US20110044582A1 - Efficient collimation of light with optical wedge - Google Patents

Abstract

Embodiments of optical collimators are disclosed. For example, one disclosed embodiment comprises an optical waveguide having a first end, a second end opposing the first end, a viewing surface extending at least partially between the first end and the second end, and a back surface opposing the viewing surface. The viewing surface comprises a first critical angle of internal reflection, and the back surface is configured to be reflective at the first critical angle of internal reflection. Further, an end reflector is disposed at the second end of the optical waveguide, and includes a faceted lens structure to cause a majority of the viewing surface to be uniformly illuminated when uniform light is injected into the first end and also to cause a majority of the injected light to exit the viewing surface.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS
• This application claims priority to U.S. Provisional Application Ser. No. 61/235,922, entitled EFFICIENT COLLIMATION OF LIGHT WITH OPTICAL WEDGE, filed Aug. 21, 2009, the entire disclosure of which is herein incorporated by reference.
BACKGROUND
• An optical collimator is a device which collects rays from a point source of light such as a light bulb or light emitting diode and causes those rays to emerge in parallel from a surface. Examples of collimators include lenses or curved mirrors found in a flashlight or car headlamp. In these examples, a volume of space exists between the point source and the surface from which the collimated light exits. In some use environments, this space may be inconvenient, as it may increase the overall size of an optical device that utilizes the collimator.
SUMMARY
• Accordingly, various embodiments are disclosed herein that relate to optical collimators. For example, one disclosed embodiment provides an optical collimator comprising an optical waveguide having a first end including a first light interface, a second end opposing the first end, a viewing surface that includes a second light interface extending at least partially between the first end and the second end, and a back surface opposing the viewing surface. The viewing surface comprises a first critical angle of internal reflection with respect to a normal of the viewing surface, and the back surface is configured to be reflective at the first critical angle of internal reflection. Further, an end reflector is disposed at the second end of the optical waveguide, and includes a faceted lens structure comprising a plurality of facets angled to cause a majority of the viewing surface to be uniformly illuminated when uniform light is injected into the first end and also to cause a majority of the injected light to exit the viewing surface.
• This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Furthermore, the claimed subject matter is not limited to implementations that solve any or all disadvantages noted in any part of this disclosure.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 shows embodiments of an optical device and optical wedge positioned within the optical device.
• FIG. 2 is a schematic, plan view showing an embodiment of an optical wedge.
• FIGS. 3 and 4 show ray traces through a sectional view of the embodiment ofFIG. 2.
• FIG. 5 shows a schematic, magnified cross-sectional view of an end reflector of the embodiment ofFIG. 2.
• FIGS. 6 and 7 show the ray traces ofFIG. 2 as paths through a stack of replicates of the embodiment ofFIG. 2.
• FIGS. 8 and 9 show ray traces through a perspective view of an embodiment of an optical wedge comprising reflective sides.
• FIG. 10 shows an embodiment of a method for collimating light.
DETAILED DESCRIPTION
• Various embodiments of optical collimators are disclosed herein in the form of wedge-shaped light guides, or optical wedges. An optical wedge is a light guide that conducts light between one light interface at an end of the wedge and another light interface at a face of the wedge via total internal reflection. The embodiments disclosed herein each utilize a folded optical path to allow light to fan out to a desired size before collimation, which may allow the reduction in size of the volume between a light source and surface (e.g. wedge face) where the collimated light exits. Such optical wedges may find various uses, including but not limited to as a backlight for a liquid crystal display (LCD).
• The subject matter of the present disclosure is now described by way of example and with reference to certain illustrated embodiments. In the accompanying figures, it will be noted that the views of the illustrated embodiments may not be drawn to scale, and the aspect ratios of some features may be exaggerated to make selected features or relationships easier to see.
• FIG. 1 shows an embodiment of anoptical system10 which may be configured to provide both display and input functionality forcontroller16 via a large-format, touch-sensitive display surface12.Controller16 may be any device configured to provide display data to and receive input data from the optical system. In some embodiments, the controller may comprise all or part of a computer; in other embodiments, the controller may be any device operatively coupled to a computer via a wired or wireless communications link.Controller16 comprisesmemory14 andprocessor15.Memory14 may be used to store instructions for execution byprocessor15, including routines for controllingoptical system10.
• To provide display functionality,optical system10 may be configured to project a visible image onto touch-sensitive display surface12. To provide input functionality, the optical system may be configured to capture at least a partial image of objects placed on the touch-sensitive display surface—fingers, electronic devices, paper cards, food, or beverages, for example. Accordingly, the optical system may be configured to illuminate such objects and to detect the light reflected from the objects. In this manner, the optical system may register the position, footprint, and other properties of any suitable object placed on the touch-sensitive display surface.
• Optical system10 includesoptical wedge100,light director20,light valve22,diffuser24, andlight source102.Light source102 andlight valve22 may be operatively coupled tocontroller16 and configured to provide a visual display image to touch-sensitive display surface12.Light source102 may be any illuminant configured to emit visible light, such as one or more light emitting diodes, for example. Light fromlight source102 is projected throughoptical wedge100 and directed tolight valve22 vialight director20. In some embodiments,light director20 may comprise a film of prisms configured to direct light in a direction normal tolight valve22. The numerous light-gating elements oflight valve22 may be used to modulate light fromlight director20 with respect to color and intensity. In some embodiments, the light valve may comprise a liquid-crystal display device, but other light-modulating devices may be used as well. In this manner, the light source and the light valve may together create a display image. The display image is projected throughdiffuser24 and is thereby provided to touch-sensitive display surface12.
• Optical system10 may be further configured to provide input functionality to controller16. Accordingly, the illustrated optical system includesdetector38,infrared emitters72, andilluminating light guide74.Detector38 may comprise a camera, such as an infrared-sensitive digital camera, for example, or any other suitable image sensing device.Infrared emitters72 may comprise one or more infrared light-emitting diodes, for example, or any other suitable light source. The illuminating light guide may be any optic configured to receive an injection of infrared light at one ormore entry zones76 and to transmit infrared light reflected off of objects touching the display screen throughexit zone78.
• For example, infrared light may be injected byinfrared emitters72 intoentry zone76 ofilluminating light guide74. The infrared light may travel throughilluminating light guide74 via total internal reflection and may leak out along the touch-sensitive display surface12 (e.g. due to diffusing elements, not shown, arranged along the touch-sensitive display surface12) until striking one or more objects in contact with touch-sensitive display surface12, such asobject40. A portion of the infrared light may reflect off of the one or more objects and exitilluminating light guide74 atexit zone78. The infrared light may travel fromexit zone78, throughdiffuser24 andlight valve22, and strike a surface ofoptical wedge100, which may be configured to direct incident infrared light ontodetector38. It will be understood however, that numerous other illumination configurations are possible, and are within the scope of the present disclosure.
• Referring next toFIG. 2,optical wedge100 may be configured to collimate light fromlight source102 disposed adjacent to athin end110 ofoptical wedge100, such that collimated lightexits viewing surface150 ofoptical wedge100, as shown by the ray traces inFIG. 2. The term “viewing surface” indicates thatviewing surface150 is closer to a viewer than a back surface (not visible inFIG. 2) which is opposite ofviewing surface150. Each of the viewing and back surfaces is bounded bysides130 and140,thin end110, andthick end120. InFIG. 2,viewing surface150 faces a viewer of the page and the back surface is hidden by this view ofoptical wedge100.
• Optical wedge100 is configured such that light rays injected into a light interface ofthin end110 may fan out as they approachthick end120 comprisingend reflector125. The light rays are delivered toend reflector125 via total internal reflection fromviewing surface150 and the back surface. In the preferred embodiment,end reflector125 is curved with a uniform radius of curvature having center ofcurvature200, andlight source102 injecting light at the focal point ofend reflector125, the focal point being at one half the radius of curvature. Atthick end120, each of the light rays reflects off ofend reflector125 parallel to each of the other light rays. The light rays travel fromthick end120 towardthin end110 until the light rays intersectviewing surface150 at a critical angle of reflection ofviewing surface150 and the light rays exit as collimated light. In an alternative embodiment,end reflector125 may be parabolic or have other suitable curvature for collimating light.
• In other embodiments, a plurality of light sources may be disposed adjacent to and alongthin end110. The use of a plurality of light sources may increase the brightness of the collimated light exitingviewing surface150 compared to the use of a single light source. In such embodiments, to correct for field curvature and/or spherical aberration, it may be desirable to slightly shortensides130 and140 ofoptical wedge100 so that a light source to either side ofcenter line210 may stay in the focal point ofend reflector125. Shorteningsides130 and140 may makethin end110 convex, as illustrated bycurve115. A suitable curvature may be found by using a ray-tracing algorithm to trace rays at a critical angle of reflection ofviewing surface150 ofoptical wedge100 back throughoptical wedge100 until the rays come to a focus nearthin end110.
• FIGS. 3 and 4 show ray traces through a schematic cross-sectional view ofoptical wedge100.FIG. 3 shows the path of afirst ray300 throughoptical wedge100, andFIG. 4 shows the path of asecond ray400 throughoptical wedge100, whereinrays300 and400 represent rays located at opposite sides of a cone of light that is input intothin end110 ofoptical wedge100. As can be seen inFIGS. 3 and 4,ray300exits viewing surface150 adjacent tothin end110 ofoptical wedge100, whileray400exits viewing surface150 adjacent tothick end120 ofoptical wedge100.
• Rays300 and400exit viewing surface150 once therays300 and400 intersectviewing surface150 at an angle less than or equal to a critical angle of internal reflection with respect to a normal ofviewing surface150. This critical angle may be referred to herein as the “first critical angle.” Likewise, rays reflect internally inoptical wedge100 when the rays intersectviewing surface150 at an angle greater than the first critical angle of internal reflection with respect to the normal ofviewing surface150. Further, rays reflect internally inoptical wedge100 when the rays intersect backsurface160 at an angle greater than a critical angle of internal reflection with respect to the normal ofback surface160. This critical angle may be referred to herein as the “second critical angle.”
• As explained in more detail below with reference toFIG. 5, it may be desirable for the first critical angle and the second critical angle to be different, such that light incident onback surface160 at the first critical angle is reflected back towardviewing surface150. This may help to prevent loss of light through theback surface160, and therefore may increase the optical efficiency of theoptical wedge100. The first critical angle is a function of the refractive index ofoptical wedge100 and the index of refraction of the material interfacing viewing surface150 (e.g. air or a layer of a cladding), while the second critical angle is a function of the refractive index ofoptical wedge100 and the material adjacent to backsurface160. In some embodiments, such as that shown inFIGS. 3-4, a layer ofcladding170 may be applied only to backsurface160, such thatviewing surface150 interfaces with air. In other embodiments,viewing surface150 may comprise a layer of cladding (not shown) with a different refractive index than backsurface160.
• Any suitable material or materials may be used as cladding layers to achieve desired critical angles of internal reflection for the viewing and/or back surfaces of an optical wedge. In an example embodiment,optical wedge100 is formed from polymethyl methacrylate, or PMMA, with an index of refraction of 1.492. The index of refraction of air is approximately 1.000. As such, the critical angle of a surface with no cladding is approximately 42.1 degrees. Next, an example cladding layer may comprise Teflon AF (EI DuPont de Nemours & Co. of Wilmington, Del.), an amorphous fluoropolymer with an index of refraction of 1.33. The critical angle of a PMMA surface clad with Teflon AF is 63.0 degrees. It will be understood that these examples are described for the purpose of illustration, and are not intended to be limiting in any manner.
• In other embodiments, backsurface160 may include a mirror. As non-limiting examples, the mirror may be formed by applying a reflective coating to backsurface160 or by placing a mirror adjacent to backsurface160. In this manner, backsurface160 may reflect incident light intersecting backsurface160. When back surface160 is configured to reflect some or all incident light, backsurface160 may be referred to herein as the “reflective back surface.” Non-limiting examples of a reflective back surface include a back surface having a mirrored surface, a mirror placed adjacent to the back surface, a back surface having a second critical angle of internal reflection with respect to a normal of the back surface, wherein the second critical angle of reflection is less than the first critical angle of reflection, or any other configuration in which the back surface is reflective to internally incident light at the first critical angle of internal reflection.
• The configuration ofoptical wedge100 andend reflector125 may be configured to cause a majority ofviewing surface150 to be uniformly illuminated when uniform light is injected intothin end110, and also to cause a majority of the injected light to exitviewing surface150. As mentioned above,optical wedge100 is tapered along its length such that rays injected atthin end110 travel to endreflector125 via total internal reflection.End reflector125 comprises a faceted lens structure configured to decrease the ray angle relative to a normal to each ofviewing surface150 andback surface160. In addition, the diminishing thickness ofoptical wedge100 fromthick end120 tothin end110 causes ray angles to diminish relative to the normal of each surface as rays travel towardthin end110. When a ray is incident onviewing surface150 at less than the first critical angle, the ray will exitviewing surface150.
• In some embodiments,light source102 may be positioned at a focal point ofend reflector125. In such embodiments,end reflector125 may be curved with a radius of curvature that is twice the length ofoptical wedge100. In the embodiment ofFIGS. 3-4, the taper angle ofoptical wedge100 is configured so that the corner atthick end120 andviewing surface150 comprises a right angle and the corner atthick end120 andback surface160 comprises a right angle. Whenthin end110 is at the focal point ofend reflector125,thin end110 is one half the thickness ofthick end120. In other embodiments, each of these structures may have any other suitable configuration.
• In the depicted embodiment,end reflector125 is spherically curved fromside130 toside140 and from viewingsurface150 to backsurface160. In other embodiments,end reflector125 may be cylindrically curved with a uniform radius of curvature from viewingsurface150 andback surface160 and a center of curvature whereviewing surface150 andback surface160 would meet if extended. A cylindrically curved end reflector may resist sag more strongly than a sphericallycurved end reflector125, which may be beneficial in large format applications. Other suitable curvatures may be used forend reflector125, such as parabolic, for example. Additionally, the curvature ofend reflector125 in the plane perpendicular tosides130 and140 may differ from the curvature ofend reflector125 in the plane parallel tosides130 and140.
• As mentioned above, it may be desirable for the critical angles of reflection ofviewing surface150 andback surface160 to be different. This may help to prevent loss of light throughback surface160, as illustrated inFIG. 5, which shows a schematic, magnified cross-sectional view ofend reflector125 of the embodiment of the optical wedge inFIGS. 2-4.End reflector125 comprises a faceted lens structure comprising a plurality of facets arranged at an angle relative to a surface ofthick end120. The plurality of facets alternate between facets facingviewing surface150, such asfacet530, and facets facing backsurface160, such asfacet540.End reflector125 conforms to a general curvature as described above, with end reflector normal542 and end reflector normal532 extending toward the center of curvature. Each of the plurality of facets has a height and an angle relative to a normal of a surface of the end reflector. For example, one of the facets facingviewing surface150 has aheight538 and anangle536 relative to end reflector normal532 and facet normal534. As another example, one of the facets facing backsurface160 has aheight548 and anangle546 relative to end reflector normal542 and facet normal544.
• The height of each of the plurality of facets may affect the uniformity and the brightness of collimated light exitingviewing surface150. For example, larger facets may create optical paths that differ from the ideal focal length, which may cause Fresnel banding. As such, in embodiments where such banding may pose issues, it may be desirable to make the height of each of the plurality of facets less than 500 microns, for example, so that such banding is less visible.
• Likewise, the angle of each of the plurality of facets also may affect the uniformity and brightness of collimated light exitingviewing surface150.Ray500 illustrates how facet angles may affect the path of a ray throughoptical wedge100.Ray500 is injected intothin end110, travel throughoptical wedge100 and strikes endreflector125. Half ofray500strikes facet530 facingviewing surface150. The portion ofray500striking facet530 is reflected asray510 towardviewing surface150.Ray510 intersectsviewing surface150 at an angle less than or equal to the first critical angle of internal reflection with respect to a normal ofviewing surface150, and thus exits theviewing surface150 asray512.
• The other half ofray500strikes facet540 facing backsurface160. The portion ofray500striking facet540 is reflected asray520 towardback surface160. Because of the difference between the critical angles ofviewing surface150 andback surface160,ray520 intersects backsurface160 at an angle greater than the second critical angle of internal reflection with respect to a normal ofback surface160, and thus reflects asray522 towardviewing surface150.Ray522 then intersectsviewing surface150 at an angle less than or equal to the first critical angle of internal reflection with respect to a normal ofviewing surface150, and thus exits asray524. In this manner, a majority (and in some embodiments, substantially all) of the light that reflects fromend reflector125exits viewing surface150.
• Due to light being separately reflected by facets facingviewing surface150 and facets facing backsurface160, overlapping, superimposed first and second images arranged in a head-to-tail orientation may be formed atviewing surface150. The degree of overlap between these images may be determined by the angles of thefacets530 and540. For example, the two images are completely overlapping when each facet has an angle relative to a normal of a surface of the end reflector of three-eighths of a difference between ninety degrees and the first critical angle of reflection, as explained in more detail below. In this instance, substantially all light input intooptical wedge100 exits theviewing surface150. Varying the facets from this value decreases the amount of overlap between images, such that only one or the other of the two images is displayed where the angles of the facets are ¼ or ½ of the difference between 90 degrees and the first critical angle of reflection. Further, varying the angles of the facets from three-eighths of the difference between ninety degrees and the first critical angle of reflection also causes some light to exit from the thin end ofoptical wedge100, rather than from viewingsurface150. Where the angles of the facets are ¼ or ½ of the difference between 90 degrees and the first critical angle of reflection, the viewing surface also may be uniformly illuminated, but half of the light exits from the thin end ofoptical wedge100, and is therefore lost. It will be understood that, depending upon the desired use environment, it may be suitable to use facet angles other than three-eighths of the difference between ninety degrees and the first critical angle of reflection to produce collimated light. Such use environments may include, but are not limited to, environments in which any regions of non-overlapping light (which would appear to have a lower intensity relative to the overlapping regions) are not within a field of view observed by a user.
• In an alternative embodiment, the faceted lens structure ofend reflector125 may comprise a diffraction grating. The grating equation may be used to calculate an angle of diffraction for a given angle of incidence and a given wavelength of light. Since the angle of diffraction is dependent on the wavelength of the light, an end reflector comprising a diffraction grating may be desirable when the injected light is monochromatic.
• FIGS. 6 and 7 illustrate the travel of light throughoptical wedge100 as paths of rays through a stack of optical wedges, each optical wedge being a replicate of the embodiment ofoptical wedge100 to further illustrate the concepts shown inFIG. 5. Tracing rays through a stack of replicates of an optical wedge is optically equivalent to tracing a ray's path within an optical wedge. Thus, in this manner, each internal reflection of a ray is shown as the passage of the ray through a boundary from one optical wedge to an adjacent optical wedge. InFIG. 6, the viewing surface is shown asviewing surface620 of a topmost wedge in the stack ofoptical wedges600. The back surface is shown asback surface630 of a bottommost wedge in the stack ofoptical wedges600. The thick ends of the stack ofoptical wedges600 join to form what is approximately acurve640 centered on theaxis610 where all the surfaces converge. InFIG. 6, the thick end of each wedge is shown as having the same general curvature. However, it will be understood that the thick end of each wedge may have any other suitable curvature.
• FIG. 6 also depicts two rays oflight650 and660 located at opposite sides of a cone of light that is injected into a thin end of theoptical wedge stack600. For eachray650 and660, after reflection from the end reflector, half of the ray emerges near the thick end of the optical wedge stack600 (and hence from the represented optical wedge), as shown bysolid lines652 and662, and half of the ray emerges from the thin end of the optical wedge stack, as shown by dashedlines654 and664. Rays injected at any angle between these two extremes will also be split by the faceted pattern in the end reflector, and emerge from the viewing surface and back surface of the optical wedge in a similar manner. The rays exitingviewing surface620 parallel torays652 and662 are represented byshaded area602. As mentioned above, it will be understood that rays shown as being emitted throughback surface630 of the optical wedge may instead be reflected by the back surface and then out of the viewing surface by utilizing a cladding (not shown) on the back surface of the optical wedge that has a lower refractive index than a cladding (not shown) utilized on a viewing surface of the optical wedge. In this manner, substantially all light that is injected into the thin end of such an optical wedge may be emitted from the viewing surface of the optical wedge.
• For the viewing surface to be uniformly illuminated (e.g. where the images reflected fromfacets530 and540 are fully overlapping), a ray injected at the thin end and travelling horizontally toward the end reflector, coincident with a normal of the end reflector, reflects off of a facet facing the viewing surface and travels to the center of the viewing surface, intersecting the viewing surface at the critical angle of the viewing surface.FIG. 7 shows a schematic depiction of a path of such a ray through a stack ofoptical wedges700.Ray710 is injected atthin end702 of the optical wedge and reflects offend reflector704 asray715.Ray715 travels to the center ofviewing surface706, intersectingviewing surface706 at critical angle ofreflection730 relative to viewing surface normal72. The sum ofangles732 and734 is the difference of 90 degrees and critical angle ofreflection730. When the thin end of the optical wedge is one half the thickness of the thick end of the optical wedge, the center point of the wedge is three-fourths the thickness of the optical wedge. Using a paraxial approximation,angle732 is three-fourths of the difference of 90 degrees and critical angle ofreflection730.Horizontal line722 is parallel to injectedray710 soangle740 is equal toangle732. From the law of reflection, the angle of incidence is equal to the angle of reflection so the facet angle may be one half ofangle740. Therefore, for the viewing surface to be uniformly illuminated, each facet facing the viewing surface may form an angle relative to a normal of a surface of the end reflector of three-eighths of a difference between 90 degrees and critical angle ofreflection730, as mentioned above.
• Any suitable light source may be used to inject light intooptical wedge100. Examples include, but are not limited to, light emitting diodes (LED). It will be noted that light radiates from a bare LED in a Lambertian pattern. However, for increased optical efficiency relative to a bare LED, it may be desired for light to be injected into the optical wedge so that all rays are at angles between the two solid line rays650 and660 shown inFIG. 6, i.e. at angles relative to the plane of the optical wedge which are between 0° and half the difference of ninety degrees minus the critical angle. Therefore, the LED may be placed at the focal point of a concentrator designed so that its exit thickness approximately equals the thickness of the thin end of the wedge and the angular range of its emission approximately equals the range shown byrays650 and660.
• In some embodiments, a plurality of light sources may be positioned adjacent to and along a thin end of the optical wedge to increase an intensity of output collimated light. The output fromoptical wedge100 of such an array of light sources may be analyzed by analyzing each of light sources and then combining the results using the superposition principle. This may help in the design of a system that produces uniform collimated light using such an array of light sources, as illustrated byFIGS. 8 and 9, which show a schematic view of paths of rays through an example optical wedge.Optical wedge100 inFIGS. 8 and 9 comprisesthin end110,thick end120,sides130 and140, andviewing surface150 havingcenterline850.Thick end120 includesend reflector125.Sides130 and140 may be reflective.Light sources802 and902 are disposed adjacentthin end110 equidistant fromcenterline850.
• InFIG. 8, a cone of light bounded byrays810 and830 is injected atthin end110 bylight source802.Ray830 intersectsend reflector125 and is reflected asray840.Ray810 intersectsend reflector125 and is reflected asray820 after an additional reflection fromside140. As shown inFIG. 8, the collimated light emitted from viewingsurface150 may not be uniform in this configuration. For example, a region betweenray820 andside140, labeled “Reflection”, may be brighter than a region betweenray820 andside130 due to the rays reflected fromside140 being emitted from the viewing surface in addition to rays reflected directly fromend reflector125 in the region betweenray820 andside140. Further, a region betweenside130 andray840, labeled “Shadow”, may be dimmer than a region between840 andside140 due to a shadow caused byray840 being reflected away fromside130.
• InFIG. 9,light source902 is the same distance fromcenterline850 aslight source802, but positioned on an opposite side ofcenterline850. A cone of light bounded byrays910 and930 is injected atthin end110 bylight source902.Ray930 intersectsend reflector125 and is reflected asray940.Ray910 intersectsend reflector125 and is reflected asray920 after an additional reflection fromside130. As described above with respect toFIG. 8, the collimated light emitted from viewingsurface150 may not be uniform in this configuration. A region betweenray920 andside130, labeled “Reflection”, may be brighter than a region betweenray920 andside140. Further, a region betweenside140 andray940, labeled “Shadow”, may be dimmer than a region between840 andside140.
• Whenlight sources802 and902 are positioned similar distances fromcenterline850, the borders of the “Reflection” region inFIG. 8 may be matched to the borders of the “Shadow” region inFIG. 9. Likewise, the borders of the “Shadow” region inFIG. 8 may be matched to the borders of the “Reflection” region inFIG. 9. The regions of shadow and reflection may cancel each other out if the brightness oflight sources802 and902 are similar such that the light injected atthin end110 by each light source is of similar brightness and uniformity.
• FIG. 10 shows anexample method1000 for collimating light via an optical waveguide. The optical waveguide may comprise a first end, a second end opposite the first end and comprising an end reflector comprising a plurality of facets, a viewing surface extending between the first end and the second end, and a back surface opposing the viewing surface. The viewing surface may have a first critical angle of reflection and the back surface may have a second critical angle of reflection, wherein the first and second critical angles of reflection are different. In one embodiment, the optical waveguide is the optical wedge ofFIG. 2, where the thin end of the optical wedge is the first end of the optical waveguide and the thick end of the optical wedge is the second end of the optical waveguide. In an alternate embodiment, the optical waveguide may have a constant thickness, e.g. the first end and the second end are the same thickness. The optical waveguide may include a cladding on the viewing and/or back surface with a refractive index that varies linearly between the first end and the second end. This embodiment will behave similarly to an optical wedge when light is injected into the first end of the optical waveguide. In yet another embodiment, the optical waveguide may have a constant thickness, a refractive index that varies linearly between the first end and the second end, and claddings on the viewing and/or back surface of constant refractive index. This embodiment will also behave similarly to an optical wedge when light is injected into the first end of the optical waveguide.
• Returning toFIG. 10, at1010, light may be injected into the first end of the optical waveguide, and then at1020, the light may be delivered to the end reflector via total internal reflection. At1030, the light may be internally reflected off of the end reflector. The light internally reflected off of the end reflector may be reflected from a first set of facets and a second set of facets, each of the first set of facets having a normal that points at least partially toward the viewing surface, and each of the second set of facets having a normal that points at least partially toward the back surface. Furthermore, in some embodiments, each of the first set of facets may have an angle of three-eighths of a difference between 90 degrees and the critical angle of reflection and each of the second set of facets may have an angle of three-eighths of the difference between 90 degrees and the critical angle of reflection. In other embodiments, the facets may have other suitable angles that do not cause unsuitable variations in light intensities.
• Due to the angle at which facets on the end reflector are angled, at1040, a first portion of light may be emitted from the viewing surface, the first portion of light intersecting the viewing surface at the first critical angle of reflection. At1050, a second portion of light may be internally reflected from the back surface at an angle equal to the first critical angle of reflection when the second critical angle of reflection is less than the first critical angle of reflection. At1060, the second portion of light may then be emitted from the viewing surface after internally reflecting from the back surface.
• Among the potential uses of such a flat panel collimator is that of illuminating a liquid crystal panel. A liquid crystal display is an inexpensive way of displaying video and comprises a liquid crystal panel behind which is placed a backlight. Past wedge backlights have utilized a slim transparent wedge with light sources along the thick end and films which direct light through the liquid crystal panel to the viewer so that they may see the displayed image. Considerable effort is taken to ensure that emission from the backlight is sufficiently diffuse so that the displayed image can be seen from a wide field of view. For example, some past wedges were filled with scattering sites. With diffuse illumination, however, it is difficult to use the liquid crystal panel in anyway other than as a conventional display.
• There exist many applications where it is desirable to project a video image. This may be done by placing a lens in front of a liquid crystal display. However, if the illumination is diffuse, the lens has to be large and therefore expensive. A flat panel collimator can be a slim way of illuminating a small liquid crystal panel or other spatial light modulator with collimated light which may be condensed through a small projection lens. If the spatial light modulator is reflective, as in the case of a digital micromirror device, no beam splitter or other space for illumination is needed. Therefore, the projection lens may be brought as close to in the light modulator as desired.
• In some applications, it may be desired to project an image only a few millimeters onto a screen. This may be done in the same way that the sun projects the shadow of trees onto the ground: illuminate a large liquid crystal panel with collimated light, and its shadow, e.g. an image, can be formed on a diffuser spaced a few millimeters away from the liquid crystal panel. One application for this is where it is desired that there be a video image on every key of a keyboard. Were a separate display screen to be formed on each keyboard key, the cost of so many small displays may be prohibitive. However, using a collimating optical wedge backlight as described above, transparent keys may be provided with diffusive surfaces and placed over a liquid crystal panel with a collimated backlight. In this manner, an image may be projected up to each key from different areas of a single large but low cost panel.
• Another example application for shadow projection is in the projection of an image onto a diffuser where fingers or objects which touch the diffuser are to be sensed with an infrared camera behind. Devices such as Microsoft's SURFACE, developed and sold by the Microsoft Corporation of Redmond, Wash., comprise a video projector, infrared lamp, camera and diffuser. The projector creates a video image on the diffuser, and the lamp illuminates objects nearby so that they appear blurred when off the diffuser but sharp at the moment of touch. The imaging optics can be made slim by pointing the camera at the diffuser via an optical wedge, such as the embodiments described above. If the liquid crystal display is illuminated by diffuse light, the projected image may be spatially separate from the diffuser, and therefore may be blurred. Therefore, the liquid crystal panel may be illuminated with collimated light as disclosed above so that a visible image without blurring forms at the diffuser. In some embodiments, the panel for providing collimated visible illumination and detecting the infrared image are the same, and the end reflector comprises facets at an angle according to this disclosure that reflect visible light but transmit infrared light, and beyond these are placed facets or equivalent which reflect infrared light and are angled so as to form a single unambiguous image.
• It will be understood that the configurations and/or approaches described herein are exemplary in nature, and that these specific embodiments or examples are not to be considered in a limiting sense, because numerous variations are possible. The subject matter of the present disclosure includes all novel and nonobvious combinations and subcombinations of the various processes, systems and configurations, and other features, functions, acts, and/or properties disclosed herein, as well as any and all equivalents thereof.

Claims (20)

1. An optical collimator comprising:

an optical waveguide having

a first end comprising a first light interface;

a second end opposite the first end;

a viewing surface comprising a second light interface extending at least partially between the first end and the second end and having a first critical angle of internal reflection with respect to a normal of the viewing surface;

a back surface opposing the viewing surface, the back surface being configured to be reflective to internally incident light at the first critical angle of internal reflection; and

an end reflector arranged at the second end of the optical waveguide, wherein the end reflector comprises a faceted lens structure comprising a plurality of facets angled to cause a majority of the viewing surface to be uniformly illuminated when uniform light is injected into a first end and also to cause a majority of the injected light to exit the viewing surface.

2. The optical collimator ofclaim 1, wherein the first end of the optical waveguide is a thin end and the second end of the optical waveguide is a thick end.

3. The optical collimator ofclaim 1, wherein the end reflector is spherically curved.

4. The optical collimator ofclaim 1, wherein the plurality of facets of the faceted lens structure of the end reflector includes a plurality of facets facing the viewing surface and a plurality of facets facing the back surface, each facet facing the viewing surface being positioned adjacent to a facet facing the back surface.

5. The optical collimator ofclaim 4, wherein each facet facing the viewing surface forms an angle relative to a normal of a surface of the end reflector of three-eighths of a difference between 90 degrees and the first critical angle.

6. The optical collimator ofclaim 4, wherein each facet facing the back surface forms an angle relative to a normal of a surface of the end reflector equal to three-eighths of a difference between 90 degrees and the first critical angle.

7. The optical collimator ofclaim 4, wherein each facet facing the viewing surface has a height of less than 500 microns and each facet facing the back surface has a height of less than 500 microns.

8. The optical collimator ofclaim 1, wherein the back surface includes a second critical angle of internal reflection with respect to a normal of the reflective back surface, wherein the second critical angle of reflection is less than the first critical angle of reflection.

9. The optical collimator ofclaim 1, wherein the viewing surface of the optical waveguide includes a cladding.

10. The optical collimator ofclaim 1, wherein the back surface of the optical waveguide includes a cladding.

11. The optical collimator ofclaim 1, wherein the back surface includes a mirror.

12. The optical collimator ofclaim 1, wherein the optical waveguide further comprises a first reflective side and a second reflective side, the first reflective side opposite the second reflective side, each reflective side extending from the first end to the second end and from the viewing surface to the back surface.

13. An optical collimator comprising:

an optical wedge having

a thin end comprising a first light interface;

a thick end opposite the thin end;

a viewing surface comprising a second light interface extending at least partially between the thin end and the thick end and having a first critical angle of internal reflection with respect to a normal of the viewing surface;

a back surface opposing the viewing surface and having a second critical angle of internal reflection with respect to a normal of the back surface, wherein the second critical angle of reflection is less than the first critical angle of reflection; and

an end reflector arranged at the thick end of the optical wedge, wherein the end reflector comprises a faceted lens structure comprising a plurality of facets arranged at an angle relative to a surface of the thick end, the plurality of facets alternating between facets facing the viewing surface and facets facing the back surface, and the facets having an angle relative to a normal of a surface of the end reflector of three-eighths of a difference of ninety degrees and the first critical angle.

14. The optical collimator ofclaim 13, wherein the end reflector is spherically curved.

15. The optical collimator ofclaim 13, wherein the end reflector of the optical wedge is cylindrically curved.

16. The optical collimator ofclaim 13, wherein each facet facing the viewing surface includes a height of less than 500 microns and each facet facing the back surface includes a height of less than 500 microns.

17. The optical collimator ofclaim 13, wherein the optical wedge further comprises a first reflective side and a second reflective side, the first reflective side opposite the second reflective side, each reflective side extending from the thin end to the thick end and from the viewing surface to the back surface.

18. A method of collimating light via an optical waveguide, the optical waveguide comprising a first end, a second end opposite the first end and comprising an end reflector, a viewing surface extending between the first end and the second end, and a back surface opposing the viewing surface, the method comprising:

injecting light into the first end of the optical waveguide;

delivering the light to the end reflector via total internal reflection;

internally reflecting the light off of the end reflector;

emitting a first portion of light from the viewing surface at a critical angle of reflection;

internally reflecting a second portion of light from the back surface at an angle equal to the critical angle of reflection, and then emitting the second portion of light from the viewing surface after internally reflecting the second portion of light from the back surface.

19. The method ofclaim 18, wherein reflecting light off the end reflector comprises reflecting light from a first set of facets and a second set of facets, wherein each facet of the first set of facets comprises a normal that points at least partially toward the viewing surface, and wherein each facet of the second set of facets comprises a normal that points at least partially toward the back surface.

20. The method ofclaim 19, wherein each of the first set of facets has an angle of three-eighths of a difference between 90 degrees and the critical angle of reflection and each of the second set of facets has an angle of three-eighths of the difference between 90 degrees and the critical angle of reflection.

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