventually resulting in light decoupling fromwaveguide4 intobuffer layer6. The dihedral angles of the shallow prismaticcorrugations representing features8 are so selected as to result in extracting at least a substantial part of light fromwaveguide4 by means of incremental deflections.
• Layer20 preferably having the same or greater refractive index thanlayer6 receives light emerging fromwaveguide4 andlayer6 and further redirects it out fromsystem2. For this purpose, the facets of each linear prism oflight extraction layer20 should be positioned to prevent light reflection back intolayer20. In one embodiment,such system2 may be utilized as a broad-area luminaire emitting light at an angle with respect to a surface normal. In one embodiment,such system2 may be utilized as a front-light in which case, for example, an image print or painting (not shown inFIG. 12) may be externally attached to layer20 or disposed in an immediate proximity to said layer.
• Each linear prism oflayer20 may also be configured to intercept light rays propagating at a first angle with respect to a surface normal and redirect them at a greater angle with respect to the same normal so that light emitted bysystem2 is collimated at least in a plane perpendicular to the longitudinal axis of the array of prisms. As illustrated inFIG. 12 by example ofray74, at least some rays may be emitted fromsystem2 in a perpendicular direction with respect to the surface plane.
• Theprismatic layer20 may be made by a variety of means. In a non-limiting examplesuch layer20 may be made in the form of a microstructured sheet or film and then laminated ontolayer6. Alternatively,layer20 may be deposited ontolayer6 first and the prismatic array may be embossed in a subsequent step. In a further non-limiting example, the fabrication ofsystem2 may include initially forming a complete layered structure (includingwaveguide4 andlayers6 and20) with smooth external surfaces and subsequently providing microstructures inwaveguide4 andlayer20 in a single step.
• FIG. 13 illustrates a front-light implementation ofsystem2 wherelayer20 comprises a screen having at least one light scattering surface to be illuminated by the light propagating inwaveguide4.Such layer20 may be formed, for example, by providing an opaque or semi-opaque scattering layer on top ofsurface14. The opaque light scattering layer may be provided by a variety of means and may include, for example, white paint or pigment, colored paint or pigment, back-scattering film, surface texture, ink, phosphorescent or fluorescent substance, liquid crystals, etc. In a further non-limiting example,layer20 may comprise a screen containing any print, image, logo, text, symbols, pattern, or the like features. When the screen includes an image print, the print may be formed directly onsurface14 using screen printing, digital printing, offset printing, ink spraying, hand painting, machine painting or the like processes. Alternatively, the print may be formed on the surface of an external film, paper or any other suitable substrate which can be laminated or otherwise attached to surface14. Such external film can be made of an opaque material in which case the printed surface should facelayer6. Alternatively, the external film with an image print can be made from an optically transparent material in which case the print may face eitherlayer6 or away fromlayer6.
• Referring further toFIG. 13, rays72 and76 having sufficiently high out-of-plane angles exit fromwaveguide4 intobuffer layer6 at different locations alongaxis200 where they are scattered bylayer20 generally towards a normal toaxis200. In contrast,ray72 illustrating the main bulk of light rays propagating inwaveguide4 at lower out-of-plane propagation angles, continues to be guided by means of TIR fromsurfaces10 and12 until it obtains the sufficient out-of-plane angle to exit intolayer6 and to be extracted fromsystem2 bylayer20.Layer20 may be configured to scatter light primarily towardssurface10 and may optionally incorporate a reflective layer to reflect any stray light escaping towards the opposing direction.
• According to at least some embodiments, the slope of the light reflecting faces16 ofsurfaces relief features8 can be made low enough in order not to perceptibly affect the visual appearance ofsurface10 or the light-scattering screen oflayer20 compared to the case whensurface10 is perfectly smooth and flat. Additionally, the surfacedepressions forming features8 may be made substantially shallow so as not to significantly deflect light rays propagating at low angles with respect to a normal to the prevailing plane ofsystem2.
• FIG. 14 further illustrates the operation ofsystem2 ofFIG. 13, wheresystem2 has a planar front-light configuration. For the purpose of illustrating the front-light operation ofsystem2, it is assumed that an observer is viewingsystem2 from a normal viewing angle.
• Obviously, eachsloped face16 will slightly alter the light propagation path between the viewer and the light scattering screen compared to the case wheresurface10 would be perfectly smooth and planar. Slope angle α offaces16 with respect to plane202 ofsurface10 will define how much light will deviate from “an ideal” path along the surface normal. Accordingly, at any non-zero angle α, an actuallight path402 from a light emitting/scattering point atsurface14 to a viewer'seye660 will be different from a hypothetical light path coinciding with a normal800 toplane202. Particularly,path402 will deviate by a deviation angle δ from normal800 and result in the observer viewing a different area oflayer20 which is offset from the respective “on-axis” area by an offsetdistance406. This offsetdistance406 depends on angle α, as well as on the refractive indices and thicknesses ofwaveguide4 andlayer6. Iflayer20 comprises a high-fidelity image print and angle α is high, the observer may experience seeing the neighbouring image pixels compared to the case of viewing the same print through a perfectly flat transparent plate.
• However, it will be appreciated that angle α may be selected to be sufficiently low so that deviation angle δ will also be low resulting in a negligibly small offsetdistance406 so that the observer will not experience a perceptible change in the visual image quality. By way of example and not limitation, angle α may take particular values of 1 angular degree or less. It can be shown that at such slope angles offaces16 and with using some common transparent materials forwaveguide4 andbuffer layer6, the deviation angle δ will also be about 1 degree or less, in which case the offsetdistance406 will generally not exceed 1.5-2% of the combined thickness ofwaveguide4 andlayer6. Particularly, if the thickness of the respective transparent layers ofsystem2 is about 5 mm, offsetdistance406 will generally be less than 100 microns at near-normal viewing angles.
• Furthermore, the slopes offaces16 can be made identical to each other in which case the light path deviations caused by the plurality ofindividual features8 will simply translate the entire image, as viewed by the observer, perpendicularly to the surface normal by a small distance and thus will also not cause the loss of perceptive image fidelity. In the illustrated front light configuration ofsystem2, faces18 can be made perpendicular or near-perpendicular to surface10 so that the visible aperture offaces18 will be negligibly small, also being substantially smaller than the visible aperture of faces16. This should ensure that faces18 do not notably interfere with image viewing.
• As substantially all of the light propagating throughsystem2 can be distributed alongwaveguide4 and emitted towards the image print along the propagation path, the efficiency ofsystem2 as a front light can be made fairly high. As explained above, surface relief features8 preliminary extract light intobuffer layer6 where the extracted light illuminateslayer20.Layer20, in turn, scatters light towards the observer and permanently extracts at least a substantial portion of light fromsystem2. Since the unwanted light leakage throughsurface10 is eliminated or at least substantially reduced by providing sufficiently low slopes offaces16 and by providing an asymmetry in refractive indices of the media adjacent tosurfaces10 and12 ofwaveguide4, a frontlight employing system2 may be used for displaying images in higher fidelity, bright illumination, and improved contrast compared to conventional edge-lit front lights.
• FIG. 15 illustrates a variation oflight scattering layer20 which is configured to scatter light into both hemispheres from its plane.Such layer20 may be exemplified by a textured matte-finish surface of an optically transmissive plate or film.Such layer20 may also be exemplified by a semi-transparent layer of white or colored paint, ink, phosphors or dye deposited onto an optically transmissive surface. The appropriate light scattering features oflayer20 may be formed directly onsurface14 or they may be provided on a transparent or translucent substrate which can be attached to surface14.
• In a backlight variation of this invention,layer20 may be provided with light diffusing features which diffuse and forward-scatter light emerging fromlayer6 toward the viewer. By way of non-limiting example, thelight diffusing layer20 ofFIG. 15 can be configured to provide a bright uniform glow from its surface. It may also be associated with a translucent image print, see-through LCD screen, colored film and the like, makingsystem2 suitable for edge-lit signage and general illumination applications.
• FIG. 16 shows an embodiment ofsystem2 in which surface relief features8 are exemplified by smooth and shallow linear undulations or corrugations extending generally perpendicular tolongitudinal axis200 ofwaveguide4 and providing surface waviness in a cross-section parallel toaxis200. In the illustrated embodiment, faces16 and18 both have smooth curved surfaces which smooth conjugates between each other.
• Similarly to the sharp-corner surface relief features8 discussed above, the smooth linear undulations may be formed by casting or extrusion ofwaveguide4 from an optically transmissive polymeric material or formed in a flat sheet of glass or polymer by micro-replication or material removal. In another example applicable to both the planar and cylindrical geometries ofwaveguide4, the smooth undulations or corrugations of surface relief features8 may be formed by laser ablation or thermal evaporation. In the case ofwaveguide4 made from acrylic, a CO₂laser with the operating wavelength of about 10 microns may be used to selectively ablate the surface material and produce the required features. Optional polishing may include, for example, buffing, flame polishing or thermal annealing. In further examples, thesmooth surface10 may be subjected to any other suitable surface modification process such as embossing, imprinting or etching in order to produce the suitable surface relief features8. In yet further examples, various processes involving heat sources may be used to modifysurface10 accordingly by means of material melting softening, thinning, stretching, etc.
• The surface undulations may be made periodic and having a constant pitch and/or slope. Alternatively, the width or slope of each undulation may be made variable in a cross-section along the propagation path. The amplitude of the undulations may also be made constant or variable. Particularly, if a constant pitch is employed, the amplitude or surface slope may be made increasing along the propagation path in order to compensate the gradual light depletion inwaveguide4. A useful variation of surface relief features8 may include shallow surface undulations having a variable slope of faces26 which increases along the intended optical path. It will be appreciated that the increase of the slopes of undulations or corrugations alongwaveguide4 will increase the rate of light extraction from the waveguide along the optical path thus compensating the light depletion and resulting in an improved uniformity across the waveguide's surface. Particularly, the individual slopes offaces16 may be selected to provide light uniformity within 20-30% across the light emitting surface ofsystem2.
• Furthermore, undulations orcorrugations forming features8 may be made essentially random within predefined ranges of width, height and/or slopes. The distribution of surface relief features8 alongaxis200 may also be made random or ordered. The randomization or quasi-randomization offeatures8 may have a particular advantage for simplifying the fabrication process as well as for reducing the glare fromsurface10 in the endproducts employing system2. The light extracting properties ofwaveguide4 essentially equivalent to making smooth undulations insurface10 may also be provided by making the thickness ofwaveguide4 variable along the propagation path.
• Accordingly, eachsurface relief feature8 represented by smooth surface undulations or corrugations may be configured to have areflective face16 facing the light source and an opposingface18 facing away from the light source. Eachface16 may be shaped so that at least a portion of its surface is generally inclined at the appropriate angle α with respect to the prevailingplane202 ofsurface10. InFIG. 16, slope angle α is illustrated by the angle betweenplane202 and a tangent30 to face16 ofsurface relief feature18. Similarly to at least some of the above described embodiments, the acceptable range of angle α may be selected from the following relationship: 0<α<φ_TIR2-φ_TIR1.
• Eachface16 is designed to introduce an additional out-of-plane angle to light propagating inwaveguide4 and extract light propagating at near-critical TIR angles intolayer6.Layer20 is provided to finally extract light fromsystem2 and scatter or direct the extracted light out of the illumination system.
• Also, in a continuing similarly to the above described embodiments employing sharp-cornered shallow depressions and planar faces16 and18, the depth of each corrugation or undulation can of features18 can be made sufficiently small relatively to the width so as to result in very low slope angles that faces16 and18 make withplane202. On one hand, it allows for distributing light along a considerable length ofwaveguide4 since each feature18 redirects only a small fraction of light propagating inwaveguide4 and extracts only a portion of light striking its surface allowing the rest to be guided further through the waveguide by means of TIR. On the other hand, low slope angles provide for low deviation angles δ and small offsetdistances406 for the viewer, which makessystem2 particularly suitable for low-distortion edge-lit front lights.
• FIG. 17 shows an embodiment ofwaveguide illumination system2 in which anexternal cladding layer22 is provided on top ofsurface10.Layer22 may be made from the same or similar material thatbuffer layer6 and may have anexternal boundary122 with the outside medium such as air.Cladding layer22 may provide protection ofsurface10 from abrasion, scratches, contamination or optical contacting with other bodies or substances which may adversely impact the light guiding properties. Ordinarily, the refractive index ofcladding layer22 should not exceed the refractive index ofbuffer layer6 to provide for maximum isolation of light inwaveguide4. However, it should be understood thatlayer22 may also have any other suitable refractive index. It will be appreciated that even when the refractive index oflayer22 is the same lower than that oflayer6,system2 may still operate in the manner described above. Although at least some of the light rays propagating inwaveguide4 and obtaining angles lower than critical TIR angle φ_TIRCwith respect to the surface normal may transiently exit intolayer22, theboundary122 oflayer22 with the even lower-index outside medium will ensure that these rays will reflect fromsurface122 by means of TIR. The rays reflected fromsurface122 can then return back towaveguide4 where they can ultimately escape throughbuffer6 and can be further extracted fromsystem2 bylayer20.
• FIG. 18 illustrates an embodiment ofwaveguide illumination system2 wheresystem2 has a cylindrical configuration. By way of example,waveguide4 may be represented by a large-core polymer optical fiber (LCPOF). LCPOF may also include an optional cladding layer (not shown inFIG. 18) surrounding the fiber core. Surface relief features8 are formed in the side of thecylindrical waveguide4 opposite to the intended light emission direction. The opposing side ofcylindrical waveguide4 is provided withbuffer layer6 having the refractive index lower than the refractive index of the fiber's core.Light extraction layer20 is provided on top oflayer6 and may comprise, by way of example, a light turning film designed to accept light emerging at low angles from the fiber and redirect it at a higher angle with respect to the fiber's longitudinal axis. Eachfeature8 may haveface16 facing the light source andadjacent face18 facing away from the light source. The slope of each face16 with respect to thelongitudinal axis200 ofwaveguide4 is sufficiently low in order to prevent light escape through that face and in order to cause the extraction of relatively small portions of light intolayer6 along the propagation path according to the principles discussed above.
• In operation, light rays initially propagate in the fiber's core ofwaveguide4 at propagation angles permitting for TIR from the longitudinal walls of the fiber, that is at the incidence angles with respect to a surface normal greater than critical TIR angle φ_TIRC. Each ray having a sufficient out-of-plane propagation angle eventually strikes one or more faces16 which progressively communicate greater out-of-plane propagation angles to the ray at each interaction. As any ray reaches the minimum out-of-plane angle sufficient for suppressing TIR at the boundary withbuffer layer6, it can escape intolayer6 and can be further directed by the light turning film oflayer20. Particularly, the light turning film may be configured to emit light in a relatively narrow range towards a perpendicular to the fiber's axis at least in a cross-sectional plane parallel to said axis. Additionally, since the fiber ordinarily has a circular or elliptical transversal cross-section, the cylindrical configuration of thewaveguide4 may also provide at least some light collimation in the plane perpendicular to thelongitudinal axis200. Therefore, it will be appreciated thatsystem2 having a cylindrical configuration may be configured to collimate light in one or two dimensions and emit the collimated light perpendicular to the fiber along its entire length thus providing an efficient side-emitting fiber illuminations system.
• It should be understood, however, that the application of cylindrical configurations ofwaveguide4 is not limited to the side emitting fibers but also includes various light pipes, edge illuminators or any suitable illumination systems which may benefit from the elongated shape of the light distributing waveguide. In a cylindrical configuration,waveguide4 may have any suitable shape in a cross-section perpendicular to thelongitudinal axis200 of the waveguide. Suitable cross-sectional shapes may include but are not limited to: circular, elliptical, square, rectangular, hexagonal, trapezoidal or other shapes having any number of sides each having straight or curved profiles. The cross-sectional shape may also be formed by a profile which contour can be made variable along the longitudinal axis ofwaveguide4.
• FIG. 19 depicts an embodiment ofwaveguide illumination system2 illustrating the incremental increasing the out-of-plane angle of the guided light by multiple TIR reflections from respective surface relief features8. Referring toFIG. 19, a light ray emitted by edge-coupledlight source400 initially propagates inwaveguide4 at an incidence angle with respect to surface12 far exceeding the critical TIR angle φ_TIRC. Surface relief features8 formed insurface10 are distributed along the longitudinal axis of the waveguide.
• The plurality offeatures8 alters a generally planar cross-sectional outline ofsurface10. Eachfeature8 is formed by a shallow recess or depression insurface10 and has two opposing adjacent faces,16 and18.Face16 is facing the light source and has a low slope angle with respect to the prevailing plane ofsurface10.Face18 is facing away from the light source and has a generally higher slope with respect to the prevailing plane ofsurface10.
• As the light ray randomly encounters features8 on its path, it strikes the respective faces16 and reflects from them by means of TIR. Since the angle of reflection is equal to the angle of incidence with respect to a normal to face16, the ray incrementally obtains a greater out-of-plane angle at each interaction withfeatures8 and continues to propagate along the longitudinal axis ofwaveguide4. This process is continuing until the angle of incidence to surface12 exceeds the TIR angle at that surface in which case the ray escapes intobuffer layer6.
• It will be appreciated that, depending on the initial propagation angle, light may propagate different distances inwaveguide4 until it exits intobuffer layer6, even if the slopes of reflective faces are kept constant. Considering that the conventional light sources have at least some beam divergence, at least a substantial portion of light input through the waveguide's edge can be effectively distributed along the longitudinal axis ofwaveguide4 and extracted fromsurface12 at generally low emergence angles with respect to that surface.
• As the light ray decoupled from the core ofwaveguide4 further propagates throughlayer6, it reaches light extractinglayer20. The light turning film oflayer20 intercepts light emerging fromwaveguide4 and redirects it at a normal angle with respect to the prevailing plane ofsystem2. Thus, light becomes effectively extracted fromsystem2 with collimation. This operation makessystem2 particularly suitable for making directional illumination systems, such as, for example, side-emitting large-core fibers and planar edge-lit LED panels.
• Various parameters of surface relief features8 may be varied to fine tune the light distribution and emission from the surface ofwaveguide4. These parameters include but are not limited to: width, height or slope of reflective faces26, general shape and distribution offeatures8 along the propagation path, etc. An optional mirrored surface may be provided alongsurface10 ofwaveguide4 to reflect any stray light that may escape from the waveguide towards a direction opposing to layer20.
• It should be understood that the differential between the stepped drop in refractive indices outwardly atsurface10 and12 of thewaveguide4 is important to force light to escape from the waveguide's core generally throughsurface12 and not throughsurface10. As illustrated above, such differential can be easily obtained by providingbuffer layer6 having a lower refractive index than the core ofwaveguide4 but higher than that of the outside medium. Since the addition of the buffer layer can generally lower the acceptance angle ofwaveguide4 compared to the bare waveguide core surrounded by low-n air on both sides, a light collimating feature may be associated withlight source400 or with the light input edge of the waveguide in order to narrow the natural divergence of light beam emanated by the light source.
• FIG. 20 shows acollimating element440 attached to the light input edge ofwaveguide4.Element440 can be made from transparent material and may have any optical configuration suitable for coupling light fromsource400 intowaveguide4 at a limited range of propagation angles permitting for TIR from bothsurfaces10 and12.Element440 may be attached to the light input edge using an optical adhesive. Alternatively, collimatingelement440 may be provided as an integral part ofwaveguide4 and formed by tapering the light input edge ofwaveguide4 accordingly. Collimatingelement440 is preferably configured to have opposing concave walls reflecting light by means of TIR. However, particularly when the divergence of light fromsource400 is too high for TIR and/or whensource400 is coupled to the light input edge using a refractive medium such as optical adhesive or encapsulant, the concave walls ofelement400 may be mirrored for increasing the acceptance angle of the collimating element.
• Considering thatwaveguide4 will only effectively conduct light that enters its edge within a certain acceptance cone, let's define an acceptance angle θ_maxofwaveguide4 being the half-angle of this acceptance cone. It will be appreciated by those skilled in the art that acceptance angle θ_maxcan be found from the following expression:
• $\sin {\theta }_{\max }=\frac{\sqrt{{\mathrm{<figure-callout\; label="n"\; filenames="US20140140091A1-20140522-D00000.png,US20140140091A1-20140522-D00002.png"\; state="\{\{state\}\}">n</figure-callout>}}_1^2-{\mathrm{<figure-callout\; label="n"\; filenames="US20140140091A1-20140522-D00000.png,US20140140091A1-20140522-D00002.png"\; state="\{\{state\}\}">n</figure-callout>}}_2^2}}{{\mathrm{<figure-callout\; label="n"\; filenames="US20140140091A1-20140522-D00018.png,US20140140091A1-20140522-D00022.png"\; state="\{\{state\}\}">n</figure-callout>}}_0},$
• where n₁is the refractive index of the core ofwaveguide4, n₂is the refractive index ofbuffer layer6 and n₀is the reflective index of the medium light is traveling through before enteringwaveguide4. Whensource400 is coupled to the light input edge ofwaveguide4 through a layer of air (n₀≈1), sin θ_max=√{square root over (n₁²−n₂²)}. Accordingly, a numerical aperture (NA) ofwaveguide4 can be defined as NA=n₀sin θ_max, or, in the case of source to waveguide coupling through air, NA=sin θ_max=√{square root over (n₁²−n₂²)}.
• As illustrated inFIG. 20, collimatingelement440 may be useful to inject far off-axis rays intowaveguide4 so that they can propagate by means of TIR in the waveguide's core and can also be distributed and emitted throughsurface14 as explained in the examples above. Collimatingelement440 should be preferably configured to provide at least some initial divergence of light upon entering intowaveguide4. Particularly, when it is desired thatsystem2 can begin emitting light within a short distance from the light input edge, collimatingelement440 may be configured to provide the initial divergence approximating the acceptance cone with the half angle being close to the acceptance angle θ_maxofwaveguide4.
• FIG. 21 shows an embodiment ofwaveguide illumination system2 comprisinglight source400,waveguide4,buffer layer6 andlight extraction layer20.Waveguide4 has a wedge shape with the light input edge being wider than the opposing terminal edge and with the thickness ofwaveguide4 gradually decreasing along the light propagation path. The taper ofwaveguide4 defined by slope angle α is relatively low so thatwaveguide4 still has a generally planar configuration.Surface10 is generally smooth and may have one or more flat portions and at least one inclined surface portion defining an extendedsurface relief feature8 and itsreflective face16 facing the light guided inwaveguide4.Buffer layer6 is provided on opposingside waveguide4 and is attached to surface12 with a good optical contact.Layer6 is further followed bylight extraction layer20.
• The refractive index ofbuffer layer6 is lower than the refractive index ofwaveguide4 which creates a differential in the refractive index drop atsurfaces10 and12 and enables the preference for light escaping throughsurface12 when the light is bent to sufficiently high out-of-plane angles.Light extraction layer20 is exemplified by a light turning film which turns light by almost 90 degrees so that the light rays emerging fromlayer6 at low angles with respect to surface14 can be directed generally towards a normal to the prevailing plane ofsystem2.Layer20 may also comprise a light scattering surface, screen, image print, etc., as discussed above.
• Similarly to the above-described principles, light rays propagating in a waveguide mode and having different out-of-plane angles will emerge fromsystem2 at different locations along the propagation path, depending on the slope offace16, resulting in light distribution and extraction along the extent of the light emitting region ofwaveguide4. It will be appreciated that slope angle α can be made sufficiently small (about one degree or less) which will result in a small angular divergence of light emerging intobuffer layer6. Accordingly, the light turning film oflayer20 can be configured to turn light emerging frombuffer layer6 by up to 90° away from the surface plane while preserving the small divergence. As the light beam turned by the light turning film overcomes TIR and emerges fromlayer20, it will have a well defined directionality and due to being confined within a finite angular range. Thus,system2 depicted inFIG. 21 can be configured to emit light from the entire light emitting region with improved light collimation.
• FIG. 22 illustrates a front-light variation ofsystem2 employing a wedge-shapedwaveguide4 in whichlayer20 comprises a screen that back-scatters light generally towards a normal to the prevailing plane ofwaveguide4, so that at least a substantial part of the scattered light can pass throughwaveguide4 and exit fromsystem2 throughsurface10.Screen2 may be formed by a bright, light scattering paint or film for illumination purposes. Alternatively,screen2 may comprise an image print viewable throughlayer6 andwaveguide4 and deposited ontosurface14 or onto an intermediate substrate using any suitable printing process.
• Referring toFIG. 22, light can propagate considerable distances alonglongitudinal axis200 in a waveguide mode while undergoing multiple TIR interactions with opposingsurfaces10 and12. Upon each interaction with a sloped portion ofsurface10, light is progressively communicated a broader angular spread with respect toaxis200, as illustrated by the example ofray72. Upon reaching a sub-TIR angle with respect tosurface12, at least the uttermost out-of-plane rays, as exemplified byrays76 and72, will exit from the core ofwaveguide4 at different locations alongaxis200, depending on the initial propagation angles. Accordingly,light extraction layer20 extracts the emergent light by means of scattering which causes at least portions of the scattered light to overcome TIR atsurface10 and exit fromsystem2 towards a normal to the surface.
• FIG. 23 illustrates a further variation ofsystem2 in whichlayer6 has corrugatedouter surface14 andlayer20 conforms to this corrugated shape thus also forming a corrugated boundary between the two layers. Additionally, light extractinglayer20 has a specularly reflective surface which can be obtained, for example, by mirroringsurface14 or by depositing a specularly reflective film or foil ontolayer6.Corrugations18 can be made identical to each other and having a constant pitch.
• Referring toFIG. 23,facets124 facing the light source may be advantageously configured to reflect light emerging fromwaveguide4 at angles approximately perpendicular to the prevailing plane ofsystem2, as illustrated byray74, thus providing an efficient light turning structure of a reflective type. According to the principles discussed above,waveguide4 may be configured to emit light intobuffer layer6 within a narrow angular cone. In turn, such reflective light turning structure may be configured to redirect the emitted light towards surface normal while preserving the narrow beam divergence. It will be appreciated that this will result insystem2 emitting a highly collimated beam fromsurface10.
• It will be understood thatsystem2 may include any other variations of light extractinglayer20 which may be configured to permanently extract light fromsystem2 and direct and distribute it according to the specific application.Light extraction layer20 may also incorporate any other light directing structures which change the propagation path of light emerging fromwaveguide4 at low angles with respect its major surface. Various modifications oflayer20 may include lens arrays, prism arrays, mirrors, diffusers, retroreflective elements, scattering elements, color changing elements or layers, etc.
• FIG. 24 explains the light collimation function of certain embodiments ofsystem2 in further detail. InFIG. 24,light source400 emits light having a relatively broadangular distribution900. When such light enters the light input edge or end ofwaveguide4, it propagates within the waveguide along itslongitudinal axis200 undergoing multiple reflections fromsurfaces10 and12 by means of TIR. The interactions of light withsurface10 include reflections from surface relief features8 (not shown) which gradually increases the out-of-plane angle of light propagation and eventually results in light escape fromwaveguide4 into low-n buffer layer6.
• As explained above, when the characteristic slope angle α of surface relief features is sufficiently low, the angular distribution of light escaping intolayer6 is also very narrow as it generally subtends an angular range from zero to a small angle which depends on slope angle α. Accordingly, whenlight extraction layer20 turns the emerging light by up to 90 degrees, the angular distribution of light emitted from the broad surface ofsystem2 will also be relatively narrow. It will be appreciated that the light beam emerging fromlayer2 may experience some broadening of the angular distribution compared to its propagation in the bulk materials ofsystem2 generally having refractive indices considerably greater than a unity. Nevertheless, it will also be appreciated that the angular distribution of light emitted bysystem2 can be made substantially narrower than that ofsource400.
• This is illustrated inFIG. 24 by reference toangular distributions912,914,916, and918 exemplifying collimated light emerging from the broad-area surface ofsystem2 at different distances fromsource400. Each theangular distributions912,914,916, and918 may be made narrower than thebroad distribution900 oflight source400 at least in a plane in which the reflection by surface relief features8 occurs withinwaveguide4. Therefore,system2 may be configured to not only distribute light from a compact source over a large area and emit such light from said large area, thus resulting in a reduced glare, but also provide an improved collimation and/or directionality of the emitted light compared to the light beam emitted by the source.
• It will be appreciated that the light-collimating embodiments ofsystem2 illustrated above do not generally require applying any opaque or mirror layer ontosurface10 in order to prevent light escaping through that surface. Due to the combined function of surface relief features8, which provide incremental out-of-plane deflection of light rays propagating alongaxis200,buffer layer6, which preliminary extracts only the uttermost deflected rays fromwaveguide4, and light extractinglayer20, which finally extracts the pre-extracted light fromsystem2, virtually no light may be allowed to exit throughsurface10 even though said surface can have a very high optical transmissivity.
• This is in a sharp contrast to the conventional illumination systems employing a waveguide, such as edge-lit backlights or lighting luminaires, for example. In such conventional systems, a substantial portion of light escapes through an unwanted side of the waveguide (usually at least 25% and up to 50%) which requires using a special diffuse or specular reflector to be attached to that surface. The reflector typically includes a sheet of highly reflective material which redirects (with some reflection loss) the escaping light towards the other side of the waveguide. It will be appreciated that the use of an opaque reflector layer introduces additional losses (compared to TIR) and precludes the possibilities of transmitting light in a transversal direction or using the system as a front light.
• On the contrary, according to at least some embodiments of the present invention,waveguide illumination system2 can maintain high transversal transmissivity and allow for a generally unimpeded light passage along in a perpendicular direction with respect to its major surfaces. Therefore,such system2 may be used for transmitting light from a different light source in a transversal direction.
• By way of example and not limitation,system2 can be implemented as a light-collimating edge-lit luminaire which may also be positioned horizontally in the light path of a daylighting system, such as a skylight located above the luminaire. In such configuration,system2 can be configured to provide illumination by distributing and emitting light emanated by one or more LEDs attached to the edge ofwaveguide4 and, additionally, to transmit light from the skylight perpendicularly through its body thus forming a combined solar/electric lighting luminaire.
• FIG. 25 illustrates an exemplary embodiment ofsystem2 which is used at the light exit end of askylight502.Skylight502 can be ordinarily designed for rooftop installation and illumination of the building interior with the natural sunlight through an opening in the roof or ceiling underneath the skylight's light-collecting aperture.
• Referring toFIG. 25,skylight502 includes a dome-shapeddiffuser sheet260, and optionalreflective side walls512 and514.Diffuser sheet260 can be made from an optically clear plastic material and may have at least one microstructured surface to improve light diffusion.Walls512 and514 are preferably covered with a sheet or film of secularly reflective material to aid in light channeling fromsheet260 to the light-emitting opening below. The light emitting opening ofskylight502 may have approximately the same aperture as the light-receiving aperture ofsheet260. Alternatively,skylight502 may be made in a tapered version, in which case the above apertures may have different transversal dimensions with respect to each other.
• Referring further toFIG. 25,waveguide illumination system2 is implemented in the form of a, optically transmissive planar panel and includeswaveguide layer4, low-n buffer layer6 and light extractinglayer20 formed by a light turning film.Light source400 including a strip of high-brightness LEDs is positioned adjacent to the light input edge ofwaveguide4. At least a substantial portion ofsurface10 is provided with sufficiently shallow surface relief features8 each including low-slope face16 andadjacent face18. The orientation of surface relief features8 is such that each face16 facessource400 and each face18 is turned away fromsource400.System2 having the above layered structure and the form factor of a relatively thin panel is positioned belowskylight502 in the respective light-emitting opening of the skylight so that the prevailing plane ofwaveguide4 extends perpendicularly to avertical axis44.
• In electric lighting operation,waveguide4 receives light from the linear array of LEDs at its light input edge and guides said light by means of TIR. Accordingly, light injected through the light input edge propagates in a waveguide mode alongaxis200 towards the opposing terminal end ofwaveguide4. According to the principles described above, surface relief features extract light intobuffer layer6 along the propagation path. In turn, the light turning film of light extractinglayer20 finally extracts light downwards. Optionally,system2 may be configured to provide a prescribed degree of collimation and emit directional light alongvertical axis44. Accordingly, since light escape throughsurface10 is minimized or eliminated, substantially all of the light emitted bysource400 and distributed alongwaveguide4 is emitted into the building interior. With the exception of light which is absorbed or scattered during propagation inwaveguide4 orlayers6 and20, practically no additional light is lost in the system. Importantly, no light is directed back towardssheet260 which would otherwise constitute a major energy loss in the case whenwaveguide4 would employ conventional microstructures or other types of prior-art light extraction features.
• In daylighting operation,system2 receives a diffuse beam of sunlight emerging fromsheet260 and transmits it further downwards through its body. Since all respective layers and surfaces ofsystem2 are optically transmissive, the sunlight transversally passes through the panel without undergoing substantial reflection, backscattering or attenuation. When additional diffusing of daylight is necessary or when masking the portions of skylight disposed above the light emitting opening is desired,system2 may further comprise one or more light diffusing layers operating in the transmissive mode. In view of the above-described operation within a skylight, it will be appreciated thatsystem2 can emit artificial light from its broad-area surface and also doubles as a skylight luminaire by allowing the daylight into the building interior and optionally providing enhanced light diffusion.
• It will be appreciated that a unique operation ofsystem2 is obtained, at least in part, by employing a two-stage light extraction mechanism. The first stage includes incremental light deflection by surface relief features8 and the second stage includes beam turning by light extractinglayer20.Buffer layer6 separates these stages from each other and provides the functional differential in the refractive index drop at the opposing sides ofwaveguide4. By employing these features, in combination,system2 suppresses light extraction through the unwanted side ofwaveguide4 and can be configured to emit light primarily through the designated side or surface. When desired, as illustrated above, it may also be configured to almost completely shut-off light emission from the unwanted side despite being optically transmissive and allowing light to pass transversely through its body.
• FIG. 26 shows an alternative arrangement of the layers aroundwaveguide4, wheresurface10 is configured to be substantially flat butsurface12, instead, has surface relief features formed by smooth and shallow corrugations or undulations. Accordingly,buffer layer6 andlight extraction layer20 adjacent to surface12 may conform to the shallow relief features ofsurface12 or may, alternatively, have flat external surfaces.
• Accordingly, the refractive index ofbuffer layer6 being greater than that of the outside medium and lower than that ofwaveguide4 provides the functional difference between TIR angles φ_TIR1and φ_TIR2atsurfaces10 and12, respectively. Therefore, the incremental angular bias of light propagation caused by the undulations of surface relief features8 along the propagation path in thewaveguide4 will result in light escaping primarily throughsurface12 whilesurface10 will continue to reflect substantially all light by means of TIR.
• By way of example and not limitation,layer20 of the embodiment ofFIG. 26 may be formed by a scattering layer or a print formed directly onsurface14 oflayer6 or formed on a separate substrate which can be laminated ontosurface14. When laminatinglayer20 ontosurface14, a soft roller may be used for pressinglayer20 againstsurface14 in order to fill the shallow depressions insurface14 and avoid forming of air gaps.
• The amplitude of the undulations forming surface relief features8 may ordinarily be very small so that the relief oflayer20 will be virtually unnoticeable when viewed from a distance. Additionally, the shallow surface relief features8 will deflect light propagating between the visible surface oflayer20 and viewer'seye660 by only a small amount causing no perceptible visual distortions.
• Ray72 illustrates a light path inwaveguide4 at relatively low out-of-plane propagation angles allowing for TIR at bothsurfaces10 and12. Each TIR ofray72 fromface16 of therespective feature8 will generally result in a greater out-of-plane angle thus introducing additional angular bias and widening the angular distribution of light guided inwaveguide4. Accordingly,ray72 may eventually obtain an incidence angle less than the TIR angle with respect to surface12 when striking one of the successive faces16 and exit intolayer6 where it will be scattered bylayer20 towards the viewer.Ray74 illustrates a ray path of the light being extracted fromsystem2 as it already obtained a sufficiently high angle with respect to the longitudinal axis ofwaveguide4. Asray74 exits fromwaveguide4 intobuffer layer6, it strikes the light scattering surface oflayer20 and can be directed towards the viewer thus providing the illumination function of an edge-lit front light.
• FIG. 27A throughFIG. 27I illustrate various shapes of surface relief features8. InFIG. 27A,feature8 is shown to includeface16 inclined at a low slope angle α to the prevailing plane ofsurface10 and opposingface18 inclined at a substantially greater slope angle to said plane. InFIG. 27B, face18 is shown being perpendicular to the prevailing plane ofsurface10. It should be understood that the illustrative case ofFIG. 27B may also include variations offace18 having angles which are close to 90° but not exactly normal. Particularly, when therespective feature8 is formed insurface10 by replication processes like injection molding, compression molding, embossing, or extrusion processes, face18 may have a suitable draft angle of 3-4° with respect to a surface normal. Such draft angle may be necessary, for example, to facilitate mold release.
• InFIG. 27C,surface relief feature8 is shown to additionally comprise a flattop portion860 which extends parallel to the plane ofsurface10 and may also be configured to reflect by means of TIR. InFIG. 27D thetop portion860 has a curved shape in a cross-section.
• FIG. 27E showsfeature8 in which faces16 and18 are disposed symmetrically and have about the same slope angle with respect to the plane ofsurface10. Such a symmetric configuration offeatures8 may be selected, for example, for the case wherewaveguide4 is illuminated from both opposing edges or terminal ends.
• InFIG. 27F,surface relief feature8 is formed by a smooth undulation or shallow recess insurface10, where faces16 and18 are formed by the opposing side surfaces of the undulation.FIG. 27G showssurface relief feature8 formed by a funnel-shaped shallow cavity insurface10. It should be understood that a relatively small portion offace16 facing light source400 (not shown inFIG. 27G) may have an arbitrary high slope angle. Such portion may include, for example, the small area immediately adjacent to the vertex of the funnel. However, it is generally preferred that at least a major portion offace16 still has relatively low slope angle α with respect to the prevailing plane ofsurface10.
• It will be appreciated that there is a great variety of possible shapes that can be used for surface relief features8. Accordingly, any suitable profile or any suitable perturbation or irregularity of otherwise smooth andflat surface10 may be used to form individual surface relief features8, including any ordered or random surface relief structures, bumps, recesses, grooves, corrugations, surface waviness, etc., provided that they can introduce the required additional angular bias for light propagating inwaveguide4 along its path and cause controlled light leakage intolayer6.
• FIG. 27H showssurface relief feature8 formed by a textured micro-relief portion ofsurface10. The textured portion may be surrounded by non-textured, flat portions ofsurface10 thus creating a distinct, well defined microstructured area of therespective feature8. The texture may be formed in a random or ordered manner, provided that each elementary micro-relief feature has a low aspect ratio (low height to width) so that the respective surface slope angles do not exceed a certain maximum angle which would cause light leakage fromwaveguide4 throughsurface relief feature8. According to one embodiment, considering that the walls of micro-relief features of the surface texture may have a variable slope from top to bottom, the walls of each micro-relief feature should have a sufficiently low prevailing slope to provide the above discussed operation. Additionally, each micro-relief feature should have a generally smooth surface on the scale which is comparable to the wavelength of light ofsource400. This requirement is aimed at minimizing or preventing light scattering and/or reflecting at higher than the prescribed angles with respect to the longitudinal axis ofwaveguide4 or the plane ofsurface10.
• It should be understood thatsurface relief feature8 are not limited to recesses or undulation-type surface microstructure but may also be formed by light-deflecting surface protrusions or indentations of the appropriate shape and even by varying the thickness ofwaveguide4 along the light propagation path, provided the prevailing slope of the respective surface structures is less than the prescribed maximum angle.
• By way of example and not limitation, the surface of the each textured area ofFIG. 27H may include an array of microlenses. According to the principles discussed above, each of the microlens should have a sufficiently low profile ensuring that the slope of the microlens walls with respect to surface10 does not exceed the maximum allowed angular value so thatsystem2 may operate in the manner described in the above embodiments.
• InFIG. 27I, an alternative configuration ofsurface relief feature8 is illustrated which includes a low-aspect-ratio, smooth-walled protrusion insurface10. Referring to a projection ofsurface relief feature8 onto a plane parallel to surface10, such protrusion may have an elongated shape, a round shape or any other suitable shape or outline. In a non-limiting example, a two-dimensional protrusion may be formed by depositing a micro droplet of UV-curable polymer ontosurface10. The polymer should preferably have a low surface tension allowing for the droplet to obtain the desired low-profile shape before curing. Alternatively, surface10 ofwaveguide4 may be specially treated or coated to increase its surface tension and obtain the desired surface wettability.
• Referring toFIG. 27I and to the example of forming surface relief feature by a UV-curable droplet, slope angle α may be associated with the angle at which the liquid contacts the surface. This contact angle is commonly known as the wetting (or dihedral) angle that a liquid droplet makes to a solid surface. Accordingly, in the illustrated example, the wetting angle of the droplet with respect to surface10 should be kept sufficiently low in order to form a low-profilesurface relief feature8 which could deflect light by a sufficiently small angle with respect to its original propagation direction.
• It is noted that surface relief features8 may be arranged insurface10 in a variety of ways. For example, surface relief features8 may be formed in a parallel array of strips or bands extending perpendicular tolongitudinal axis200 ofwaveguide4. The respective strips or bands can be made substantially straight. Alternatively, they may have a constant curvature or even some waviness.
• FIG. 28 shows an embodiment ofsystem2 in which waveguide4 has a shape of a rectangular plate or slab and in which surface relief features8 are arranged in a parallel linear array which longitudinal axis is perpendicular toaxis200.FIG. 28 also shows an exemplary configuration oflight source440 which includes a strip of LEDs where each LED is provided with individualcollimating element440. Eachcollimating element440 intercepts light emitted by the respective LED and collimates said light towardsaxis200. Collimatingelement440 may be configured to provide light collimation in at least one plane or dimension. By way of example and not limitation, collimatingelement440 ofFIG. 28 may have a linear geometry and may be characterized by a longitudinal axis and an optical plane both of which extending perpendicular to the plane of the drawing. It will be appreciated that suchlinear collimating element440 will provide light collimation in a plane which is parallel to the respective plate or slab. In a further non-limiting example, collimatingelement440 may have a round aperture and may be configured to collimate light in two orthogonal dimensions such as those parallel and perpendicular to the prevailing plane of the rectangular plate or slab.
• FIG. 29 shows an alternative exemplary arrangement of surface relief features8 acrosssurface10 and also shows an alternative configuration of a plurality ofcollimating elements440. Referring toFIG. 29, eachsurface relief feature8 is formed by a textured area as explained in relation toFIG. 27H and is separated from the adjacent surface relief features8 by a spacing area in an ordered array pattern. The size of eachsurface relief feature8 may be varied. Particularly, the size may increase with the distance fromlight source400 to compensate the light depletion inwaveguide4 alongaxis200. Alternatively, the density of surface relief features8 in the array may increase with the distance fromsource400 for the same reason. Referring further toFIG. 29, an array of lightcollimating elements440 is arranged on a single transparent substrate and form a solid piece structure which can be placed between the LED strip andwaveguide4. Accordingly, lightcollimating elements440 ofFIG. 29 may also be configured to emit light alongaxis200 and provide light collimation in one or two dimensions.
• FIG. 30 depicts an embodiment ofillumination system2 in whichbuffer layer6 is provided on themicrostructured surface10 ofwaveguide4. Light extractinglayer20 provided on the external surface of thebuffer layer6, accordingly so thatsurface12 is exposed to the outside medium.Layer6 should provide a good optical contact betweenwaveguide4 and light extractinglayer20. It may be formed as a conformal thin coating (conforming to the shape of surface relief features8) of a constant thickness. Alternatively, as shown inFIG. 30,layer6 can be made of a sufficient thickness to fill the depressions insurface10 and provide a smooth external surface. Thelight extracting layer20 ofFIG. 30 can be configured to provide strong backscattering of light emerging fromwaveguide4 with a highly diffuse reflectivity in whichcase system2 can be advantageously used, for example, for general lighting applications.Layer20 may also be associated with an image print in which configuration thesystem2 can be used, for example, as a front light, decorative light or as an illumination cover for signage or painting/printing arts.
• In operation, referring toFIG. 30,ray72 propagating inwaveguide4 strikes surface10 where it is reflected from one of surface relief features8 by means of TIR due to the relatively low propagation angle with respect toaxis200. The slope offace16 with respect to the prevailing plane ofsurface10 is sufficiently low so as to result in the incremental increase in the out-of-plane angle ofray74 without causing it to exit throughsurface10. As a result of TIR fromface16,ray72 obtains slightly lower incidence angle with respect to normal800 and further propagates towards the opposingsurface12 ofwaveguide4.
• Since the drop in refractive index outwardly fromwaveguide4 at itssurface12 is greater than that atsurface10, the incidence angle ofray72 intosurface12 is not sufficient to overcome TIR at that surface. Therefore,ray72 continues to propagate in the waveguide mode through the core ofwaveguide4.
• In contrast, rays74 and76, deflected by the preceding surface relief features8 (not shown),strike surface10 at smaller angles with respect to normal800, said angles being less than the critical angle of TIR at the interface betweenwaveguide4 and low-n layer6. As a result, rays74 and76 exit from the core ofwaveguide4 andstrike layer20 which scatters the extracted rays back towards the viewer'seye660.
• FIG. 31 depicts an embodiment similar to that ofFIG. 30 except thatlight extracting layer20 is configured to transmit and diffuse light emerging fromwaveguide4 towards the opposing side. In such configuration,system2 can be used, for example, for backlights, transmissive sign displays or area illumination.
• Referring toFIG. 30 andFIG. 31,exemplary rays74 and76 illustrate light emerging fromwaveguide4 and illuminatinglayer20. As it is further illustrated, the emerging light can be further directed bylayer20 to provide either front light or back light operation depending on the configuration of the light scattering/diffusing layer. Furthermore, the surface oflayer20 can be illuminated from one or more additional directions and by one or more additional light sources such as, LEDs, lighting luminaires or sunlight. By way of example and not limitation, whenlayer20 comprised a transmissive light diffuser, it can be illuminated from the appropriate edge ofwaveguide4 by a strip of LEDs and by a beam of sunlight incident ontolayer20 perpendicular to its surface, in whichcase system2 ofFIG. 31 may have operation similar toFIG. 25
• Referring toFIG. 31, a distantlight source890 is illustrated which illuminatessystem2 perpendicularly to its plane.Ray892 passes through the transparent layers ofwaveguide4 andbuffer layer6 after which it strikeslayer20. In turn,layer20 scattersray892 similarly torays74 and76, all of which are extracted from the waveguide toward a viewer600. Whenlight source890 is exemplified by sunlight, such as that delivered from a skylight or a similar device,system2 can be used as a luminaire for hybrid lighting and can provide combination of natural and artificial illumination. In this case, light fromsource400 can be advantageously made dimmable in response to the change in the sunlight intensity so that constant lighting in the room can be maintained. The corresponding layers ofsystem2 may be configured to provide illumination pattern which is generally symmetrical with respect to no normal800.
• FIG. 32 depicts an embodiment ofsystem2 in which light extractinglayer20 is made from an optically transmissive material and includes V-shapedprismatic grooves380 which slope is selected to redirect light emerging fromwaveguide4 into a perpendicular direction with respect to the prevailing propagation path of light in the waveguide. It is preferred that a refractive index n₆oflayer20 is at least equal or greater than that ofbuffer layer6 to prevent TIR at the boundary between the two layers.
• Accordingly, rays74 and76 emerging first intobuffer layer6enter layer20 and strike respective sloped faces ofgrooves380 where said rays are reflected by beans of TIR towards along a normal to the prevailing plane ofwaveguide4.Ray78 strikes an interface with the air pocket formed bygroove380 and is reflected by TIR back intowaveguide4 and towardssurface10. However, sincesurface10 represents a boundary with the outside medium (air) which has substantially lower index thanwaveguide4,ray78 will eventually reflect fromsurface10 and can thus have a further chance of being fully extracted fromsystem2 byother grooves380 along the propagation path.
• It will be appreciated that the arrangement ofFIG. 32 can be configured to efficiently collimate light at least in the plane of the drawing that is a plane which is parallel tolongitudinal axis200 ofwaveguide4 and which is also perpendicular to the prevailing plane ofwaveguide4. Accordingly, whenwaveguide4 ofsystem2 is illuminated by a non-collimated or weakly collimated beam of light at its light input edge or end, the illustrated embodiment ofsystem2 can emanate highly collimated light at least in the above-mentioned plane. It will also be appreciated that the degree of such collimation can be adjusted in a broad range, for example, by adjusting the slopes offaces16 offeatures8 and/or the slopes or configuration ofgrooves380. Moreover, additional angular distribution effects for the emitted light can be achieved by varying the respective slopes offaces16 and/orgrooves380 along thelongitudinal axis200 ofwaveguide4.
• It will be further appreciated thatsystem2 may include various collimated elements attached to the input edge or end ofwaveguide4 which can provide additional means for controlling the angular spread or distribution of light emitted from the respective broad surface ofsystem2. Particularly, whensystem2 employs a planar configuration ofwaveguide4 and discrete light sources such as LEDs,collimating elements440 such as those illustrated inFIG. 28 andFIG. 29 may be used to additionally collimate the incident beam in a plane parallel toaxis200 and also parallel to the prevailing plane ofwaveguide4. In this case,system2 can emit light which is collimated in two orthogonal planes or dimensions rather than just in a single plane or dimension. Additionally, it is noted that the degree of collimation and angular distribution of light emitted bysystem2 may be independently controlled in each of the orthogonal planes or dimensions thus providing a virtually unlimited number of combinations for the emitted beam parameters. In a non-limiting example, when used as an area illumination device such as ceiling-mounted panel luminaire,system2 may be configured to illuminate a well defined area below the luminaire while emitting much fewer or no light towards non-functional directions.
• According to one embodiment, slope angle α of each face16 may be limited to about three angular degrees in order to maximize the collimation power ofsystem2 and/or minimize the light leakage through surface relief features8. More particularly, according to certain embodiments, slope angle α of each face16 may be less than two and a half degrees, less than two degrees, less than one and a half degrees, and less than one degree. According to one embodiment, the minimum slope angle α of each face16 may be about one half of a degree.
• The slope of the reflecting walls ofgrooves380 may be selected to redirect light emerging fromwaveguide4 at any suitable angle with respect to a normal to the prevailing plane or longitudinal axis ofsystem2. Particularly,grooves380 oflight extraction layer20 may be configured to redirect the emergent light so as to result insystem2 generally emitting light at an off-normal angle from its major broad-area surface. Such an off-normal angle may take particular values of, for example, thirty degrees, forty five degrees or sixty degrees. Considering thatsystem2 may be configured to emit a major portion of collimated light in a limited range of angles with respect to the surface normal, such range may be between zero and thirty degrees, between fifteen degrees and forty five degrees, between thirty degrees and sixty degrees, for example. Such off-normal illumination may be useful for a number of applications ofsystem2. By way of example and not limitation,system2 emitting off-normal collimated beam may be used to illuminate a portion of a wall from a planar lighting fixture mounted flush with a ceiling, in an application like accent illumination of wall-mounted fine art drawings and the like.
• It should be understood thatgrooves380 may be substituted by any other features capable of redirecting light emerging fromwaveguide4 toward a prescribed direction.FIG. 33 shows another non-limiting example of light redirecting features associated withlayer20 and exemplified by sharp-angle slits382 in the surface oflayer20. Eachslit382 has a relatively narrow base and extends deep into the body oflayer20 at an angle with respect to surface normal. Such configuration of light redirecting features may be more advantageous from the point of view of enhancing the light extraction efficiency. As it can be seen fromFIG. 33,ray78 is extracted fromsystem2 in the first pass as opposing to the configuration illustrated inFIG. 32. The deep andnarrow slits382 may be made by any suitable means, including but not limited to laser ablation, slitting with a sharp blade, material cracking, etc. The process of forming of such slits should preferably allow for forming optically smooth walls which can efficiently reflect light by means of TIR. It is noted that, while straight profiles of the slit walls are illustrated inFIG. 33, such walls may also have any curvilinear or stepped profile as well.
• In a further non-limiting example,grooves382 may be replaced by narrow undercuts made in a surface oflayer20. Each undercut may have parallel or nearly-parallel walls. and may be formed, for instance, by laser ablation, a sharp blade or by any other means of material removal or cutting. Similarly, to the above example ofslits382, at least the light-redirecting wall of each undercut should preferably have an optically smooth surface allowing for TIR. The light-redirecting wall of each undercut should make a dihedral angle with the surface oflayer20 which is suitable for light extraction towards a normal direction with respect to the prevailing plane ofsystem2.
• FIG. 34 illustrates an embodiment ofsystem2 in which surface relief features have a variable slope with respect to the prevailing plane ofwaveguide4. Accordingly,system2 ofFIG. 34 includeswaveguide4 defined by opposing TIR surfaces10 and12, low-index buffer layer6 attached to surface12,light extraction layer20 attached to the buffer layer, andlight source400 coupled to an edge or terminal end of the waveguide.Surface10 is provided with an array of micro-stepped surface relief features8. Eachfeature8 has a TIR face facing the light source and inclined at a sufficiently low angle with respect to the prevailing plane ofsurface10 and/orwaveguide4 so that light leaks out ofwaveguide4 primarily throughbuffer layer6 andlight extraction layer20. The slope of individual TIR faces increases along the light propagation path as a function of the distance fromlight source400. This can be useful, for example for compensating of light depletion by theprevious features8 along the waveguide and for providing a uniform light output from the light emitting surface oflayer20. According to one embodiment, the slope angle of the TIR faces varies between approximately one half of a degree and three degree. According to one embodiment, the slope angle of the TIR faces increase with the distance fromsource400 linearly. According to one embodiment, the slope angle of the TIR faces increase with the distance fromsource400 according to a non-linear function in which the rate of slope increase accelerates with the distance fromsource400.
• It will be appreciated thatsystem2 ofFIG. 33 is designed to receive light from one side and emit it from the respective broad-area surface. Such configuration exhibits certain geometrical asymmetry with respect to a normal to the prevailing plane of the system, which, in turn, my potentially result in an unwanted asymmetry in light distribution in the prevailing plane of light extraction and collimation. In order to eliminate such asymmetry,system2 may be configured in a symmetrical configuration which is also illuminated from the opposing sides by symmetrically disposed light sources.
• FIG. 35 shows a two-sided implementation ofsystem2 which includes two symmetrical segments alongaxis200. Accordingly,waveguide4 also has two symmetrically disposed segments and two opposing light input edges or ends which are illuminated with the respectivelight sources400 and410. Each of the waveguide segments is provided with an array of surface relief features8 formed insurface10.
• Each of surface relief features8 is represented by a low-profile, linear asymmetric micro-prism protruding from a base atsurface10 outwardly fromwaveguide4. The tips or ridges of the micro-prisms may have sharp corners or they may also be slightly rounded or even have small flat portions.
• The larger-area facets of the micro-prisms are tilted outwardly from the central portion ofwaveguide4 and are configured for incremental deflection of propagating in a waveguide mode. Each of the larger-area facets should preferably have an optically smooth, polished surface to preserve TIR atsurface10 and minimize light loss.
• The dihedral angle of the larger-area facets of the micro-prisms is variable alongaxis200, increasing along the intended light propagation path from the opposing edges or ends ofwaveguide4 towards the mid-portion ofwaveguide4. This ensures that the rate of light deflection increases as a function of the distance from the respective light source and at least partially compensates light depletion inwaveguide4 by the preceding micro-prisms. In will be appreciated that the dihedral angles of the microprisms may be particularly tailored to provide for a nearly-constant rate of light extraction from the surface ofwaveguide4 alongaxis200. Additionally, the rate of progressive light extraction alongaxis200 by the prismatic surface relief features8 should preferably be sufficient to ensure that substantially all or at least a substantial portion of light injected intowaveguide4 may be extracted.
• Accordingly, the opposing lateral sides of light extractinglayer20 may be configured to receive light from the respective portions ofwaveguide4 and further direct the extracted light towards surface normal.
• Referring further toFIG. 35, each of the opposing segments ofsystem2 may be configured to emit a slightly asymmetric beam with respect to a normal to the emitting broad-area surface. However, since the opposing segments ofsystem2 are symmetrical and emit beams that mirror each other with respect to a surface normal, the resulting light distribution representing a superposition of the respective light beams will also become symmetrical with respect to the plane of symmetry ofsystem2 regardless of the distance from the emitting surface.
• Accordingly, by varying the angular distribution and directionality of the individual beams emitted by the opposingsystem2 segments ofFIG. 35, the resulting light beam may be focused by pointing the respective beams toward converging directions or a target. Alternatively, the individual beams may also be defocused by pointing them beams toward diverging directions or away from a certain target.
• It will thus be appreciated that the structure and operation ofsystem2 allow for an unprecedented control of the beam emitted from its broad-area surface compared to the conventional art employing waveguide illumination systems. In accordance with at least some embodiments and the principles of light collimation described above in reference, for example, toFIG. 9 andFIG. 10, the angular distribution of light emerging fromwaveguide4 can be made very narrow at least in one plane or dimension. In turn,light extraction layer20 may be configured to direct the narrow-distribution light emerging fromwaveguide4 towards a plurality of distinct directions. Such directions may be parallel, convergent or divergent. The divergent or convergent directions may also follow a particular angular pattern. Alternatively, the direction of light emission from the light emitting region ofsystem2 may be randomized within a limited range thus providing an improved light diffusion without sending light into non-functional directions.
• In an illustrative example, each ofangular distributions912,914,916, and918 ofFIG. 24 may be directed towards a common area located at a distance fromsystem2 along a normal toaxis200. As a result, particularly when the respective distributions are sufficiently narrow, the intensity of the light beam in the respective area can be made greater than in the intensity of the surrounding areas. Thus,system2 may be configured to additionally focus light emitted from its surface.
• Such focusing can be achieved by the appropriate configuring thelight extraction layer20. For instance, whenlayer20 employs light redirecting features such asprismatic grooves380 ofFIG. 32 orslits382 ofFIG. 33, the slope of the respective features may be varied accordingly so as to provide such focusing of the emitted light. In an exemplary case, referring toFIG. 32, the TIR walls ofrespective grooves380 redirectingrays74 and76 can make slightly different dihedral angles to the prevailing plane ofwaveguide4 so as to result in said rays converging at a certain distance fromsystem2.
• FIG. 36 shows an embodiment ofillumination system2 in a front light configuration in which there is provided a light-guiding layer formed byplanar waveguide4 andlight extraction layer20 positioned immediately adjacent to the broad-area surface12 of the waveguide.Waveguide4 has high optical transmissivity of the material and also high transparency at least in a direction perpendicular to the waveguide's plane. The waveguide is configured for transmitting light from edge to edge by means of TIR from at least the opposingsurfaces10 and12 both of which are made smooth (polished) and highly transparent. The side walls ofwaveguide4 can also be made smooth and capable of reflecting light by means of TIR.Light source400 is coupled to the light input edge of the waveguide. The opposing (terminal) edge can be provided with a mirrored surface to reflect light back into the waveguide and recycle rays that have not been extracted from the waveguide in a single pass.
• Layer20 comprises a screen containing an image print which is opaque for the incident light and includes at least one region which has good light scattering or reflective properties. The image may contain any text, graphics, symbols or patterns and can be exemplified, for example, by a front-illuminated display that can be found in the signage industry.
• Waveguide4 andlayer20 ofFIG. 36 should be preferably positioned very close to each other, including the case when they are disposed in a physical contact by at least portions of their surfaces. However, a direct optical contact without any intermediate layer (such as air) should be avoided in order to prevent the suppression of TIR ofsurface12 and unwanted premature light leakage fromwaveguide4.
• Waveguide4 is exemplified by a transparent plastic matrix with a plurality of very fine optically transmissive glass or plastic particles which are incorporated into the matrix in very small proportions by volume and have forward-scattering properties. The forward-scattering particles are meant to mean such scattering particles that scatter light substantially in a forward direction and have a very low or negligible scattering far sideways and in the reverse direction.
• The forward-scattering particles are made from an optically transmissive material which refractive index differs from that of the main bulk ofwaveguide4 by a predetermined amount. A minimum difference in refractive indices is required for the particles to effectively scatter light rays that propagate sufficiently close to them. Particularly, it is preferred that the refractive index of the particles differs by more than 0.02 but less than 0.4 from the refractive index of the body ofwaveguide4. The forward-scattering particles should preferably be finely separated from each other by considerable distances in order to provide for consistent and predictive light scattering as well as maintain high visual transparency and transmissivity of the waveguide.
• By way of example and not limitation, the core ofwaveguide4 can be made from PMMA (n_core=1.49) and the particles can be made from polystyrene (n_part≈1.59) or FEP (n_part≈1.34). The particles can have a simple, single-component structure or they can have a more complex morphology and may be composed by a core and a shell made from different materials. Depending on the other parameters involved, the particles can have a mean diameter less than, equal to or greater than the wavelength of visible light.
• The concentration of forward-scattering particles should be sufficiently low to keepwaveguide4 optically thin along the perpendicular to the prevailing plane of the waveguide. At the same time, the particle concentration should be high enough to makewaveguide4 optically thick alonglongitudinal axis200 and yet highly transmissive.
• The term “optical depth” or “optical thickness” is commonly directed to mean the quantity of light removed from a light beam by scattering or absorption during its path through a medium. In the context of this invention, as applicable to waveguides including non-absorbing, forward-scattering particles, this term can be more narrowly directed to mean the quantity of light that has been perceptibly scattered from the original propagation path of the light beam. The original propagation path is the path of the light beam in the homogeneous medium of the waveguide in the absence of the scattering particles.
• Particularly, if I₀is the reference intensity of radiation in a homogeneous, weakly-absorbing medium ofwaveguide4 and I is the observed intensity of light propagating along the same optical path, then an optical depth τ of the medium in the presence of forward-scattering particles can be defined by the following expression: I=I₀e^−τ.
• In the optically thin case, that is referring to the case of light propagating along the perpendicular to the plane ofwaveguide4, τ<<1 and e^−τ≈1≈τ, so that the above expression can be simplified as follows: I=I₀(1−τ). Accordingly, it at least some embodiments of the present invention,waveguide4 can be configured to perceptibly scatter less than 5% of light propagating along the waveguide's normal and more preferably scatter less than 2% of said light. In other words, the column density of light scattering particles should be low enough so that τ is less than 0.02-0.05 along the normal to the waveguide's prevailing plane and so that the high transparency of the waveguide is maintained.
• At the same time, the concentration and light scattering parameters of forward-scattering particles should be selected to ensure that at least a substantial part of light is removed fromwaveguide4 by means of scattering as such light propagates through the column of material alonglongitudinal axis200. According one embodiment, it is preferred that at least approximately half of the light beam input intowaveguide4 through an edge is scattered as it propagates towards the opposing terminal edge. According to one embodiment, it can be preferred that at least 80% or light is scattered along its longitudinal propagation inwaveguide4, which corresponds to an optically thick case of τ≧1.6.
• An important relevant parameter for estimating the required concentration of light scattering particles is an effective cross-section σ which defines the area around the particle where light is likely to be scattered. In general, the scattering cross-section is different from the geometrical cross-section of a particle, and it depends upon the wavelength of light and the permittivity, shape and size of the particle. Another important parameter defining the type of scattering and the effective cross-section of dielectric particles of diameter d in a refractive medium is the so-called size parameter x.
• $x=\frac{\pi d·n_{\mathrm{med}}}{\lambda },$
• where n_medis the refractive index of the refractive medium and λ is the wavelength of the propagating light.
• The general case for an arbitrary value of x is called Mie scattering in the relevant art. In the case of relatively large particles compared to the wavelength (x>>1), the total cross-section tends toward a geometric limit of σ=πd²/4. As the particles become smaller, the forward scattering diffraction peak can be observed, in which case the cross section will be become twice the geometric limit.
• In the case of very small particles (x<<1), the so called Rayleigh limit, the total cross-section is given by
• $\sigma =2/3\pi {\mathrm{<figure-callout\; label="d"\; filenames="US20140140091A1-20140522-D00000.png,US20140140091A1-20140522-D00002.png"\; state="\{\{state\}\}">d</figure-callout>}}^2{x^4\left(\frac{m^2-1}{m^2-1}\right)}^2,$
• where m is the ratio between the refractive index of the particle and the refractive index of the medium.
• Considering light propagating inwaveguide4 having sparsely distributed scattering particles and assuming that the particles do not shadow each other, the optical depth can be related to the scattering cross-section through the following expression: τ=NLσ, where L is the propagation path length and N is the number density of light scattering particles.
• The relationship between an optical depth along the waveguide's longitudinal axis τ₈₁and an optical depth perpendicular to the prevailing plane of the waveguide τ_⊥ defines the relative difference in waveguide transparency between the respective directions. It can also be used to define the relationship between the thickness and the length of the waveguide for the given scattering efficiency of the particles and particle concentration.
• The scattering particles should be particularly configured to be substantially invisible to the naked eye from a distance and should not introduce a perceptible blur or haze to the bodies or images behindwaveguide4. Additionally, the scattering particles should be configured to not introduce a substantial glare or loss in image contrast whenwaveguide4 is lit bysource400 or by any other external light source.
• In a non-limiting example, the core ofwaveguide4 may include an Acrylite® Endlighten acrylic sheet (e.g. available from CYRO Industries, Rockaway N.J.) which combines visual transparency and light-scattering properties. In a further non-limiting example,waveguide4 may include a Plexiglas® ELiT acrylic sheet. The sheets can be made using extrusion, casting or any other suitable process and can be configured to provide 90-92% light transmission along surface normal.
• In operation, referring further toFIG. 36, the light input edge ofwaveguide4 receives light emanated bysource400 and refracts light into the body of waveguide allowing at least a substantial part of the emitted light to propagate in a waveguide mode. Such light injected through the light input edge ofwaveguide4 propagates towards the opposing terminal edge by means of transmission and TIR fromsurfaces10 and12 until it is incrementally scattered by a the light scattering particles to a sufficient angle to overcome TIR atsurface12 and exit towards light extractinglayer20. The image print oflayer20 back-scatters the out coupled light into all directions so that at least a portion of the scattered light can reach the viewer'seye660. Thus, the observer can clearly see the image print which is brightly lit by the highly transparent top layer represented bywaveguide4.
• FIG. 37 illustrates the structure and operation of the embodiment ofFIG. 36 in further detail by depicting an exemplary path ofray74 emanated bysource400.Waveguide4 comprises a large plurality of forward-scatteringparticles36 made from a transparent refractive material that has different refractive index thanwaveguide4. Each scatteringparticle36 is configured to provide strong forward scattering propertied so that each interaction of light ray with such particle can change its propagation direction by up to a relatively small, predetermined amount. The bend angle introduced by eachparticle36 should be low enough to generally prevent light exiting fromwaveguide4 at high angles with respect to the prevailing plane of the waveguide. Instead, each scatteringparticle36 should be configured to provide incremental deviation of rays along the propagation path and eventually result in light rays exiting fromsurface12 at low angles with respect to the surface, thus providing means for illuminatinglayer20 by the decoupled light.
• Particles36 are preferably finely distributed through the volume ofwaveguide4 in a concentration low enough to maintain high transparency of the waveguide along normal800. At the same, time, the concentration should be sufficient to extract most of the light fromwaveguide4 by means of forward scattering and by means of eventual communicating incidence angles greater then TIR atsurface12.
• Due to the probabilistic nature of scattering and particle encounter,ray74 may undergo multiple scattering events until it reaches the incidence angle greater than the TIR angle atsurface12. It will be appreciated that at certain concentrations oflight scattering particles36, a portion of light beam may remain non-extracted upon reaching the terminal edge ofwaveguide4. Therefore, the terminal edge may be provided withreflective surface602 which will reflect the above portion of light back into the waveguide and promote a more complete light utilization. In certain cases, employingreflective layer602 may also result in an improved uniformity of light emitted bysystem2.
• It will be appreciated by those skilled in the art that whenwaveguide4 is surrounded by air, the smooth waveguide/air interfaces atsurfaces10 and12 may appear functionally identical with respect to light propagating by TIR within the waveguide. Accordingly, with the absence of TIR suppression or at one of the surfaces, the propagating light has essentially equal probability of exiting through either surface. As a result, about 50% of light may exit fromwaveguide4 without illuminating the image print oflayer20. Although the emergence angle of light exiting throughsurface10 can be made sufficiently low so as not to interfere significantly with viewing the illuminated image print from a normal direction, the loss of the respective portion of light may be unwanted for a variety of reasons.
• In order to eliminate or at least substantially reduce this light loss and enhance the brightness of the image print, a suitable intermediate outcoupling layer can be provided betweenwaveguide4 andlayer20, as illustrated inFIG. 38.
• Referring toFIG. 38, the intermediate outcoupling layer is exemplified by highlytransparent buffer layer6 disposed in a good optical contact with bothwaveguide4 andlayer20. Similarly to at least some of the embodiments discussed above in reference to, for example,FIG. 5 andFIG. 6,buffer layer6 has a lower refractive index thanwaveguide4. At the same time, the refractive index oflayer6 should be greater than the refractive index of the surrounding air or otherwise of the material adjacent to surface10.
• Buffer layer6 allows for TIR atsurface12 at sufficiently high incidence angles, that is when the propagation angle is sufficiently low with respect tolongitudinal axis200. Similarly to the embodiments described in reference toFIG. 5 andFIG. 6,surface10 can be characterized by the first TIR angle φ_TIR1andsurface12 can be characterized by the second TIR angle φ_TIR2.
• Accordingly, when the incidence angle ontosurface12 becomes lower than φ_TIR2as a result of scattering byparticles36 and incremental increase of the out-of-plane angle, light can escape fromwaveguide4 through the respective surface and illuminate the image surface oflayer20. Since the refractive index of air is lower than that oflayer6, the TIR angle φ_TIR1surface10 is lower than the second TIR angle φ_TIR1. This results in the prevailing light escape paths throughlayer6 and no light or at least much less light leaking throughsurface10.
• Referring further toFIG. 38,system2 may also include collimatingelement440 associated with the light input edge ofwaveguide4. Collimatingelement440 may be attached to the light input edge ofwaveguide4 with or without good optical contact or it can be spaced apart from the edge by a small air gap or by an intermediate optically transmissive layer. For maximizing the light input, collimatingelement440 may be coupled to the waveguide edge using an optical adhesive or encapsulant. Collimatingelement440 may be configured to narrow the angular light distribution emitted bylight source400. This may ensure that the second TIR φ_TIR2is not exceeded inwaveguide4 prematurely for at least some rays and that no substantial portion of light exits from thewaveguide4 near the light input edge. Since the embodiment ofFIG. 38 may be configured to admit up to about twice as much light onto the print oflayer20 and eliminate or at least substantially suppress light loss through the opposingsurface10 compared to the embodiment ofFIG. 37, it may be advantageously selected for at least some applications where more complete light utilization and improved image contrast are important.
• FIG. 39 depicts an embodiment ofsystem2 configured as a backlight and more particularly as a two-sided backlight. An additional pair of the buffer layer and the light extraction layer is provided onsurface10, as indicated byreference numerals506 and520, respectively.
• Similarly, thelayers506 and520 are laminated to surface10 with a good optical contact with or without intermediate adhesive layers. Both light extraction layers20 and520 may be configured as transmissive diffusers and can have any suitable color, tint, light transmissive and scattering characteristics. Either one of thelayers20 and520 may also be configured to display an images or any suitable pattern or text.
• Accordingly, light propagating throughwaveguide4 and scattered byparticles36 can be evenly distributed along thesurfaces10 and12 and will illuminate bothlayers20 and520 thus providing the desired operation of a two-sidedbacklight having waveguide4 sandwiched between the opposing light extracting layers.Ray74 ofFIG. 39 exemplifies light exiting fromwaveguide4 throughsurface12 andray76 exemplifies light emerging fromsurface10.
• FIG. 40 illustrates the effect of addinglayer6 betweenwaveguide4 andlight extraction layer20 for an exemplary case of anisotropic scattering particle536 embedded into the material of the waveguide. Aray720 exemplifies light propagating in a waveguide mode throughwaveguide4.
• Referring toFIG. 40,ray720 is scattered into all directions by theparticle536. Whilesurfaces10 and12 receive an about equal number of scattered rays, the low-n layer6 enables the asymmetry in TIR angles betweensurfaces10 and12 and results in more light extracted throughsurface12 towardslayer20 compared to the amount of light escaping fromwaveguide4 throughsurface10.
• FIG. 41 illustrates the advantage of employing anisotropic scattering particles by example of a single forward-scatteringparticle36 operating in conjunction with the low-n layer6. As shown inFIG. 41,ray720 is forward-scattered byparticle36 at scattering angles not exceeding the complementary angle to the first TIR angle φ_TIR1. At the same time, the maximum scattering angle is greater than the complementary angle to the second TIR angle φ_TIR2which causes at least some uttermost scattered rays to obtain sub-TIR incidence angles with respect to surface12 and exit from waveguide intolayer6. Sincelayer6 is optically transmissive, all light rays exiting intolayer6 reachlight extraction layer20 which, in turn may redirect or redistribute said rays and direct them into the prescribed directions. Accordingly, by limiting the scattering angle by a predetermined value, the unwanted light escape throughsurface10 can be eliminated or at least substantially reduced compared to the case ofFIG. 40 whereisotropic particle536 is employed.
• A scattering angle δ can be defined as an angle between the original propagation direction that a light ray has before encounteringscattering particle36 and the direction of a scattered ray resulting from ray interaction with the particle. Accordingly, referring toFIG. 42, ifray720 has out-of-plane propagation angle β before scattering, it may obtain a new out-of-plane propagation angle β+δ thus broadening the angular distribution of light propagating inwaveguide4.
• In an operational aspect of this invention, forward-scatteringparticles36 may be configured to provide function similar to surface relief features8 of at least some embodiments discussed above (see, e.g., the discussion on broadening the angular distribution of propagating light beam in reference toFIG. 7 andFIG. 8). More particularly, it may be appreciated that an analogy may be drawn between the scattering angle δ ofFIG. 42 and the angular increment2α ofFIG. 8. Although angle δ ofFIG. 42 is obtained by using a light scattering mechanism and a specially configuredparticle36 embedded into the bulk ofwaveguide4 material while angle2α is obtained usingsurface relief feature8 having a characteristic slope α with respect to the waveguide surface, both of these angular values represent a relatively small incremental deviation of the respective light rays from the original propagation directions. Either one of the above-compared light deviation mechanisms may be configured directed to provide a controlled leakage of light through the designated major surface ofwaveguide4. Additionally, it is noted that by selecting the appropriate parameters of surface relief features8 and forward-scatteringparticles36 of the respective embodiments, the angular spread and angular/spatial distribution of light extracted fromwaveguide4 can also be controlled, which may be used, for example, for providing a light collimating function ofsystem2.
• It will be appreciated by those skilled in the art that, although each individual interaction of a light ray with forward-scatteringparticle36 has a random character, by selecting the material, refractive index, size, concentration and/or other parameters ofparticles36, the light scattering pattern of the particles may be tailored to provide a controlled magnitude and rate of spreading of the light beam along the propagation path inwaveguide4. Aslayer6 frustrates TIR for at least those rays that have incidence angle intosurface12 greater than the critical TIR angle φ_TIRC, light will primarily exit fromwaveguide4 towardlayer20 and not toward the opposing side of the waveguide.
• By applying Mie scattering calculations to dielectric spheres in refractive medium and considering an exemplary case of PMMA waveguide4 (n≈1.49) and polystyrene particles (n≈1.59) a scattering phase function can be obtained for various particle sizes. The scattering phase function, or phase function, gives the angular distribution of scattered light intensity at a given wavelength.
• FIG. 43 shows a scattering phase function plot for an exemplaryspherical particle36 having a 5 μm in diameter, calculated according to Mie theory for 532 nm wavelength. As it can be seen from anangular distribution972 of scattered light intensity, the scattering angle δ generally does not exceed 5° for the most part of light beam. More particularly, about 90% of scattered light will be deviated from the original propagation path by 5° or less. Therefore, each interaction of light beam with such a narrowly-scatteringparticle36 will broaden the angular distribution by a small increment which can be sufficient for useful light extraction throughsurface12 and theadjacent layer20 but generally insufficient for light escape through opposingsurface10.
• FIG. 44 shows a similar plot calculated for 0.9 μm diameter ofparticle36 where the scattered light intensity has a differentangular distribution974. The analysis ofangular intensity distribution974 indicates that about 90% of the scattered light beam is confined within ±17° scattering cone and that 95% of the beam is contained within ±20° scattering cone, respectively. Thus, statistically, scattering angles δ will generally be greater for the case of 0.9 μm diameter ofparticle36 than for the case a larger, 5μm particle36. Accordingly, light rays havingangular distribution974 will be extracted fromwaveguide4 much faster and will travel much shorter distances alongaxis200 than light rays havingangular distribution972. It will be thus appreciated that the particle size can be used for controlling the rate of light extraction and overall light output for a given area ofsystem2. The size ofparticles36 therefore represents an important system parameter that generally needs to be factored into the system design along with the other parameters discussed above for the respective embodiments.
• According to at least some embodiments of the present invention, the size and other parameters of forward-reflectingparticles36 can be selected to result in the scattering angles δ that generally do not exceed the difference in TIR angles atsurfaces10 and12. In other words, δ<φ_TIR2−φ_TIR1, where φ_TIR2is the critical TIR angle at the interface ofwaveguide4 with buffer layer6 (e.g.,surface12 inFIG. 38) and φ_TIR1is the critical TIR angle at the interface ofwaveguide4 with the outside medium (e.g.,surface10 inFIG. 38). This can minimize light escape into air through the waveguide surface opposing the interface betweenwaveguide4 andbuffer layer6.
• It should be understood that this invention is not limited to the cases wheresystem2 has the shape of a rectangular plate, sheet or film or an elongated rod and may be applied to the case whensystem2 has any other suitable shape. Particularly,system2 can have any basic geometric form, a free-form or any combination thereof. Additionally, any two-dimensional planar shape ofsystem2 can be bent in any suitable way to form a three-dimensional shape. This can be used, for example, to provide a broader illumination pattern or create artistic or decorative effects. Similarly,system2 having an elongated or rod-like geometry can be bent or formed to create a two-dimensional or three-dimensional shape.
• FIG. 45 depicts an example ofsystem2 in an axisymmetrical configuration wherelight source400 is positioned at the central area ofwaveguide4 and surface relief features8 are represented by nested radial rings.Waveguide4 may have anarea290 which is clear and void of anyfeatures8.
• FIG. 46 shows an alternative round configuration ofsystem2 in which there is a central opening inwaveguide4 where a ring of multiplelight sources400 is attached to the inner edge of the waveguide. In configurations depicted inFIG. 45 andFIG. 46, light injected intowaveguide4 in the central area and propagating radially away from the center is emitted from the broad surface of the waveguide by the combined function of surface relief features8,buffer layer6 andlight extraction layer20, according to the principles described above. In view of the foregoing description of the invention, it should be understood that the above-described forward-scatteringparticles36 can also be used in place or in addition to surface relief features8.
• It should be understood that at least some of the configurations ofsystem2 shown in a cross-section in the foregoing embodiments may also be implemented in an axisymmetrical form obtained by the revolution of the respective cross-section around an axis. It should also be understood that this invention is not limited to the light input through an edge intowaveguide4, but can also be applied the case where light can be input at any suitable location ofwaveguide4, including an arbitrary location acrosssurfaces10 or12. The light can be input by embeddinglight source400 into the envelope of waveguide.Light source400 can be alternatively attached to a broad-area surface ofwaveguide4 or spaced apart from the body of the waveguide. Whensource400 is externally attached towaveguide4, a suitable collimating or non-collimating light coupler can be used to inject light into waveguide's core.
• At least some of the foregoing embodiments were described upon the case where a difference between the refractive indices at the opposing walls or surfaces ofwaveguide4 was used to force light escape towards a designated side of the waveguide. However, this invention is not limited to this and may be applied to the case when any other suitable means are used for suppressing light leakage through the unwanted side of the waveguide. Particularly, a specularly reflective layer, such as a mirrored surface, may be provided on the side of the waveguide opposing to the light emitting side to return any stray light back to the waveguide.
• FIG. 47 shown an embodiment ofsystem2 comprisingplanar waveguide4 sandwiched between alight turning film620 and a sheet-formspecular reflector630.Waveguide4 has generally planar opposing broad-area surfaces10 and12.Surface12 has a plurality of shallow surface relief features8 represented by shallow steps formed in said surface.
• Light source400, which may be represented by one or more LEDs, cold cathode fluorescent lamp (CCFL) or any other source, is positioned near the light input edge ofwaveguide4. Collimating element is provided to collect light propagating fromsource400 away from the light input edge and inject such light into the waveguide.
• Each surface relief feature has aplanar face16 which is configured to reflect light propagating at relatively low out-of-plane angles by means of TIR, as illustrated by a. The slope angle α of the respective faces16 is sufficiently low so as to generally result in multiple interactions of light rays before they can obtain the sufficient out-of-plane angle to overcome TIR at eithersurface10 or12. This is illustrated by the example of a light path of aray372 inFIG. 47. The slope offace16 of the respectivesurface relief feature8 is insufficient to communicate a sub-TIR incidence angle with respect to either surface10 or12. Accordingly,ray372 can continue propagating in the waveguide mode until its interactions with the subsequent surface relief features8 result in reaching the minimum out-of-plane angle to suppress TIR.
• Reflector630 is positioned adjacent to surface10 in such a way that there is at least minimal air gap between the two.Reflector630 has a high specular reflectivity and is configured to reflect light emerging fromsurface10 at high exit angles with respect to surface normal back towardswaveguide4.
• Light turning film620 has a prismaticstructure facing waveguide4. The grooves of the prismatic structure are aligned parallel to each other in an linear array which longitudinal axis extends generally perpendicular tolongitudinal axis200 ofwaveguide4.
• By way of example and not limitations,light turning film620 may be exemplified by the Transmissive Right Angle Film (TRAF) which is commercially available from 3M. The TRAF film has a polyester backing substrate with the prismatic structure made from modified acrylic resin. It has the acceptance angle of 0° to 20° with respect to the plane of the film and a nominal thickness of about 155 μm. Accordingly, surface relief features8 may be configured to gradually increase the out-of-plane angle of light propagating in the waveguide mode and eventually result in light exiting from the waveguide core towards TRAF at such an out-of-plane angle which will be within the acceptance angle of the TRAF. In turn, TRAF may intercept and further redirect the emerging light away from the emitting surface by an angle of approximately 70° thus resulting in light emission from the broad-area surface ofsystem2 in a perpendicular direction.
• It will be appreciated that surface relief features8 may be configured to match any other acceptance angle oflight turning film620, simply by adjusting slope angle α of the respective faces16. Particularly, each face16 may have such a slope angle α which will ensure that most light will emerge fromwaveguide4 also at an out-of-plane angle which is within the acceptance angle offilm620.
• It will be appreciated that, in order to achieve the desired operation, slope angle α will typically be much lower than theacceptance angle waveguide4. Most light rays propagating inwaveguide4 will require multiple interactions with surface relief features8 along the propagation path in order to obtain the minimum out-of-plane angle to overcome TIR atsurface12 and yet to enterfilm620 at the prescribed low out-of-plane angles. It will thus be appreciated that most light will be extracted fromwaveguide4 by deviating from the original propagation path in an incremental fashion and that the increments to the out-of-plane angle communicated by eachsurface relief feature8 will be relatively low due to the smallness of slope angle α.
• Alight ray374 ofFIG. 47 exemplifies the final portion of the light path according to the above scenario.Ray374 emitted bysource400 and injected intowaveguide4 has a relatively high out-of-plane angle with respect to the waveguide's prevailing plane as a result of its multiple interactions with surface relief features8 along the propagation path. Particularly, the angle which is complementary to the out-of-plane angle ofray374 is lower than the critical TIR angle atsurface12 by a relatively small difference which may be overcome by an additional interaction with a singlesurface relief feature8. Asray374 strikes face16 of the respectivesurface relief feature8, it is reflected by means of TIR and obtains an increment in the out-of-plane angle. It will be appreciated by those skilled in the art that the increment in the out-of-plane angle will be twice the slope angle α offace16.
• Accordingly, asray374 is directed towards the opposingsurface12, its new incidence angle with respect to a normal to surface12 may become less than the critical TIR angle at that surface resulting in ray decoupling fromwaveguide4. Upon exiting from a greater-index material ofwaveguide4 into a low-n outside medium (such as air),ray374 will bend substantially towards the plane ofwaveguide4 due to the refraction angle being greater than the angle of incidence. Therefore, even ifray374 was making a relatively high out-of-plane angle when propagating withinwaveguide4, its respective out-of-plane angle outside ofwaveguide4 would generally be substantially lower, in accordance with the Snell's law, including near-zero angles. Accordingly, the maximum allowable angle thatray374 can make with respect to surface12 can be set to the acceptance angle offilm620 in which case substantially all rays emerging fromwaveguide4 may be turned towards a perpendicular direction. Sincefilm620 may be configured to generally preserve the angular distribution of light that it redirects and since most rays emerging fromwaveguide4 may be distributed within a relatively narrow angular cone, a highly collimated beam may thus be obtained.
• Referring further toFIG. 47, the maximum allowable slope angle α_MAXofface16 may be defined from the following reasoning for a given acceptance angle φ_ACCoflight turning film620. It is noted that this reasoning is provided by way of an illustrative example to assist the reader in understanding of the operation of this invention and should not be construed as limiting the scope of the invention.
• Since the out-of-plane angle ofray374 should preferably be equal to or lower than the acceptance angle φ_ACCupon exiting fromwaveguide4, the minimum refraction angle φ_RMINofray374 counted off from a normal to surface12 should be complementary to angle φ_ACC, that is φ_RMIN=90°−φ_ACC.
• By using the Snell's law, it can be shown that the minimum angle of incidence φ_IMINcorresponding to φ_RMINcan be found from the following relationship: n₁sin φ_IMIN=n₀sin(90°−φ_ACC) or n₁sin φ_IMIN=n₀cos(φ_ACC), where n₁is the refractive index of the material ofwaveguide4 and n₀is the refractive index of the outside medium. Accordingly, assuming the outside medium being the air with n₀≈1, sin φ_IMIN=cos(φ_AAC)/n₁.
• Since the angle of reflection is equal to the angle of reflection in respect to a normal to face16, TIR fromface16 will decrement the incidence angle or ray ontosurface12 by 2α. Now, considering that the minimum incidence angle thatray374 may take with respect to a normal to surface12 while propagating in a waveguide mode is the critical TIR angle φ_TIRCand that the incidence angle of reflected ray should not exceed φ_IMIN, obtain α_MAX=(φ_TIRC−φ_IMIN)/2.
• For the case illustrated inFIG. 47 and in view of the above assumptions, φ_TIRC=arcsin(n₀/n₁)≈ arcsin(1/n₁). Accordingly, slope angle α offace16 may be selected to not exceed α_MAX, where
• ${\alpha }_{\mathrm{MAX}}=\frac{1}{2}\left(\arcsin \left(1/n_1\right)-\arcsin \left(\cos \left({\phi }_{\mathrm{ACC}}\right)/n_1\right)\right)$
• For instance, ifwaveguide4 ofFIG. 47 is made from PMMA (acrylics) having refractive index n₁≈1.49 andlight turning film620 includes TRAF having the acceptance angle φ_ACCof 20°, it will give α_MAX≈1.5°. It is noted however, that greater slope angles may also be selected forfaces16 in which case at least some rays may emerge fromwaveguide4 at out-of-plane angles beyond the acceptance angle offilm620. Accordingly, a portion of light emitted fromsystem2 may have a prescribed degree of collimation and another portion of the emitted light may have a more diffuse angular distribution. It will be appreciated that, when the primary function ofsystem2 ofFIG. 47 is to emit a collimated light beam while minimizing light scattering, slope angle ααshall either not exceed the maximum angle α_MAXor exceed it only by a relatively small amount. Particularly, the slope angle α of each face16 with respect to prevailing plane ofwaveguide4 may be made less than two angular degrees. It can be shown that, at such low angle, at least a major portion of light escaping throughsurface12 will still have out-of-plane angles less than 20 degrees, which is within the acceptance angle of the TRAF film. Accordingly, whenfilm620 is exemplified by the TRAF film, practically all of the emerging light may be redirected towards the surface normal while generally preserving the narrow, 20-degrees angular spread and resulting insystem2 emission of a collimated beam perpendicularly to its plane.
• Light turning film620 may also be provided with light scattering properties or associated with an external light scattering layer. This may be useful, for example, for smoothing out the irregularities in light distribution that may be present in the collimated beam. In another example, the light scattering features associated withlight turning film620 may also be useful for additional spreading of the collimated beam emitted bysystem2 over a broader angular range.
• At least some of the foregoing embodiments were described upon the case where surface relief features8 are arranged in parallel rows in a surface ofwaveguide4. However, this invention is not limited to this and may be applied to the case where surface relief features are arranged in an array which can have any other configuration. For example, as illustrated inFIG. 48, surface relief features may be formed insurface10 in an annular stepped arrangement radially extending fromsource400 and being symmetrical with respect toaxis200. However, it is noted that any other suitable pattern may also be used for arrangingfeature8 insurface10, which may also include symmetric, asymmetric, intermittent, ordered or random patterns. For instance, any of the surface distribution patterns of light extracting features of the prior art devices, also including those shown inFIG. 1 throughFIG. 3, may be used to distribute surface relief features8 across a major surface ofwaveguide4.
• The foregoing embodiments were described upon the case where the light deflecting elements are exemplified by either surface relief features8 formed in a major surface ofwaveguide4 or forward-scatteringparticles36 distributed though the body of the waveguide. However, this invention is not limited to this and may be applied to the case where the light deflecting elements have any other type provided thatsystem2 has the same basic operation.
• For instance, the light deflecting elements may include diffraction or holographic structures distributed along either one or both ofsurfaces10 and12. Such structures may be configured to deflect light by small angles in an incremental fashion along the propagation path inwaveguide4 and to cause controlled light leakage from the waveguide at relatively low out-of-plane angles.
• In a further instance, the light deflecting elements that can provide the incremental light deflection along the optical path inwaveguide4 may also be formed by a corrugated boundary between two different transmissive materials incorporated into the body of the waveguide. An illustrative example of such waveguide is shown inFIG. 49. Accordingly,waveguide4 ofFIG. 49 has a planar slab shape and is formed by two optically transmissivedielectric layers642 and644 disposed in optical contact with each other. The refractive index oflayer642 is n₁and the refractive index of layer644 is n₃₁which is different than n₁. The difference in the refractive indices creates a refractive optical interface betweenlayers642 and644 which bends all rays entering it at any non-zero incidence angles. In the case illustrated inFIG. 49, n₃₁<n₁.
• The boundary betweenlayers642 and644 is not planar and includes prismatic corrugations each formed by a pair ofplanar facets670 and672. The corrugations may be adjacent to each other and form a continuous corrugated boundary between the respective layers. It is noted, however, that the corrugations may also be alternated with flat portions of the boundary and may also be distributed alongaxis200 according to any suitable pattern.Facet670 makes adihedral angle766 with the prevailing plane ofwaveguide4 and facet673 makes adihedral angle768 with the same plane. According to one embodiment,angle766 is greater thanangle768 and may take particular values up to 90 degrees. According to one embodiment, angles766 and768 may be identical or nearly identical. According to one embodiment,angle766 may be less thanangle768.
• In operation, aray722 propagating inlayer642 ofwaveguide4 at an out-of-plane angle β strikesfacet670 of the inter-layer boundary and refracts into the lower-index layer644.Ray722 further strikes surface12 ofwaveguide4 and reflects from the surface by means of TIR.Ray722 further refracts atfacet672 and enterslayer642 again. Whendihedral angle768 offacet672 is lower than thedihedral angle766 offacet670, the consecutive refraction ofray722 atfacets670 and672 will result in a new out-of-plane propagation angle β+ω which. It will be appreciated that the angular increment ω to the out-of-plane angle is a function of the difference in refractive indices betweenlayers642 and644 and the difference betweendihedral angles766 and768. Accordingly, it will also be appreciated thatsuch waveguide4 with a corrugated inter-layer boundary may be configured to provide an incremental increase of the out-of-plane angle αlong the propagation path. In turn, this may eventually result inray722 exiting fromwaveguide4 and being intercepted and redirected by the light extraction layer (not shown) according to the principles described above.
• Various surface treatment techniques other than microstructuring or replication may also be used to produce suitable light deflecting elements which could incrementally bend light in small angular increments along the propagation path. For example selective UV exposure or chemical processing may be used to cause repetitive variations in the refractive index along the waveguide's surface or through the body of the waveguide. The variations of the refractive index along the propagation path may create a number of optical interfaces which can bend light by a relatively small angle upon each interaction with the propagated light. As light rays accumulate a sufficient increment to the out-of-plane angle to overcome TIR, they can exit fromwaveguide4 where they can be further redirected by light extractinglayer20.
• System2 may also incorporate any number of auxiliary layers serving various purposes, such as, for example, providing additional mechanical strength, environmental resistance, peel resistance, improved visual appearance, color, etc. Any optical interface between a layer formed by a lower refractive index transmissive medium and a layer formed by a higher refractive index transmissive medium may also be provided with an intermediate optically transmissive layer, for example, for promoting the optical contact or adhesion between the layers. The intermediate layer should preferably have a refractive index which is greater than the lower of the two refractive indices at the given optical interface.
• It may be appreciated that thesystem2 may be implemented in a sheet form and may provide efficient light distribution across a large-area and emission of a collimated beam from its entire surface, which may find utility in various lighting devices. Particularly,system2 may be employed for making directional lighting luminaires and fixtures having a wide emission area, compact form and reduced glare.
• FIG. 50 shows an exemplary embodiment of a low-profiledirectional luminaire1050 which incorporatessystem2 including at least edge-litplanar slab waveguide4 andlight extraction layer20 configured for light collimation.Such luminaire1050 may be configured in the form of a panel installable in a similar manner to the fluorescent troffers or non-collimating direct-lit and edge-lit LED lighting panels. By way of a non-limiting example,luminaire1050 may be designed for drop in ceilings in commercial and institutional buildings and have dimensions common for such applications. For instance, similarly to the drop-in tiles of the common suspended grid ceilings, the outer dimensions ofluminaire1050 may be 2′×2′, 2′×4′, or 1′×4′ so that the luminaire may directly replace the tile panels in the grid.
• Referring further toFIG. 50luminaire1050 includes two linear arrays of high-brightness LEDs878 and880, respectively, attached to the opposing edges ofwaveguide4. Each LED array (or LED strip) may be mounted on a heat sink (indicated as452 and454) to dissipate the heat generated byLEDs878 and880. Each of theheat sinks452 and454 may be made from aluminum in the form of a rectangular block, a hollow rectangular pipe, a channel or any structural profile having the appropriate cross-section. Various conventional finned or finless heat sinks may also be employed.
• Each ofLEDs878 and880 may be provided with some kind of individual collimating or beam shaping optics. For instance, each LED may include a dome shaped lens which can aid in light input intowaveguide4 and reducing light spillage.
• Luminaire1050 may include a metal orplastic housing462 configured to hold various luminaire components together particularly includingwaveguide4, light extractinglayer20, LED strips, heat sinks, wiring (not shown), etc. When housing462 is made from a plastic material, the material may be opaque or transmissive/translucent.Housing462 may enclose just the perimeter area ofsystem2 or may also cover the non-luminous back surface of the system. In either case, especially whenhousing462 is made from an opaque or poorly transmitting material, it should have an opening corresponding to the light emitting area ofsystem2. The dimensions of the opening should preferably be at least slightly smaller than the dimensions ofwaveguide4 to ensure thatsystem2 and any of its components can be sored securely insidehousing462. In other words, anoutline1002 of the waveguide should be sufficiently large compared to the opening so that the waveguide won't fall through the opening under normal operation and use ofluminaire1050. Alternatively, or in addition to that, the opening may be covered by an optically transmissive sheet of plastic material. The transmissive sheet may also be provided with light diffusing properties and may be made a part of the optical assembly ofsystem2.
• Luminaire1050 may be provided with any number or light diffusing layers or otherwise beam-shaping layers. For example, an opaque light diffusing sheet of a reflective type may be provided on the back ofwaveguide4 to recycle and homogenize stray light. A light diffusing sheet of a transmissive type may be provided on the opposing (light emitting) side of the waveguide to smooth out possible non-uniformities of the emitted beam and/or further control beam spread.
• Luminaire1050 ofFIG. 50 is also shown to include anLED driver554 representing a self-contained power supply that has outputs matched to the electrical characteristics of the strips ofLEDS400 and880.LED driver554 may be attached tohousing462 or it may also be incorporated into the housing. Alternatively,LED driver554 may be located at a distance from the main body ofluminaire1050 and may be electrically connected to the LED arrays via a power cord of a suitable length.
• LED driver554 may ordinarily be current-regulated and configured to deliver a consistent current over a range of load voltages.LED driver554 may also be configured to provide dimming of the LEDs by means of pulse width modulation (PWM) circuits or by any other suitable means. The LED driver may also have more than one channel for separate control of the opposing LED arrays or for separate control of individual LEDs or LED groups within the arrays. The respective LEDs or LED groups, in turn, may be configured to emit light in different color or different color temperatures thus allowing for obtaining various static or dynamic illumination effects and/or for just varying the color of light emitted byluminaire1050.
• The structure ofsystem2 which is incorporated intoluminaire1050 ofFIG. 50 may be selected from any of the foregoing embodiments of any their variations. Particularly, by employing a light-collimating embodiment ofsystem2 withplanar waveguide4,luminaire1050 may be configured to emit a highly directional light beam into a prescribed angular range or towards a well defined area to be illuminated.
• Depending on the application, the beam angle ofluminaire1050 may be limited to any particular value which is considerably less than a full 180-degree in any plane. It will be appreciated that sincesystem2 may provide light collimation at least in a longitudinal plane which is parallel toaxis200 and perpendicular to the prevailing plane ofwaveguide4, the directionality ofsuch luminaire1050 can be substantially enhanced in comparison to the conventional edge-lit lighting panels which typically emit light according to a highly diffuse, lambertian pattern. As it has been illustrated in reference to the above-described embodiments ofsystem2, beam collimation may also be provided in a transversal plane (a plane perpendicular to axis200), for example by using and appropriately configuring the collimating elements which may be associated with individual LEDs (see, e.g.,FIG. 28 andFIG. 29). Accordingly, the edge-litluminaire1050 employingsystem2 and fewer LEDs may be configured to provide highly directional, wide-area beam patterns normally attainable only by using direct-lit LED panels having large two-dimensional arrays of discrete light sources with individual optics. It is essential thatsystem2 may be configured to emit substantially all of the light extracted fromwaveguide4 into functional directions with minimum or no light rejection.
• Considering that, in many practical applications, the light beam emitted bysystem2 and/orluminaire1050 may not have sharply defined boundaries, the beam angle may be defined as two times the vertical angle at which the intensity is 90% of the maximum beam intensity. In turn, the vertical angle may be defined as an angle between the center of the emitted directional beam and the direction in which the beam intensity is evaluated. For example, with theluminaire1050 pointed downward, a vertical angle of 0° may thus describe the center of a directional beam emitted along the surface normal.
• By way of example and not limitation, the beam angle ofdirectional luminaire1050 may be limited at least in one plane to less than 120 degrees. By way of further non-limiting examples, the beam angle may be limited to 90 degrees, 75 degrees, 60 degrees, 45 degrees, 30 degrees, or 15 degrees. It is noted, however, that any other practical limits for the beam angle may be also established depending on the desired illumination pattern andsystem2 can be configured accordingly. It will also be understood that the beam angle may be controlled independently in each of the longitudinal and transversal planes which are orthogonal with respect to each other. In one embodiment, the beam angle may be made the same or similar in both planes. In one embodiment, the beams angles may be made different in the respective planes. In one embodiment, the collimation may be provided in only one of the above planes. For example, the beam angle in the longitudinal plane may be 60 degrees and the beam angle in the transversal plane may be up to a full 180 degrees with a diffuse lambertian or gaussian pattern.
• Further details of operation ofwaveguide illumination system2 shown in the drawing figures as well as its possible variations and further applications will be apparent from the foregoing description of preferred embodiments. Although the description above contains many details, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. Therefore, it will be appreciated that the scope of the present invention fully encompasses other embodiments which may become obvious to those skilled in the art, and that the scope of the present invention is accordingly to be limited by nothing other than the appended claims, in which reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more.” All structural, chemical, and functional equivalents to the elements of the above-described preferred embodiment that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Moreover, it is not necessary for a device or method to address each and every problem sought to be solved by the present invention, for it to be encompassed by the present claims. Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element herein is to be construed under the provisions of 35 U.S.C. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for.”

Claims (20)

What is claimed is:

1. An illumination system, comprising:

an optical waveguide having at least one major surface and configured to guide light towards at least one predetermined direction along said major surface in response to a total internal reflection;

a plurality of light deflecting elements distributed within said waveguide along said direction and configured to incrementally deflect light rays from said direction in at least one plane perpendicular to said major surface; and

light extracting means optically coupled to said major surface and configured for progressively extracting light from said waveguide in response to light incidence onto said major surface at an angle lower than the angle of a total internal reflection.

2. An illumination system as recited inclaim 1, wherein said waveguide has a planar configuration having two opposing broad-area surfaces extending parallel to each other.

3. An illumination system as recited inclaim 1, wherein said waveguide has a cylindrical configuration having a longitudinal dimension substantially greater than a transversal dimension.

4. An illumination system as recited inclaim 1, wherein said waveguide comprises at least one light input edge or end and further comprises at least one light source optically coupled to said at least one light input edge or end.

5. An illumination system as recited inclaim 1, further comprising at least one light emitting diode configured to inject light into said optical waveguide.

6. An illumination system as recited inclaim 1, further comprising at least one light source configured for inputting light into an edge or end of said optical waveguide, wherein said light source is selected from the group of elements consisting of light emitting diodes, compact fluorescent lamps, cold-cathode fluorescent lamps, halogen lamps, mercury-vapor lamps, sodium-vapor lamps, metal halide lamps, electroluminescent lamps, and lasers.

7. An illumination system as recited inclaim 1 further comprising at least one light source and at least one collimating element associated with said at least one light source.

8. An illumination system as recited inclaim 1, further comprising a reflective layer adjacent to a major surface of said waveguide.

9. An illumination system as recited inclaim 1, wherein said light extracting means comprises an optical element selected from the group of elements consisting of light turning films, light scattering layers or surfaces, screens, image prints, and textured surfaces.

10. An illumination system as recited inclaim 1, wherein said light extracting means is configured to direct light generally towards a normal to said major surface.

11. An illumination system as recited inclaim 1, further comprising a buffer layer disposed between said optical waveguide and said light extracting means and having a lower refraction index than said optical waveguide.

12. An illumination system as recited inclaim 1, wherein each of said light redirecting elements comprises at least one surface relief feature.

13. An illumination system as recited inclaim 1, wherein each of said light redirecting elements is selected form the group of surface relief features having a relatively low aspect ratio and consisting of prisms, lenses, prismatic grooves, corrugations, undulations, depressions, indentations, recesses, protrusions, and extensions.

14. An illumination system as recited inclaim 1, wherein each of said light redirecting elements comprises a surface facet making a sufficiently low dihedral angle with respect to the prevailing plane of said optical waveguide, said dihedral angle being below a predetermined maximum angle.

15. An illumination system ofclaim 1 in a front light configuration, wherein said waveguide has the form of a planar panel and is made substantially transparent at least along a transversal direction with respect to a prevailing plane of said panel.

16. An illumination system ofclaim 1 in a backlight configuration, wherein said waveguide has the form of a planar panel.

17. An illumination system ofclaim 1 within day lighting device, wherein said system is configured for a generally unimpeded transversal light passage and is further configured to emit artificial light from a broad-area surface.

18. An illumination system as recited inclaim 1, wherein said system is configured for emitting at least a substantial part of the emitted light in the form of a collimated beam.

19. An illumination system, comprising:

a planar optical waveguide having a first broad-area surface and an opposing second broad-area surface extending generally parallel to said first broad-area surface;

a buffer layer of an optically transmissive material attached to said first surface, said buffer layer having a refractive index lower than the refractive index of said waveguide;

a plurality of light deflecting elements within said waveguide configured to extract light into said buffer layer; and

a light extracting layer attached to said buffer layer and configured to extract light from said buffer layer.

20. A directional edge-lit lighting fixture, comprising:

a planar sheet of an optically transmissive material configured to guide light along a predetermined direction in response to optical transmission and a total internal reflection;

at least one strip of light emitting diodes attached to an edge of said sheet; and

means for extracting light from a major surface of said sheet and for collimating the extracted light at least in a plane parallel to said predetermined direction;

whereby said directional edge-lit lighting fixture can emit light within a pre-defined angular range being substantially smaller than 180 degrees in said plane.

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