US20210149094A1 - High incidence angle retroreflective sheeting - Google Patents

Abstract

A retroreflective sheeting which comprises a front sheet of an optically transmissive material and a reflective back sheet bonded to the front sheet. The front sheet includes a plurality of parallel channels defining surfaces which reflect light using a total internal reflection and configured to retroreflect high-incidence-angle light rays at least in one plane. The retroreflective sheeting may have a plurality of retroreflective areas with different orientations of the channels and/or different retroreflective properties with respect to light incident onto the retroreflective sheeting at high incidence angles.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS
• This application is a division of application Ser. No. 15/915,357 filed on Mar. 8, 2018, which is a continuation of application Ser. No. 14/791,411 filed on Jul. 4, 2015, which claims priority from U.S. provisional application Ser. No. 62/024,444 filed on Jul. 14, 2014, 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 THEINVENTION1. Field of the Invention
• The present invention relates to retroreflective materials which are effective at off-axis light incidence and are particularly effective within a range of incidence angles of about 20° to about 85°. More particularly, this invention relates to retroreflective materials such as sheeting or films useful for marking surfaces which are positioned at relatively low angles with respect to light rays directed towards such surfaces.
2. Description of Background Art
• Various types of optical retroreflectors used to direct light back toward the light source employ prismatic cube corner elements or tiny glass beads. Retroreflective materials are useful since they appear brightest to observers located near the light source, such as a car's driver near headlights, for example, and can be effective in a broad range of incidence or viewing angles.
• However, the retroreflective properties and, hence, the apparent brightness of conventional retroreflective materials diminishes rapidly with the increase of the incidence angle. For example, the cube corner retroreflective sheeting, which provides some of the highest reflectivity of any known retroreflective sheeting, loses nearly all of its reflectivity when the incidence angle becomes greater than about 40°, which limits its use.
• It is therefore an object of the present invention to provide an improved retroreflective sheeting structure that enhances retroreflectivity at high entrance angles.
• Raised retroreflective elements or retroreflectors consisting of array of individually assembled retroreflective elements have been proposed which lack the utility and cost benefits of continuous retroreflective sheeting. Accordingly, practical retroreflective materials are needed that could be implemented within a compact form factor of a continuous sheet or film and which could maintain high brightness at high incidence angles. It is therefore another object of the present invention to provide an improved retroreflective structure that provides advanced retroreflectivity at high incidence angles within a sheet-form material.
• The design and optical efficiency of retroreflective sheeting, while improved over the last two decades, may still be improved even further in terms of the overall light return. For example, the state-of-the-art encapsulated glass bead sheeting reflects only 14% of light back toward its source, the most common truncated cube corner sheeting reflects a maximum of about 32%, and the most advanced full-cube sheeting reflects a maximum of about 58%. It is therefore another object of the present invention to provide an improved retroreflective sheeting structure that enhances the overall light return at least for some observation and/or incidence angles.
• These needs and others are met within the present invention, which provides an improved retroreflective sheeting structure for redirecting angle incident at high entrance angles, particularly achieving relatively high retroreflectivity at incidence angles above 20° and achieving superior retroreflectivity at incidence angles of about 40°-85°. The improved structure employs internal TIR surfaces and a rear reflective surface to efficiently intercept and retroreflect light towards the source. Such structure can be made thin and flexible, finding utility in various retroreflective systems such as road pavement marking, high visibility sheeting or films, light redirecting materials, light rejecting materials, and the like.
BRIEF SUMMARY OF THE INVENTION
• The present invention solves problems of retroreflecting light at high incidence within a thin sheet-form structure having a top optically transmissive layer and a bottom reflective layer. The top layer includes at least one parallel array of straight and narrow channels formed within the layer's material. The channels may also be arranged in two parallel array oriented perpendicular to each other. The channels have smooth parallel surfaces which are configured to reflect light propagating through the top layer by means of a total internal reflection (TIR). The channels may be spaced according to a predefined ratio with respect to the channel depth to maximize retroreflectivity for a particular angular range. The bottom reflective layer may include a mirrored surface and may also be configured to include linear prismatic surface corrugations extending perpendicular to the channels of the top layer. Each of the corrugations should preferably be shaped in the form of an isosceles right-angle linear prism which retroreflects light at least in one plane which is perpendicular to the longitudinal axis of the prism and parallel to the linear channels of the top layer. The retroreflective sheeting may be preferably operable in an orientation in which the incident light enters the surface of the sheet-form structure at an off-normal angle and in which the channels of the top layer are perpendicular or near perpendicular to the incident light direction.
• In at least one embodiment, the invention features a retroreflective sheet structure having a top layer of an optically transmissive material and a bottom reflective layer. The top layer has a plurality of channels formed within its optically transmissive material. Each channel is configured to reflect light by means of a total internal reflection. The channels can be arranged into various configurations. In one implementation, the channels are arranged into a parallel array. In one implementation, the channels are arranged into a first parallel array and a second parallel array having an orientation perpendicular to the first parallel array. In one implementation, the first and second array are disposed in a staggered arrangement.
• In at least one embodiment, each of the channels is defined by a pair of opposing walls extending between opposing major surfaces of the top layer parallel to each other and perpendicular to the major surfaces.
• In various implementations, the optically transmissive material of the top layer is selected from the group of polymeric materials consisting of optically clear or translucent plasticized polyvinyl chloride, thermoplastic elastomers, polyurethanes, and silicones. In further implementations, the bottom reflective layer comprises a refractive material selected from the group of consisting of glass, poly(methyl methacrylate), polycarbonate, polystyrene, rigid polyvinyl chloride, polyester, and cyclic olefin copolymer. In yet further implementations, the bottom reflective layer includes a mirrored surface.
• In at least one embodiment, the bottom reflective layer comprises a linear array of isosceles prisms having substantially perpendicular sides arranged side-by-side to form a plurality of peaks and grooves, wherein the perpendicular sides of the isosceles prisms make an angle of approximately 45° with a surface plane of the bottom reflective layer. In at least one implementation, the orientation of the channels is perpendicular to the orientation of the isosceles prisms. In at least one implementation, a depth of the channels is at least 5 times greater than a height of the isosceles prisms.
• In at least one embodiment, a distance between adjacent channels of formed in the top layer of the retroreflective structure is at least 6 times an average width of the channels. In at least one embodiment, a distance between adjacent channels of formed in the top layer of the retroreflective structure is at least 10 times an average width of the channels. In at least one embodiment, a depth of each said channel is at least ten times an average width of the channel. 14. In at least one embodiment, the plurality of channels is arranged into an parallel array in which the channels are spaced from each other by a distance that is greater than a depth of each channel by a factor of at least 1.2 and at most 1.8.
• In at least one embodiment, the retroreflective sheet structure is formed into an elongated shape and each of the plurality of channels is aligned parallel to a longer dimension of the elongated shape. In at least one embodiment, the retroreflective sheet structure is formed into an elongated shape and each of the plurality of channels is aligned parallel to a shorter dimension of the elongated shape.
• In various embodiments, the walls of the channels can have different surface roughness characteristics. In one implementation, a root mean square surface profile roughness parameter of the walls of the channels is at most about 60 nanometers at a sampling length of between 20 and 100 micrometers. In one implementation, a root mean square surface profile roughness parameter of at least a substantial portion of the surface of each of the channels is at least about 10 nanometers at a sampling length of between 20 and 100 micrometers.
• In at least one embodiment, the thickness of the top layer of the retroreflective sheet structure is between 100 micrometers and 2 millimeters.
• In at least one embodiment, at least one of the top and bottom layers comprises light diffusing features. In at least one embodiment, either one of said top and bottom layers has a predefined visually conspicuous color or tint.
• In at least one embodiment, at least one edge of the retroreflective sheet structure is made impermeable to moisture and/or air. In at least one embodiment, the retroreflective sheet structure includes at least one layer of optically clear adhesive bonding together the top and bottom layers.
• In at least one embodiment, the retroreflective sheet structure comprises a prismatic surface as a top layer. In different implementations, the top prismatic surface is formed by a plurality of linear microprisms that can have asymmetric or symmetric configurations in which the sloped faces of the microprisms are configured to receive at least light rays propagating at extremely high angles with respect to a surface normal or near-parallel to a surface of the sheet structure and refract such rays towards the bottom reflective layer.
• In at least one embodiment, the invention features a retroreflective material, comprising a thin, flexible film of a transparent polymeric material, comprising a structured surface on one side, a smooth surface opposite the structured surface on the other side, and a plurality of linear reflectors formed between the structured and smooth surfaces and extending generally perpendicular to the smooth surface, wherein the structured surface includes a linear array of miniature isosceles prisms having substantially perpendicular sides arranged side-by-side to form a plurality of peaks and grooves, the perpendicular sides of said prisms make an angle of approximately 45° with said smooth surface opposite said structured surface. In at least one implementation of the retroreflective material, the plurality of linear reflectors is arranged in a parallel array extending generally perpendicular to the linear array of miniature isosceles prisms.
• In at least one embodiment, the invention features a retroreflective sheeting configured to retroreflect more than 60% of light incident onto the surface of the sheeting at an off-normal incidence angle.
• In at least one embodiment, the invention features a retroreflective sheeting having omnidirectional retroreflective operation. In various implementations, such omnidirectional retroreflective sheeting having a plurality of areas each configured for a different azimuth acceptance angle. In one implementation, such areas are arranged into circular segments each being a rotated copy of an adjacent segment. In one implementation, the omnidirectional retroreflective sheeting further includes areas having conventional retroreflectors such as on cube corners, full-cubes or glass beads.
• In at least one embodiment, the invention features a slat of a window or door blind, comprising a strip of an optically clear plastic material having a plurality of linear reflectors embedded into the clear plastic material; and a reflective layer underneath the strip, wherein each of the linear reflectors has at least one surface configured to reflect light by means of a total internal reflection. In at least one implementation, each of the linear reflectors is aligned parallel to a longitudinal dimension of the strip.
• Further embodiments and elements of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon.
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 fragmentary, schematic cross section view of a layered retroreflective sheet material, according to at least one embodiment of the present invention.
• FIG. 2 is a fragmentary, schematic cross section view of a top layer of a retroreflective sheet material, according to at least one embodiment of the present invention.
• FIG. 3 is a schematic perspective view of a retroreflective sheet, according to at least one embodiment of the present invention.
• FIG. 4 is a fragmentary, schematic cross section view and raytracing of a retroreflective sheet material, showing a retroreflective operation, according to at least one embodiment of the present invention.
• FIG. 5 is a fragmentary, schematic cross section view and raytracing of a retroreflective sheet material, showing a retroreflective operation with a reverse path of a light ray, according to at least one embodiment of the present invention.
• FIG. 6 is a fragmentary, schematic perspective view and raytracing of a retroreflective sheet material, showing a retroreflective operation of the material at least in one plane, according to at least one embodiment of the present invention.
• FIG. 7 is a schematic perspective view of a retroreflective sheet configured for retroreflecting light in orthogonal angular dimensions, according to at least one embodiment of the present invention.
• FIG. 8 is a schematic cross section view and raytracing of an individual linear prismatic element, showing a retroreflective operation in a cross-sectional plane, according to at least one embodiment of the present invention.
• FIG. 9 is a fragmentary, schematic perspective view of a reflective layer including a plurality of linear prismatic elements, according to at least one embodiment of the present invention.
• FIG. 10 is a fragmentary, schematic cross section view and raytracing of a reflective layer including a plurality of linear prismatic elements, according to at least one embodiment of the present invention.
• FIG. 11 is a schematic perspective view of a reflective layer portion, showing a light ray entering a smooth surface of the layer at an off-normal angle, according to at least one embodiment of the present invention.
• FIG. 12 is a schematic bottom view of retroreflective sheet shaped in the form of a strip, showing linear prismatic elements expending perpendicular to a longitudinal axis of the strip, according to at least one embodiment of the present invention.
• FIG. 13 is a schematic bottom view of retroreflective sheet shaped in the form of a strip, showing linear prismatic elements expending parallel to a longitudinal axis of the strip, according to at least one embodiment of the present invention.
• FIG. 14 is a schematic view showing an example of using a retroreflective sheeting for road marking, according to at least one embodiment of the present invention.
• FIG. 15 is a schematic view showing an example of using a retroreflective sheeting for vehicular marking, according to at least one embodiment of the present invention.
• FIG. 16 is a schematic view of a retroreflective sheeting applied to a road warning cone, according to at least one embodiment of the present invention;
• FIG. 17 is a schematic view of a retroreflective sheeting applied to a road marker pole, according to at least one embodiment of the present invention.
• FIG. 18 is a schematic view of a retroreflective sheeting portion, showing a diverging beam of retroreflected light, according to at least one embodiment of the present invention.
• FIG. 19 is a schematic view of a retroreflective sheeting portion, showing a non-zero angle between an incident ray and a retroreflected ray, according to at least one embodiment of the present invention.
• FIG. 20 is a schematic view and raytracing of a reflective layer portion, showing a retroreflected light deviating from an incidence direction, according to at least one embodiment of the present invention.
• FIG. 21 is a schematic view and raytracing of a reflective layer portion, showing different dihedral angles of prismatic element faces with respect to a sheet surface, according to at least one embodiment of the present invention.
• FIG. 22 is a schematic fragmentary view and raytracing of a retroreflective sheeting, showing a plurality of rays retroreflected toward a source direction, according to at least one embodiment of the present invention.
• FIG. 23 is a schematic fragmentary view and raytracing of a retroreflective sheeting, showing a refractive prismatic element configured to bend high-incidence-angle light rays, according to at least one embodiment of the present invention.
• FIG. 24 is a schematic top view of an omnidirectional retroreflective sheeting, according to at least one embodiment of the present invention.
• FIG. 25 is a schematic top view of an alternative configuration of an omnidirectional retroreflective sheeting, according to at least one embodiment of the present invention.
• FIG. 26 is a schematic top view of an optically transmissive layer, showing a plurality of linear channels arranged into two parallel arrays crossed at a right angle with respect to each other, according to at least one embodiment of the present invention.
• FIG. 27 is a schematic perspective view of a retroreflective sheeting, showing a plurality of intersecting linear channels formed in a top transmissive layer, according to at least one embodiment of the present invention.
• FIG. 28 is a schematic fragmentary perspective bottom view and raytracing of an optically transmissive layer, showing individual full-cube retroreflective cells formed by a plurality of intersecting linear channels, according to at least one embodiment of the present invention.
• FIG. 29 is a schematic fragmentary view of a horizontal window blind employing retroreflective sheeting, showing a plurality of adjustable light deflecting slats in a fully open position, according to at least one embodiment of the present invention.
• FIG. 30 is a schematic fragmentary view of a horizontal window blind employing retroreflective sheeting, showing a plurality of adjustable light deflecting slats in a fully closed position, 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 and method generally shown in the preceding figures. It will be appreciated that the apparatus and method 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.
• The present invention particularly seeks to provide retroreflective sheeting capable of redirecting light back to the source with high efficiency at off-normal incidence angles and particularly at high angles of incidence. The following embodiments of the present invention are generally directed to a retroreflective sheet material which may be configurable for light redirecting operation in response to light incident onto such material from off-normal angles and may be further configurable for maintaining a certain minimum level of retroreflectivity for relatively high entrance angles or even for the case when the incident light propagates near parallel to the surface of the material.
• FIG. 1 illustrates a first embodiment of a retroreflective sheeting of the invention. In this embodiment, aretroreflective sheet2 has a layered structure and is formed by an optically transmissivetop layer4 and areflective bottom layer20.Top layer4 is defined by a firstmajor surface10 and an opposing secondmajor surface12 that extends parallel to surface10.Bottom layer20 is defined by amajor surface22 facing away fromlayer4 and an opposingmajor surface24 facinglayer4. Each of thelayers4 and20 extends both longitudinally and laterally and forms a continuous sheet or sheet-form structure. Accordingly,major surfaces22 and10 of therespective layers20 and4 define opposing outer boundaries ofretroreflective sheet2.
• Top layer4 is made from an optically transmissive material and configured for light input and output to and fromsheet2. Such material should preferably be optically clear with relatively high light transmissivity so thatlayer4 can effectively transmit substantially all of the light to and fromlayer20.
• In one embodiment,reflective layer20 may be formed by a reflective coating deposited ontosurface12 oflayer4. In one embodiment,reflective layer20 can be made from an opaque substrate material, such as metal or plastic, with itssurface24 being smooth and mirrored for specular reflectivity. Mirroring may be performed by any suitable means, for example, including but not limited to vacuum metallization ofsurface24 or laminating a mirror film onto such surface. Alternatively,layer20 may be made from an optically transmissive material and its reflectivity may be provided by mirroring theopposite surface22.
• The thickness oflayer4 should preferably be within a range typical for film or thin sheet materials. In one embodiment, the thickness oflayer4 may be within the range from 50 micrometers to 2 millimeters and more preferably within the range of 100 micrometers to 1 millimeter.
• Layer20 can be made rigid or flexible, depending on the application and can also be made to any desired thickness. From the practical point of view, the overall thickness ofsheet2 and its layers may be selected from the range of thicknesses that is typical to films or thin flexible sheets. In a preferred embodiment, the thickness ofsheet2 may be selected from the range between 100 micrometers and 2 millimeters which may further include any auxiliary layers such as an adhesive or protective layer.
• Retroreflective sheeting can be made operable withlayers4 and20 being detached from each other and even spaced apart by a relatively small distance. However, it at least one embodiment, it may be preferred thatlayer20 is disposed as close tolayer4 as possible, end even more preferred that the layers are disposed in a close contact with each other. It is noted that many applications may require monolithic retroreflective sheeting with a good structural integrity, in whichcase layer20 should be bonded, welded, laminated or otherwise attached tolayer4 with good and durable physical contact so thatchannels6 are permanently encapsulated and embedded into the material ofsheet2.
• Any suitable lamination and/or bonding technique may be used. For example, layers4 and20 may be bonded by an optically clear adhesive using roll lamination, press lamination, vacuum press lamination, encapsulation and the like. The bonding process may also include curing the adhesive layer using heat, moisture, UV light or other techniques depending on the type of adhesive used.Layers4 and20 may bonded together using an alternative technique which does not involve any adhesives. Examples of such bonding include but are not limited to heat welding, ultrasonic welding, radio-frequency (RF) welding, solvent welding, and the like.
• Layer4 includes an array of deep, narrow and substantiallyparallel channels6 formed in its body betweensurfaces10 and12 and extending perpendicularly to such surfaces into the bulk of the material that formslayer4. Eachchannel6 has afirst wall7 and an opposingsecond wall8 extending parallel to the first wall.Channels6 should be as narrow as practically possible with spacing betweenwalls7 and8 being just sufficient to prevent optical contact.
• Walls7 and8 should also be as parallel to each other as practically possible. The parallelism of such walls should preferably be within a small predefined angular range. In one embodiment, each pair ofwalls7 and8 should be parallel within 2 degrees, more preferably within 1 degree, and even more preferably within 0.5 degree.
• Each of thewalls7 and8 should also extend into the depth of the material oflayer4 substantially perpendicular to surface10. In one embodiment, the deviation ofwalls7 and8 from a normal to surface10 should preferably be within 1 degree, more preferably within 0.5 degree, and even more preferably within 0.3 degrees.
• The width ofchannel6 is defined by the distance between the respective opposingwalls7 and8. It is preferred that the width of eachchannel6 is less than 100 micrometers, more preferred that is less than 60 micrometers, even more preferred that it is less than 40 micrometers, and even more preferred that it is less than 20 micrometers. In the extreme cases, the width ofchannels6 may also be made as small as 10 micrometers or even less, such as, for example 1 to 5 micrometers. On the other hand, it may also be preferred that there is a certain minimum channel width of at least a few micrometers is maintained in order to prevent or reduce the risk of opposingwalls7 and8 ofchannels6 to close upon each other and thus create an optical contact.Channels6 are hollow and designed to internally reflect light by means of a total reflection (TIR) which requires an optical interface between a higher-index transmissive material and air having refractive index of about 1. Accordingly, the optical contact betweenwalls7 and8 is generally unwanted since it may suppress TIR.
• The depth of eachchannel6 should be substantially greater than its width. According to one embodiment of the present invention, the depth of eachchannel6 may be at least approximately ten times the average width of the channel. By way of example and not limitation, the average width of eachchannel6 may be approximately 20 micrometers and the depth of the channel may be at least 200 micrometers. In at least some embodiments, the ratio between the depth and width of channels6 (an aspect ratio) may be advantageously selected to exceed 15 or 20 times.
• On the other hand, the channel width should also be significantly less than the spacing between adjacent channels to minimize the percentage of incident light that can be intercepted by the channel ends. According to one embodiment of the present invention, a distance between adjacent channels is at least 6 times an average width of the channels, more preferably at least 8 times an average width of the channels, and more preferably at least 10 times an average width of the channels.
• Each of thewalls7 and8 should have a substantially smooth surface capable of reflecting light by means of TIR in a specular or near-specular regime while minimizing scattered light. It should be understood that the surfaces ofwalls7 and8 do not have to be absolutely smooth to provide such operation. It can be shown thatwalls7 and8 may provide good TIR reflectivity even with some non-negligible surface roughness as long as such roughness is significantly less than the wavelength. It is generally preferred that a root-mean-square (RMS) roughness parameter of the surface ofwalls7 and8 is below 0.1 micrometers (100 nanometers). According to one embodiment, the RMS surface roughness parameter should be within the range between 0.01 micrometers (10 nanometers) and 0.1 micrometers (100 nanometers), more preferably between 0.01 micrometers (10 nanometers) and 0.06 micrometers (60 nanometers), and more preferably between 0.01 micrometers (10 nanometers) and 0.03 micrometers (30 nanometers). According to one embodiment, the preferred sampling length for measuring such RMS roughness parameter can be between 20 and 100 micrometers and should not generally exceed the depth ofchannels6.
• According to a preferred embodiment,channels6 should be formed in the material oflayer4 to a depth that preserves the integrity of the layer, prevents material tearing or separation at the locations ofchannels6 and allowslayer4 to retain its continuous sheet form. Accordingly, it is preferred thatchannels6 extend into the bulk material through only a portion of the thickness oflayer4 so that all of the channels are formed on a common substrate. At least some portion of the thickness of the material should remain uncut and at least one major surface oflayer4 should be smooth and generally uninterrupted across the entire width ofsheet2.
• While forming extremely narrow, smooth-surface TIR channels with vertical walls in optically clear materials may be challenging using conventional techniques such as molding or embossing, suitable new techniques have recently been developed. By way of example, U.S. Pat. No. 8824050, published Sep. 2, 2014 and herein incorporated by reference in its entirety, discloses forming narrow TIR-quality channels in a soft optically clear material, such as plasticized PVC (PVC-P), using one or more ultra-sharp rotary blades. It also discloses examples of forming slits having TIR walls with RMS surface roughness between about 0.01 micrometers (10 nanometers) and 0.03 micrometers (30 nanometers). Likewise, a sheet or film of optically clear PVC-P or a similar material can be used for forminglayer4 and a finely honed rotary blade may be used to slit the material to a prescribed depth to producechannels6 which opposingwalls7 and8 will be capable of reflecting light by means of TIR. Since slits produced by a thin and extremely sharp rotary blade in a soft and elastic material, such as PVC-P or thermoplastic polyurethane (TPU), allow for maintaining a very small width of the formed slits and high parallelism of the opposing walls of such slits, good-quality channels6 may be produced using such technique.
• It is noted that, where a definition or use of a term in a reference, which is incorporated by reference herein is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
• It is further noted that the use of PVC-P material (also frequently referred to as plasticized PVC, flexible PVC or simply vinyl) forsheet4 and the use of surface slitting technique for formingchannels6 are not prescriptive and are described here by way of non-limiting examples. Other materials that may potentially be suitable for slitting and producingnarrow TIR channels6 include but are not limited to optically clear or translucent thermoplastic elastomers, optically clear or translucent polyurethanes, including TPU, and optically clear or translucent silicones. Furthermore, other types of optically transmissive materials can be used with an appropriate technique of formingchannels6 with the described properties. Such materials may include but are not limited to glass, poly(methyl methacrylate) (PMMA, acrylic), polycarbonate, polystyrene, rigid polyvinyl chloride, butyrate, polyester, and cyclic olefin copolymer.
• It is essential that eachchannel6 forms a highly efficient reflector embedded into the material oflayer4 and extending generally perpendicular to the layer's surface. While the technique of slitting may be preferred for making deep andnarrow channels6 in certain soft materials, such as PVC-P or TPU, it may also be possible that the certain techniques related to micro- and nano-texturing, such as hot embossing, molding, imprinting, UV-curable embossing, etching, and the like, can be adapted for producing the described structure oflayer4 in rigid materials such as PMMA, PET, clear epoxies or polycarbonate.
• FIG. 2 depictslayer4 withoutlayer20 attached to it and shows that high-aspect-ratio channels6 may be formed insurface12 without disrupting the overall planarity and smoothness of such surface. The channels may extend only partially into the material oflayer4 so that theopposite surface10 may be left uninterrupted. It will be appreciated that whensuch layer4 is subsequently co-laminated withlayer20 to formsheet2,channels6 will become fully enclosed with their forming TIR surfaces embedded into the final retroreflective structure. Such encapsulation may be advantageous for protectingchannels6 and surfaces ofwalls7 and8 from the environment and for providing dimensional stability of the embedded TIR reflectors. An additional benefit of such encapsulation can be the prevention of closing the walls ofchannels6 upon themselves.
• FIG. 3 shows rectangularlyshaped sheet2 and illustrates its retroreflective operation.Linear channels6 oflayer4 are arranged into a parallel array and extend parallel to areference line400 which, in turn, is parallel to a pair of opposing edges of the rectangular sheet. In one embodiment,reference line400 is parallel to a longer major dimension ofrectangular sheet2. In one embodiment,reference line400 is parallel to a shorter major dimension ofrectangular sheet2.
• Whilesheet2 ofFIG. 3 is shown having a generally planar appearance, it should be understood that such sheet can be made flexible and bendable to any shape. Alternatively,sheet2 may also be made rigid to maintain such planar shape or any other prescribed shape depending on the application. One or more edges ofsheet2 may be sealed using an air and/or moisture impermeable encapsulating resin or tape. Such sealing or encapsulation may be useful for preventing the delamination of the layers, and/or preventing the moisture ingress or contamination ofchannels6 with dust or dir through the edges. In one embodiment, the entire perimeter ofsheet2 may be sealed. In a further embodiment, theentire sheet2 may be encapsulated within another material or structure or overmolded with a clear plastic material.
• A preferred orientation ofsheet2 with respect to a light source and the viewer is such that light rays should entersheet2 at a relatively high off-normal incidence angle and also propagate along a direction perpendicular toreference line400. In other words, sinceline400 defines the alignment ofparallel channels6, light rays should entersheet2 perpendicular or near-perpendicular to the longitudinal orientation ofchannels6. In the context of the present invention, the term “off-normal” is meant to characterize light rays having substantially non-zero incidence angles with respect to a surface normal, in contrast to “normal” rays having incidence angles equal to or substantially close to zero with respect to the surface normal.
• In operation, an incidentlight ray62 emanated by a distant source strikes surface10 of thetop layer4 at anincidence angle32. The incidence angle is defined as an angle that the incident ray makes with a normal to the surface at the point of incidence. Referring toFIG. 2,ray62 makes incidence angle with respect to a normal160.Ray62 also makes anangle34 withsurface10 ofsheet2 at the point of incidence. It will be appreciated thatangle32 and34 are complementary.
• Ray62 is internally redirected and retroreflected bysheet2 and forms aretroreflected ray64 upon the exit fromsurface10. Theretroreflected ray64 emerges at the same angle with respect to surface10 as the angle ofincidence32 and propagates generally back towards the source.
• The retroreflective operation ofsheet2 is further explained in reference toFIG. 4 which shows a fragmentary schematic cross section ofsheet2 with the plane of the drawing being generally perpendicular toreference line400 ofFIG. 3 and thus also perpendicular to the longitudinal axis ofchannels6.Surface24 ofreflective layer20 inFIG. 4 is mirrored for high specular reflectivity.Incident ray62 striking thelight input surface10 atincidence angle32 refracts into the material oflayer4 and propagates in this layer between a pair of adjacentparallel channels6. Upon reaching theopposite surface12 oflayer4,ray62 reflects fromreflective layer20 by means of a specular reflection and further reflects fromwall7 ofchannel6 by means of TIR. As a matter of optics, the reflection angle for both specular reflection and TIR is the same as the angle of incidence. Sincewall7 is perpendicular to the reflective surface oflayer20,ray62 is further propagated parallel to the original propagation direction after the two consecutive reflections fromreflective layer20 andwall7 and forms retroreflectedray64 upon the exit fromsurface10 which propagates back toward the source. Anemergence angle42 that retroreflectedray64 forms with a normal to surface10 is equal to theincidence angle32 ofincident ray62 and acomplementary angle44 is thus equal toangle34. Accordingly, the retroreflective structure ofsheet2 shown inFIG. 4 retroreflects light at least in a plane which is perpendicular to the longitudinal axis oflinear channels6.
• It is noted that the light path ofrays62 and64 ofFIG. 4 is reversible. In other words, depending on the point of incidence intosurface10,ray62 may undergo a first reflection fromwall7 of therespective channel6 and then undergo a second reflection fromreflective layer20. This is illustrated inFIG. 5 which shows that theretroreflected ray64 may be directed back toward the source regardless of the point of entry ontosheet2.
• Referring further toFIG. 4 andFIG. 5, the parallel array oflinear channels6 oflayer4 and mirroredsurface24 oflayer20 form a linear array of long and narrow retroreflective cells each shaped as a rectangular prism with a right-angle reflection geometry. More specifically, an individual retroreflective cell is defined by a pair ofadjacent walls7 and8 together with the respective portion of mirroredsurface24 disposed between such walls.
• Referring toFIG. 5, the spacing ofchannels6 may be characterized by a distance L between adjacent channels in a plane perpendicular to the longitudinal axis of the channels. A depth D ofchannels6 may be defined as the distance from a base to a tip of the respective channel.
• According to one embodiment, it is preferred that the L/D ratio is generally greater than 1 and less than 2. In other words, the distance L between adjacent channels should be greater than the depth D of such channels but less than twice the depth D. More specifically, the maximum practical L/D ratio may also be defined based on the refractive index of the material oflayer4 in whichchannels6 are formed. In one embodiment, it may be defined as follows:
• $\frac{L}{D}<\frac{2}{\sqrt{n^2-1}},$
• where n is the refractive index of the material oflayer4. For example, if the refractive index of the material oflayer4 is 1.51, the L/D ratio should be at most around 1.8. In one embodiment, the L/D ratio should be approximately (within a 20% error) equal to 1.5 (the distance between adjacent channels being greater than the channel depth by a factor of at least 1.2 and at most 1.8).
• It may be appreciated that the combination of planar mirroredsurface24 andTIR walls7 and8 ensures retroreflectivity only in one angular dimension which is perpendicular to the common longitudinal axis ofparallel channels6. It may further be appreciated that, in an orthogonal angular dimension, such structure may have substantially no retroreflective properties. Accordingly, a light ray propagating in a plane which is parallel to the common axis oflinear channels6 may simply be reflected bysheet2 without redirecting back to the source.
• This is illustrated inFIG. 6 which shows further aspects on light redirection operation ofsheet2 whichlayer20 has a planar, specularly reflective surface. Referring toFIG. 6,ray62 enterssurface10 ofsheet2 from an off-normal direction and makesincidence angle32 with respect to normal160 at the point of entry. A plane of incidence ofray62 is defined as the plane that encompasses the propagation vector ofray62 upon the incidence ontosurface10 and surface normal160. Areference line72 indicates the projection ofincident ray62 ontosurface10.Ray62 makesangle34 with the prevailing plane ofsurface10 and withreference line72.Ray34 is complementary toincidence angle32.
• Aprimary retroreflection plane800 is defined as the plane which is perpendicular to reference line400 (perpendicular to linear channels6). Since light rays entering ontosheet2 inplane800 may be retroreflected exactly towards the source, it may be preferred that the orientation ofsheet2 is such that the actual incidence plane and the preferred incidence plane coincide or otherwise parallel to each other. Areference line70 indicates an imaginary line at whichplane800 andsheet2 intersect.
• Retroreflected ray64 makesemergence angle42 with respect to surface normal160 andforms angle44 with respect tosurface10. Areference line74 indicates the projection ofray64 ontosurface10 so thatangle44 is also the angle betweenray64 andreference line74.
• In the illustrated example, the incidence plane does not coincide withprimary retroreflection plane800 and make anangle36 with respect toplane800. Accordingly, whileretroreflected ray64 makes the same angle with respect to normal160 as the incident ray62 (the emergence angle equals to the angle of incidence) it is not pointed exactly back towards the source as it emerges at a different azimuth angle (turning angle with respect to normal160). As it will be understood fromFIG. 6, the plane of emergence ofray64 andplane800 make anangle46 with respect to each other. The total azimuthal turn angle ofretroreflected ray64 with respect toray62 is the sum ofangles36 and46.
• It can be shown that at the planar geometry of the reflective surface oflayer20,angle46 will be equal toangle36. Accordingly, the deviation ofray64 from the direction to the light source will be twice themisalignment angle36 ofsheet2 from the “ideal” orientation in whichlinear channels6 would be perpendicular to the source direction. In other words,retroreflective sheet2 employing a planar mirrored surface inlayer20 can be configured to retroreflect light in a first angular dimension, such as an elevation angle, and reflect like a mirror in a second angular dimension, such as an azimuth angle, which is orthogonal to the first angular dimension. The respective azimuth/elevation angular coordinate system may be defined, for example, as follows: the coordinate system origin is located at the point of entry ofray62 ontosurface10, the azimuth angle is a ±180° rotational angle with respect to normal160 with a zero-point in the plane of incidence, and the elevation angle is an 0°-90° angle with respect to the plane ofsurface10.
• While retroreflecting light in a single angular dimension (such as an elevation angle) may find utility in many applications, at least some application will benefit from a full retoreflectivity in both angular dimensions (e.g., elevation and azimuth) so that the retroreflected rays could be directed back towards the source even whensheet2 is not oriented with itschannels6 being perpendicular to the source direction.
• An embodiment ofsheet2 configured to retroreflect light towards the source in a broad range of orientations of the sheet with respect to the source direction in both orthogonal angular dimensions is shown inFIG. 7.
• Referring toFIG. 7,reflective layer20 ofsheet2 is formed from an optically transmissive material and includes a parallel array of linearprismatic elements50 formed insurface22. Eachprismatic element50 includes two planar faces which are perpendicular to each other and form dihedral angles of approximately 45° with respect to the prevailing plane ofsurface22. In a cross section, eachprismatic element50 has an isosceles right-angle triangular shape in which the sides corresponding to the planar faces intersect at about 90° and each make an angle of 45° with respect to the base of the triangle. Each of the linearprismatic elements50 extends perpendicularly tolinear channels6. Sincechannels6 andprismatic elements50 are formed in different layers ofsheet2, the respective perpendicular arrays ofchannels6 andelements50 are stacked on top of one another and do not physically intersect.
• It is generally preferred that the height ofprismatic elements50 is considerably less than the depth ofchannels6. In one embodiment, it is preferred that the height ofprismatic elements50 is less than the depth ofchannels6 by at least a factor of two, more preferred by a factor of five or more, and even more preferred by a factor of ten or more. One advantage of such configuration may be that the effective depth of the retroreflective cells formed by pairs ofadjacent channels6 can be kept at approximately constant depth. Another advantage may be that by makingprismatic elements50 considerably smaller, possible light leakage into an adjacent retroreflective cell can be reduced or virtually eliminated. It may also be preferred that the thickness oflayer20 is substantially less than the thickness oflayer4 and, even more preferably, considerably less than the depth ofchannels6. In one embodiment, the height ofprismatic elements50 is between 20 and 100 micrometers and the depth ofchannels6 is at least 200 micrometers.
• Suitable materials for suchmicrostructured layer20 may include but are not limited to glass, acrylic (PMMA), polycarbonate, polyurethane, polyester, certain optically clear epoxies, and the like.Prismatic elements50 may be formed from the same or a different material than the main body oflayer20. By way of example and not limitation, miniatureprismatic elements50 can be formed from polycarbonate or PMMA on a polyester film substrate.
• Layer20 may conventionally have the same or similar structure as optical lighting films utilized to transport light, as illustrated in U.S. Pat. No. 4,260,220 to Whitehead, herein incorporated by reference in its entirety. Such films are made of flexible polymeric sheets of a transparent material having a structured prismatic surface on one side and a smooth surface on the opposite side. The structured surface includes a linear array of miniature substantially right angled isosceles prisms which are arranged side-by-side to form a plurality of peaks and grooves. The perpendicular sides of the prisms make an angle of approximately 45 degrees with the smooth surface. The structure of such films enables light entering the smooth side to be reflected by the structured side as long as the angle by which the light rays deviate from the longitudinal axis of the microprisms does not exceed a certain maximum angle, which depends upon the refractive index of the film material.
• In a further non-limiting example,layer20 may comprise Brightness
• Enhancement Film (“BEF”) or Optical Lighting Film (“OLF”) available from 3M Corporation, both of which having linear, right-angle prismatic surface structures.
• It will be appreciated by those skilled in the art that the right-angleprismatic structures50 can makesurface22 reflective for rays entering onto thesmooth surface24 oflayer20 in a broad range of propagation angles. In particular, such prismatic structures can reflect light by means of TIR from the respective planar faces when the incidence angle is within a certain acceptance angle from a surface normal. The acceptance angle of such reflection varies depending on the orientation of the incidence ray with respect to the surface plane and to the longitudinal axis ofprismatic elements50.
• An advantage of employingprismatic elements50 is that, in addition to providing reflective properties forsurface22, each of such elements retroreflects light in a second plane which is perpendicular to its longitudinal axis thus complementing the single-plane retroreflection of thetop layer4.
• In operation, referring further toFIG. 7,channels6 andprismatic elements50 work cooperatively toretroreflect incident ray62 back towards the source, as indicated by the propagation path ofretroreflected ray64 which emerges parallel or near parallel to the incident ray. It is noted that such “full-angle” or “dual-axis” retroreflection occurs despite the incidence plane ofray62 is not parallel to eitherchannels6 orprismatic elements50. In this case,angle36 characterizing the azimuth deviation ofincident ray62 fromprimary retroreflection plane800 is still equal toangle46 that characterizes the respective azimuth deviation ofretroreflected ray64. However, unlike the structure ofFIG. 6, theretroreflected ray64 ofFIG. 7 is deflected into the same hemisphere with respect to plane800 as theincident ray62. Accordingly, with the elevation angle ofretroreflected ray64 being the same as that of incident ray62 (angles that are complementary to theincidence angle32 and theemergence angle42, respectively) and the azimuth turn angle ofretroreflected ray64 with respect to normal160 and the source direction being also the same as that ofray62, the incident light undergoes a full bidirectional retroreflection insheet2 and is directed back towards the source.
• The operating principle of retroreflecting light bysheet2 in the azimuthal angular dimension will now be further explained.FIG. 8 schematically shows a transversal cross section of individualprismatic element50 in a plane perpendicular to the longitudinal axis of such prismatic element.Prismatic element50 includes a firstplanar face14 and an adjacent secondplanar face16 which is perpendicular to face14. Light rays are respectively shown in a projection onto a plane of the transversal cross section ofprismatic element50. It will be appreciated that TIR is virtually lossless provided thefaces14 and16 are sufficiently smooth. Accordingly, those light rays entering the light receiving aperture ofprismatic element50 for which the TIR condition is met at both faces14 and16 can be losslessly reflected by the respective faces and reversed into retroreflected rays propagating parallel to the incident rays in such projection. As it is further illustrated inFIG. 8, various light rays incident at different angles within a certain acceptance angle are retroreflected towards the respective incident directions
• InFIG. 9, showing a portion of prismaticreflective layer20, multiple linearprismatic elements50 are disposed adjacently to each other and form a continuouscorrugated surface22 which has retroreflective properties at least in a plane perpendicular to the longitudinal axis of the prismatic corrugations.
• Turning now back toFIG. 7, the bi-directional retroreflectivity at off-normal incidence angles is achieved due to the triple reflection of light rays insheet2 at right-angle reflection geometry: one reflection from a wall ofchannel6 and two more reflections from the planar perpendicular faces ofprismatic elements50. Each of the reflection occurs by means of lossless TIR which ensures very high overall reflection efficiency.
• Sincewalls7 and8 are perpendicular to each other and tochannels6,incident ray62 undergoes three consecutive reflections from mutually orthogonal TIR surfaces insheet2 and is therefore retroreflected in a fashion which is somewhat similar to that of a truncated-cube or full-cube corner retroreflector. However, it should be understood that the retroreflector structure and the orientation of TIR surfaces insheet2 are different from the traditional cube corner retroreflectors and configured in favor of the relatively high off-axis incidence angles. It may be appreciated that such configuration ofsheet2 virtually eliminates the “dead” areas for such incidence angles.
• Referring toFIG. 10, the angle by which any ray deviates from normal160 in a cross-section perpendicular to the longitudinal axis ofprismatic elements50 must be less than a predetermined maximum angle θ_maxto prevent light incidence onto the prismatic faces at angles that are greater than the critical TIR angle. Accordingly, a normal-incidence light ray142 and off-normal rays144 and146 propagating inlayer20 at angles below θ_maxwith respect to normal160 are retroreflected while an extreme off-normal ray148 exits fromsurface22 after reflecting from one of thefaces16 and striking an opposingface14 at a below-TIR incidence angle.
• It may be appreciated that the least favorable conditions for TIR operation ofsurface22 is when the incidence plane of an incident light ray is perpendicular to the longitudinal axis of linear prismatic elements50 (perpendicular toreference line70 inFIG. 7). At such incidence andprismatic elements50 having right-angle isosceles configuration, it can be shown that the maximum angle θ_maxcan be found from the following relationship: θ_max=45°−φ_TIR, where φ_TIRis a critical angle of (TIR) that characterizes an optical interface between the material oflayer20 and the outside medium. The critical TIR angle θ_TIRis defined by the refractive properties of the material and may be found from the following expression: φ_TIR=arcsin (n₂/n₁·sin 90°)=arcsin (n₂/n₁), where n₁and n₂are the refractive indices of the material oflayer20 and the outside medium, respectively. In an exemplary case of the interface between PMMA with the refractive index n₁of about 1.49 and air with n₂of about 1, φ_TIRis approximately equal to 42°.
• Accordingly, if the surrounding medium is air, then
• ${\theta }_{\max }=45°-\arcsin \left(\frac{1}{n_1}\right),$
• which gives approximately 3° in case of acrylic (PMMA) material and about 6° for polycarbonate (PC). It can be shown that the 3° and about 6° propagation angles withinlayer20 correspond to the outside incidence angles of approximately 4° and 9° for PMMA and PC, respectively. Obviously, θ_maxwill be higher for a higher refractive index of the material oflayer20.
• As a matter of optics, light rays may deviate from the surface normal by any angle in a plane that is parallel to the longitudinal axis ofprismatic elements50 with no adverse effect on TIR operation. Moreover, θ_maxincreases with the increase of the incidence angle in this plane. Accordingly, the acceptance angle ofsheet2 having prismaticbottom layer20 can be significantly increased by operatingsheet2 at relatively high incidence angles and especially at grazing incidence of light ontosurface10.
• In determining the angular limits of retroreflective operation of the retroreflective sheet material of the present invention, the path of aray entering layer20 viasurface24 can be considered. InFIG. 11, showing a planar portion oflayer20 having corrugatedprismatic surface22 and opposingsmooth surface24, anincident ray152 makes anangle182 withsurface24 in a plane perpendicular to the prevailing plane oflayer20.Angle182 is also an angle betweenray152 and aray projection206 ontosurface24. In conjunction withray152 andsurface24, anangle184 can be defined as the angle betweenprojection206 ofray152 andlongitudinal axis70 of linearprismatic elements50 in the plane ofsurface24. Accordingly, anacceptance angle186 may be defined as the angle of the cone which limits the angular deviation of incident rays fromaxis70 for which the requirement of light reflection bylayer20 can still be satisfied. In other words,acceptance angle186 is defined by theuttermost ray paths202 and204 in the plane ofsurface24.
• When no TIR reflection bysurface22 occurs, the incident light received bysurface24 will be transmitted bylayer20. Whensurface24 is contacting with air or vacuum and n₁is the refractive index oflayer20, it can be shown thatacceptance angle186 can be found from the following expression:
• $2{\cos }^{-1}\sqrt{1-\left({\mathrm{<figure-callout\; label="n"\; filenames="US20210149094A1-20210520-D00016.png,US20210149094A1-20210520-D00019.png"\; state="\{\{state\}\}">n</figure-callout>}}_1^2-1\right)\frac{2-\sqrt{2}}{2+\sqrt{2}}}.$
• Forlayer20 layer made from acrylic (n₁=1.49),angle186 is about 54.5° (±27.2° half-angle). For polycarbonate (n₁=1.58),angle186 is about 60° (±30° half-angle).
• Whilesuch acceptance angle186 may suit many applications, it may nevertheless be increased even further, for example, by providing a higher refractive index material forlayer20 or by mirroringprismatic elements50. In one embodiment, at least a portion of themicroprosmatic surface22 oflayer20 may be mirrored with a reflective coating so that faces14 and16 will remain reflective even when the TIR condition is not met.Prismatic elements50 may be metalized, for example, with an aluminum or silver layer using a vacuum deposition method.
• Sheet2 can be attached to any suitable substrate which can have any suitable thickness, including film thicknesses. Suitable substrate materials include but are not limited to wood, glass, fabric, metal or plastic sheeting, plastic films, and various composite laminates. An adhesive layer also can be disposed behind thebottom layer20 to securesheet2 to a substrate. Whenlayer20 includes non-metalized microprismatic corrugations, such asprismatic elements50, creating air pockets between prism surfaces and the adhesive/substrate may be required to maintain TIR reflectivity. To achieve this, an intermediate support film or a mesh material may also be provided betweenlayer20 and the adhesive/substrate. The support film may also be permanently attached to layer20 by ultrasonic welding in a plurality of locations distributed oversurface22. Since welding may melt, destroy or distort microprismatic surface corrugations and suppress TIR, the cumulative area of the weld seams should be relatively small compared to the area oflayer20 so that the overall retroreflective operation ofsheet2 is not significantly impaired.
• The retroreflective material of this invention can be made in the form of large format sheets or a continuous roll and then cut to any suitable dimensions or shape. The dimensions ofretroreflective sheet2 may vary in a broad range. Particularly, the retroreflective material ofsheet2 can be cut into strips of any suitable length, width and aspect ratio. Depending on the application, the longer and shorter dimensions of such strips can be oriented relatively tochannels6 andprismatic elements50 in any suitable way. InFIG. 12,sheet2 is shown shaped in the form of a strip in which linearprismatic elements50 extend perpendicular to a longitudinal axis of the strip. InFIG. 13, linearprismatic elements50 are aligned parallel to such axis.
• Such retroreflective strips can be used, for example, in high-conspicuity pavement or road structure markers. Particularly, a suitably shaped and dimensioned retroreflective strip can be adhered to or embedded into various roadside structures to enhance the conspicuity and visibility of the structures to the drivers of approaching cars. This is illustrated inFIG. 14 which depicts a strip ofsheet2 attached to aguard rail540 extending along aroad486. Acar450 approaching the strip location illuminatessheet2 by itsheadlights228 athigh incidence angle32 with respect to normal160.
• Sheet2 receives beams oflight462 and464 from the left and right headlamps, respectively, and retroreflects each of the beams back towards the car with high efficiency and within a narrow cone including anarea252 where the car driver is located. Sincesheet2 can be particularly configured for high incidence angles, it can provide fairly high light output in the retroreflective operation despite the light receiving surface of the sheet being near parallel to the incident light beam. Accordingly, the illuminated strip ofsheet2 will appear bright to the car driver and the conspicuity and visibility of theguard rail540 will thus be enhanced.
• FIG. 15 shows an example of incorporating retroreflective strips ofsheet2 material into the side trim of atruck470. Such strips can be configured to retroreflect light back toward the source at high incidence angles, particularly including angles above 40° or so, and to appear bright to the drivers of passing cars who would normally view the sides of the truck at relatively sharp angles.
• The retroreflective material of the present invention can also be used in pavement markers, road signs, emergence vehicle markers, road worker clothing marking, etc.Sheet2 can be made flexible so that it could be wrapped around objects. In a more particular example, referring toFIG. 16, a strip of the retroreflective material ofsheet2 can be applied to a plastic or rubber road sign cone which is on roads under repair. In a further example shown inFIG. 17,sheet2 is wrapped around a road marking pole while having the same basic structure and operation. Similarly, retroreflective strips ofsheet2 material can be attached to road-side poles or tree trunks to provide visually conspicuous marking of such objects.
• When a conventional low-incidence-angle retroreflective material is used to mark a round object, only the central area of the wrapped-around strip is usually visible in a high brightness, while the peripheral left and right areas may appear dim due to the reduced retroreflectivity at high incidence angles. Accordingly, such conventional marking does not emphasize the dimensions of the round road-side structure. In contrast, the material ofsheet2 may be configured to maximize the retroreflectivity at high incidence angles, such as, for example, 60 to 85 degrees. Accordingly, whensheet2 is used to mark a round object, the opposing sides of the visible strip area will appear in increased brightness, thus providing dimensional information on the object to the driver of an approaching vehicle. It is noted thatsheet2 configured for superior high-incidence-angle retroreflection will still provide at least some retroreflectivity even at relatively low incidence angles since at least a portion of light striking the entrance aperture of the sheet will be reflected not only byreflective layer20 but also bychannels6. Therefore, the entire visible area ofsheet2 wrapped around an object may appear sufficiently bright to a car driver.
• The retroreflective sheeting of the present invention may be adapted to disperse the retroreflected light within a predetermined range of directions in one or more angular dimensions. This may be useful, for example, to include the eyes of a car driver into the retroreflected light beam, considering that the light-emitting headlights and the driver's eyes are typically not on the same line of sight when viewed from the surface ofsheet2.
• This may be accomplished by different means. In one embodiment, one of the layers or materials ofsheet2 may be provided with weak light scattering properties which would cause an increase in the divergence of the retroreflected light. In one embodiment, certain angular bias may be incorporated into various optical interfaces ofsheet2. For example, an angular bias may be added towalls7 and/or8 in the form of providing random slopes to such walls with respect to a normal to the surface ofsheet2 within a small predefined angular range (0.1°−2°). In another example, such angular bias may be implemented in the form of some surface waviness ofwalls7 and8.
• A further alternative of increasing light output towards off-axis directions (at higher observation angles) may include providing controlled surface structure or haze for either one ofsurfaces10,12,24 and22 or any combination thereof. Such light-diffusing surface structure or haze can be introduced, for example, by chemical etching and/or electroplating of the tool surface used to produce the respective layers ofsheet2. For example,surface10 may be textured with shallow microstructures to cause slight dispersion of the retroreflected beam.
• In one embodiment,channels6 can be made slightly curved in order to introduce controlled divergence into the retroreflected beam in an incidence plane, a plane which is orthogonal to the incidence plane, or both. Furthermore, it may be appreciated that the natural surface roughness ofwalls7 and8 resulting from the process of makingchannels6 by means of material slitting using a blade may also result in some angular divergence of retroreflected rays which can be advantageously exploited to increase light output into directions other than the source direction.
• Referring toFIG. 18,sheet2retroreflects incident ray62 in the form of a divergent beam which is confined within a predeterminedangular range84. Whensheet2 is used for road signs or pavement marking, the angle of such divergent beam may be selected to advantageously encompass the angular size of the approaching vehicles in a range of useful distances from the sheet so that the drivers will be able to see the respective signs or markings in increased brightness.
• Sheet2 may also be configured to retroreflect light with a predetermined angular offset with respect to the incident light direction. This is illustrated inFIG. 19 in which retroreflectedray64 makes anangle100 with respect toincident ray62. In one embodiment,angle100 can be made greater than anangle102 of the divergence cone of the retroreflected light, as shown inFIG. 19.
• The angular shift of the retroreflected beam (as defined by angle100) can be enabled using different approaches. One approach is to makewalls7 and8 ofchannels6 slightly tilted with respect to a normal by a predefined small angle. It can be shown that the angle at whichchannels6 should be tilted with respect to the “ideal” normal orientation to produce a desiredangle100 should be one half ofangle100. For example, in order to shift the retroreflected beam form the source direction by 1°,channels6 should be tilted at a 0.5° angle with respect to a normal tosheet2.
• An angular shift of the retroreflected beam in an orthogonal angular dimension may be enabled by appropriately designingprismatic elements50.
• In one embodiment, anangle254 betweenfaces14 and16 can be made slightly lower or slightly greater than 90° to steer the retroreflected light away from the original propagation direction in a controlled fashion. This is illustrated inFIG. 20 which showsangle254 being not exactly 90 degrees which causes the retroreflected ray to deviate from the direction of the respective incident ray.
• In one embodiment,prismatic elements50 can be canted to either side by a predetermined angle so that theirfaces14 and16 will make differentdihedral angles256 and258 to a prevailing plane or surface ofsheet2, which should also cause controlled deviation of the retroreflected rays from the source direction, as illustrated inFIG. 21.
• Further modifications of the structure ofsheet2 may include selecting angular and/or dimensional parameters of the plurality ofchannels6 orprismatic elements50 so as to provide various dynamic visual effects and further enhance the conspicuity of the retroreflective sheeting. For example, in one embodiment, the L/D ratio characterizing the array ofchannels6 may be selected to provide variable degree of retroreflectivity (and, hence, variable apparent brightness) as the observation angle ofsheet2 by a distant viewer changes. In an alternative embodiment, thedihedral angles256 and258 offaces14 and16 and/orangles254 may be varied according to a predetermined pattern. For example, one or both of thedihedral angles256 and268 can be varied across thesurface22 in predetermined increments which can cause retroreflection of the incident light beam into a plurality of distinct directions. As the geometry of retroreflection changes for a moving observer and/or a light source (such as is the case of a car or truck driver and headlights moving with respect to a road sign or marking), the observer may seesheet2 in variable brightness.
• It is noted that the retroreflective sheet material described in the foregoing embodiments may be configured to return almost 100% of the incident light back towards the source. This is illustrated inFIG. 22 which shows an individual retroreflective cell formed by a pair ofadjacent channels6 and a portion of reflectivebottom layer20 underneath. A plurality of high-incidence-angle light rays are distributed over an area ofsurface10 which is roughly equivalent to the entrance aperture of the retroreflective cell formed by theadjacent channels6 of thetop layer4 and thereflective surface24 of thebottom layer20.
• The illustrated example corresponds to an L/D ratio of about 1.5, a refractive index oflayer4 of about 1.51 and an incidence angle of approximately 65° (25° angle with respect to surface10). As it can be seen, at such an incidence angle,sheet2 orientation with respect to a light source, andchannels6 depth and spacing, each of the light rays incident ontosurface10 ofsheet2 is retroreflected and returned toward the source direction. Since light rays ofFIG. 22 cover practically the entire light receiving aperture of the retroreflective cell, substantially all of the light incident onto such aperture will be retroreflected.
• It will be appreciated that, in practice, the actual retroreflection efficiency will be lower than 100% due to a number of factors, such as the Fresnel reflection fromsurface10, light absorption or scattering in the transmissive materials ofsheet2 and manufacturing imperfections. Iflayer20 comprises a specular material, the reflectivity of such material will also be factored into the resulting light output efficiency. Accordingly, as a practical consideration, it may be possible to design the retroreflective sheeting of the present invention to return above 90% of light toward the source at least at optimal incidence angles. In one embodiment,sheet2 may be configured to retroreflect more than 60% of the incident light at an optimal incidence angle or within a range of incidence angles approximating the optimal incidence angle. In one embodiment,sheet2 may be configured to retroreflect at least 75% of the incident light.
• It is noted that, while the retroreflective sheeting of the present invention is primarily designed to provide enhanced retroreflectivity at high incidence angles and specific orientations with respect to the light source, it is also operable to provide retrorectivity at lower incidence angles and other orientations. For example, it can be shown that the structure ofsheet2 will reflect essentially all of the normal-incidence light back towards the source. It can also be shown that, at relatively low incidence angles, theretroreflective sheet2 structure ofFIG. 7 will retroreflect at least a portion of the incident light.
• The retroreflection efficiency offsheet2 at lower incidence angles may be increased by lowering the L/D ratio. Accordingly, by varying the L/D ratio, the structure of retroreflective sheeting may be optimized for a specific angular range.
• FIG. 23 illustrates an embodiment ofretroreflective sheeting2 which is similar to that shown inFIG. 22 but which further includes a prismatic layer on top oflayer4. Such prismatic layer can be formed by a plurality ofprismatic elements300 distributed over the surface ofsheet2. An individualprismatic element300 is illustrated inFIG. 23. Suchprismatic element300 is formed by an asymmetric triangular prism defined byfaces302,304 and306 and is made from an optically transmissive polymeric material.
• Face302 is configured for receiving light from directions that represent extremely high incidence angles, including the case when the incident rays propagate parallel or near parallel to the prevailing plane ofsheet2. In operation, face302 intercepts such extremely high incidence angle rays and refract them light into the body oflayer4. As a matter of optics, a sloped orientation offace302 reduces Fresnel reflections fromsheet2 compared to the case whensheet2 has a planar outer surface and directs the received light rays at angles more favorable for high-efficiency retroreflection. For the described operation,sheet2 may need to be oriented such that faces302 of the respectiveprismatic elements300 are facing the expected light source direction. Possible variations ofprismatic elements300 may include various base angles (dihedral angles that said faces form withsurface10 or the prevailing plane of sheet2) offaces302 and304 as well as symmetrical configurations in which faces302 and304 form the same base angle and can receive high-incidence-angle light on either one or both faces.
• The top prismatic layer includingprismatic elements300 may be formed by a sheet-form prism array molded or extruded on a flexible film substrate or directly onsurface10. Alternatively,prismatic elements300 may be formed directly in the material oflayer4.
• Sheet2 may be configured to include areas in which portions of retroreflective sheeting are disposed in different orientation with respect to each other. This may be useful, for example, for providing an omnidirectional retroreflective device.
• One embodiment of omnidirectional retroreflective sheeting is illustrated inFIG. 24 in which acircular sheet2 includes eightidentical segments1002 being a rotated copy of an adjacent segment. Eachsegment1002 haschannels6 aligned perpendicular to a radius of the circle (reference lines70 of the respective segments are aligned along the radius) covering the entire 360° angular range of the circle. It may be appreciated that, when each segment is configured for an azimuth acceptance angle of about 30° and elevation acceptance angle of 0°-90°, at least one of such segments will always operate at the prescribed acceptance angle range. Accordingly, at least 15% of the area ofsheet2 will efficiently retroreflect light regardless of the incidence angle, thus providing omnidirectional operation.
• Furthermore,sheet2 may also be operated together with conventional retroreflectors, e.g., those based on cube corners, full-cubes or glass beads. In one embodiment,sheet2 may be optimized for high incidence angles and include areas which comprise such other types of retroreflectors designed to efficiently retroreflect light incident at low, normal or near-normal angles. In one embodiment, the retroreflective sheeting of this invention may be alternated with such conventional retroreflective sheeting to provide a full angular coverage (incidence angles from 0 to almost 90°).
• An alternative configuration of omnidirectional retroreflective sheeting is shown inFIG. 25 which further includes acentral area1006 formed by a conventional cube-corner or full-cube retroreflective material configured for high-efficiency retroreflection at low incidence angles. Accordingly, the omnidirectional structure ofFIG. 25 may exhibit fairly high retroreflective efficiency for the entire 0°-90° range of incidence angles.
• FIG. 26 shows an embodiment ofretroreflective sheeting2 in whichlayer4 includes a second array ofparallel channels6 extending generally perpendicular to thechannels6 of the first array. Such perpendicular arrays ofchannels6 may be formed in the same surface oflayer4, as shown inFIG. 27. Alternatively,layer4 may be formed by two or more sub-layers superimposed on one another and the respective arrays ofchannels6 may be formed in those sub-layers.
• It will be appreciated thatsheet2 havinglayer4 with two perpendicular arrays ofchannels6 may be used for retroreflecting light in two orthogonal angular dimensions using smooth-surface specularlyreflective layer20 and without using surface corrugations such as triangularprismatic elements50.
• Referring toFIG. 27, the perpendicular grid ofchannels6 formed by the intersecting arrays essentially forms a plurality of light-channeling cells. Both of the arrays ofchannels6 are shown to be formed insurface10. However, it should be understood that such intersecting channels arrays may also be formed insurface12. Each light-channeling cell will have a shape of rectangular parallelepiped defined by four vertical walls of intersectingchannels6 and a horizontal terminal wall represented by an uncut portion ofsurface10. When suchtop layer4 is coupled to specularly reflectivebottom layer20, as further shown inFIG. 27, and with the appropriate dimensioning ofchannels6, each light channeling cell can have retroreflective properties. In other words, each light channeling cells coupled to thereflective layer20 forms a full-cuberetroreflective cell30.
• The operation of full-cube retroreflective cells30 is illustrated inFIG. 28 for a case wherechannels6 are formed insurface12 andsurface10 is made smooth. Off-normal incident ray62 entering one of thecells30 undergoes two reflections from a pair of adjacent orthogonal faces of the cell, which also formTIR walls7 and/or8, and from a mirrored surface ofreflective layer20 which is perpendicular to both of the orthogonal faces. Accordingly, it can be shown that at such rectangular geometry of reflection,ray62 emerges fromcell30 asretroreflected ray64 and propagates back to toward the source. Accordingly, the structure ofsheet2 ofFIG. 28 represents an alternative full-cube retroreflective sheeting structure which can efficiently retroreflect light, particularly at high angles of incidence (high entrance angles).
• The retroreflective sheeting of this invention can be used for light rejection in whichcase sheet2 may be configured to retroreflect at least a portion of light incident from an unwanted range of directions. Such configuration of retroreflective sheeting may find utility in various daylight control devices such as window blinds or louvers.
• For example, a strip of light directing material having the structure ofsheet2 may be incorporated into horizontal venetian blind structure or into a vane of a horizontal louver system. One or more slats of the horizontal blinds may be formed by a strip-shapedsheet2 where eachchannel6 extends parallel to the longitudinal dimension of the strip. Such horizontal slat may have notches formed at least the ends of the respective strip for passing a support cord through them and can also be operable by a manual or automatic position control mechanism that allows for changing the angle or position of the slat and thus varying the amount of transmitted and/or rejected light.
• An embodiment of a horizontal windowblind structure900 is illustrated inFIG. 29 which shows three venetianblind slats920 adjustably operable bycords912 and914. Each ofslats920 has a slightly curved shape for structural rigidity and includes a light redirecting structure ofretroreflective sheet2 attached to arigid substrate layer928. Depending on the orientation ofslats920, windowblind structure900 may be configured to reject various amounts of sunlight and prevent the associated heat from entering a building interior.Substrate layer928 is optional and may conventionally include materials and structures used in window louvers and blinds, such as for example, natural wood, faux wood, vinyl, sheet metal and the like.
• As shown inFIG. 29, whenslats920 are in a fully open position, alight ray922 exemplifying a beam of direct sunlight strikes one of theslats920 and is redirected back towards the exterior of a building by the retroreflective structure ofsheet2. The retroreflective operation is provided by the reflection ofray922 from one of thechannels6 using TIR and from the bottomreflective layer20.Layer20 can be configured to include a mirrored surface in which case it will reflect light by means of a specular reflection. Alternatively,layer20 can be provided with a microprismatic surface, such as that shown inFIG. 7, in which case it will reflect light by means of TIR from the respective faces of the micro prisms.
• Accordingly, such operation ofblind structure900 may be useful for rejecting at least a portion of the solar beam and thus reducing the heat gain and glare associated with direct sunlight. It is noted that, unlike the conventional window blinds that can only block solar heat by also blocking the view, windowblind structure900 may achieve similar solar heat rejection results while essentially preserving the view. This is illustrated by the unimpeded path of aray924 exemplifying diffuse light incident into the building interior from various outdoor objects.
• At an optimal orientation ofslats920, windowblind structure900 can potentially reject substantially all of the direct daylight and the heat associated with the direct solar beam. It may be appreciated that a relatively small portion of the direct solar beam intercepted byslats920 may still reach the building interior even at an optimal orientation and spacing ofslats920. For example, some light (˜4%) can be reflected through Fresnel reflection from the top surface ofsheet2.
• At non-optimal slat orientations, the fraction of admitted light may increase due to an increased fraction of light which is not intercepted byslats920 orchannels6 in each slat. This can essentially provide means for controlling the light and heat throughput of the blinds by manually adjusting their orientation. In one embodiment, window blind structure may be motorized and optionally provided with an automated daylight control system. Such automated system can be made operable in response to the availability and amount of the direct sunlight at the window and can be configured to open, close or rotateslats920 to various intermediate angular positions for maintaining a prescribed lighting level or maximizing heat rejection.
• In some cases, it may be desired that the admitted portion of direct sunlight is diffused in order to avoid glare or sharp shadows in the interior. Accordingly, in one embodiment, one or more surfaces or optical interfaces ofsheet2 may be provided with light diffusing features, such as surface texture. Forexample surface10 may be patterned with light matte finish to spread light reflected fromslat920 across a wider angular range.
• Whenslats920 are in a fully closed position (FIG. 30), both the direct and diffuse sunlight components are rejected, as shown by the paths ofrays922 and924. It will be appreciated thatslats920 may be adjusted to any intermediate angular orientation between the fully open and fully closed position in which case the amount of rejected and/or admitted light can be controlled for almost any solar elevation. It is further noted that, by adjusting the angular position ofslats920, the angular direction of the admitted daylight may also be varied in a broad range. This can be useful, for example, for illuminating deep portions of the building interior by steering such redirected light farther away from the window while rejecting most of the heat associated with the direct solar beam. The light diffusing finish ofsurface10 may also include various decorative finishes or ornamental patterns. It is noted thatblind structure900 may be configured so thatslats920 can be rotated almost 360 angular degrees and either retroreflective or the opposing non-retroreflective surfaces of the slats may be exposed to the incident sunlight. When the non-retroreflective sides ofslats920 are exposed to the incident sunlight by facing towards the outside of the building, windowblind structure900 may operate simply as a shading device, similarly to the conventional venetian blinds.
• The appearance ofsheet2 may be configured in a number of ways. For instance, a pigment may be added to one or more of its materials thus altering the color or transparency. In applications requiring improved conspicuity,sheet2 may be provided with a visually conspicuous color or tint. To provide a certain color tosheet2, a dye or pigment may be incorporated in the resin used to formlayer4,layer20 or any other auxiliary layer or coating that can be added tosheet2. Also,sheet2 may include any suitable image, pattern or print which can be formed onsurface10 or embedded into the structure of the sheet. Such print may be produced, for example, using thermal transfer printing, screen printing, ink jet printing, and the like.
EXAMPLE 1
• A retroreflective sheet-form structure as schematically illustrated inFIG. 7 was made by attaching a top optically transmissive layer formed by a 1-millimeter-thick sheet of optically clear thermoplastic polyurethane (TPU) to a bottom reflective layer formed by a same-sized sheet of brightness enhancementfilm BEF II 90/24 commercially available from 3M Corporation and having a thickness of about 140 μm (5.5 mils). TheBEF II 90/24 film product has a microprismatic surface structure formed by an array of right-angle isosceles microprisms having a linear configuration and prism pitch of about 24 μm (0.9 mils).
• An array of parallel TIR micro-channels was formed in the top layer by slitting a surface of the TPU sheet material to a cutting depth of approximately 500 μm using a pack of sharp rotary blades commercially available from KAI Corporation. The micro-channels were formed at a constant spacing of about 600 μm so that the L/D ratio characterizing the channels was about 1.2. In laboratory measurements of the surface roughness of the TIR walls of the micro-channels have indicated that the root-mean-square (RMS) roughness parameter ofsuch walls7 and8 was between 0.01 micrometers (10 nanometers) and 0.03 micrometers (30 nanometers).
• The top and bottom layers were oriented relatively to each other such that the channels of the top layer were aligned perpendicular to the linear microprisms of the bottom layer, the cut surface of the top layer was facing the bottom layer, and the microprisms of the bottom layer were facing away from the formed retroreflective sheet structure.
• The formed retroreflective sheet structure was then tested during night time in lights of an approaching vehicle. In a first type of tests, the retroreflective structure was attached to rigid board and disposed along a roadside in various orientations with respect to a direction to the approaching vehicle to simulate various types of road markings and their observation conditions. In a second type of tests, the retroreflective sheet structure was wrapped around a cylindrical pole to simulate retroreflective roadside markings of round objects.
• In such tests, the retroreflective sheeting had a very high apparent brightness (when viewed by a driver of the approaching vehicle) in a broad range of incidence (entrance) and observation angles. When compared side-by-side to various types of commercially available retroreflective sheeting, the new retroreflective structure had a markedly brighter appearance than any of the other tested sheeting (which included ASTM retroreflective sheeting of Types I through XI) at least at entrance angles exceeding 30-40° and also exceeded in brightness most of the tested products at lower entrance angles.
EXAMPLE 2
• A retroreflective sheet-form structure as was made as described in EXAMPLE 1 above except that the top layer was made from optically clear plasticized polyvinyl chloride (flexible polyvinyl film). The tests were conducted as described in EXAMPLE 1 above and have shown similar results.
EXAMPLE 3
• A retroreflective sheet-form structure as schematically illustrated inFIG. 3 was made by a top optically transmissive layer formed by a 0.5-millimeter-thick sheet of optically clear thermoplastic polyurethane (TPU) and attaching it to a bottom reflective layer formed by a metalized polyethylene terephthalate (Mylar®) film. The top layer included an array of parallel TIR micro-channels formed as described in EXAMPLE 1 above and spaced/dimensioned at 1.4 L/D ratio.
• The bottom layer was provided with a high-tack adhesive that was used to attach the retroreflective sheet-form structure to a thin and rigid sheet-form substrate. A strip of approximately 4 centimeters by 50 centimeters of the resulting sheet-form retroreflective structure was cut out such that the longitudinal axis of the strip was parallel to the longitudinal axis of the micro-channels formed in the top layer. The resulting strip was tested by exposing it to a parallel beam of light and measuring its reflectance at different incidence angles and orientations. The tests have shown a strong, angularly-selective retroreflection in a plane perpendicular to the longitudinal axis of the strip, particularly at angles exceeding 40-45°.
EXAMPLE 4
• A retroreflective sheet-form structure as schematically illustrated inFIG. 27 was made by a top layer of clear and flexible PVC attached to a similarly-sized sheet of Mylar® material. The top layer had a thickness of about 0.8 mm and included two parallel arrays of micro-channels disposed perpendicular to each other. The perpendicular arrays of micro-channels were formed by slitting a surface of the top layer along the respective perpendicular directions using a tightly packed assembly of rotary blades. The channel depth D was about 450 μm and the channel spacing was about 550 μm for both arrays. In laboratory tests, the resulting sheeting had high retroreflection efficiency for a broad range of off-axis incidence angles.
• Further details of the structure and operation of retroreflective sheeting of the invention, as shown in the drawing figures, as well as their possible variations 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. A high incidence angle retroreflective article, comprising:

a flexible sheet-form structure having a thickness between 100 micrometers and 2 millimeters and comprising a front sheet of an optically transmissive material, which has a front surface and a parallel back surface, and a back sheet of an optically transmissive material bonded to the front sheet and having a front surface facing the back surface of the front sheet and a parallel back surface facing away from the front sheet;

a plurality of parallel channels formed in the front sheet and each having a pair of opposing parallel walls extending perpendicular to a plane of the front sheet; and

a mirror coating formed on the back surface of the back sheet,

wherein at least one of the opposing parallel walls is configured to reflect light using a total internal reflection and has a root mean square surface profile roughness parameter of at least 10 nanometers and equal to or less than 60 nanometers at a sampling length between 20 and 100 micrometers.

2. A high incidence angle retroreflective article as recited inclaim 1, wherein the thin and flexible sheet-form structure is configured to retroreflect an incident off-axis light ray in a first plane that is perpendicular to the plurality of parallel channels and reflect said light ray without retroreflection in a second plane that is orthogonal to the first plane.

3. A high incidence angle retroreflective article as recited inclaim 1, further comprising a prism layer located in front of the front sheet, wherein the prism layer comprises a plurality of asymmetric triangular prisms each having a first sloped face and a second sloped face forming a different dihedral angle with respect to the plane of the front sheet than the first sloped face.

4. A high incidence angle retroreflective article as recited inclaim 3, wherein the prism layer is formed in a surface of a flexible sheet substrate which is bonded to the front surface of the front sheet.

5. A high incidence angle retroreflective article as recited inclaim 3, wherein the prism layer is formed in the front surface of the front sheet.

6. A high incidence angle retroreflective article as recited inclaim 3, wherein the thin and flexible sheet-form structure is configured to be operatively positioned such that the first sloped face is facing an expected light source direction.

7. A high incidence angle retroreflective article as recited inclaim 1, further comprising a prism layer positioned in front of the front sheet, wherein the prism layer comprises a plurality of symmetrical triangular prisms each having a first sloped face and a second sloped face forming approximately the same dihedral angle with respect to the plane of the front sheet as the first sloped face.

8. A high incidence angle retroreflective article as recited inclaim 1, comprising a plurality of retroreflective areas having different retroreflective properties, wherein the parallel channels within a first one of the plurality of retroreflective areas are disposed in a first orientation and the parallel channels within a second one of the plurality of retroreflective areas are disposed in a second orientation which is different than the first orientation.

9. A high incidence angle retroreflective article as recited inclaim 1, wherein the front surface of the front sheet has a light diffusing texture configured to disperse a parallel or near-parallel beam of light within a predetermined range of directions.

10. A high incidence angle retroreflective article as recited inclaim 1, wherein the optically transmissive material of the front sheet comprises plasticized polyvinyl chloride.

11. A high incidence angle retroreflective article as recited inclaim 1, wherein the parallel channels are configured at a spacing distance L and a depth D which are bounded by a relation

$\frac{L}{D}<\frac{2}{\sqrt{n^2-1}}$

in which n is a refractive index of the optically transmissive material of the front sheet.

12. A high incidence angle retroreflective article as recited inclaim 1, wherein the parallel channels are spaced from each other by a distance that is 1.5 times greater than a depth of the parallel channels and is at least 6 times an average width of the channels.

13. A high incidence angle retroreflective article as recited inclaim 1, wherein a thickness of the front sheet is between 100 micrometers and 1 millimeter.

14. A high incidence angle retroreflective article having a thin and flexible sheet-form structure and comprising:

a front sheet of an optically transmissive material;

a back sheet of an opaque material bonded to the front sheet and having a reflective front surface; and

a plurality of retroreflective areas distributed over an area of the flexible sheet-form structure and having different retroreflective properties from one another,

wherein a first one of the plurality of retroreflective areas has a first plurality of channels formed in the front sheet and each defining a first pair of opposing parallel walls configured to reflect light using a total internal reflection, wherein a second one of the plurality of retroreflective areas has a second plurality of channels formed in the front sheet and each defining a second pair of opposing parallel walls configured to reflect light using a total internal reflection, and wherein an orientation of each of the first plurality of channels within the first one of the plurality of retroreflective areas is different than an orientation of each of the second plurality of channels within the second one of the plurality of retroreflective areas.

15. A high incidence angle retroreflective article as recited inclaim 14, wherein a surface of at least one of the first pair of opposing parallel walls has a root mean square surface profile roughness parameter of at least 10 nanometers and equal to or less than 60 nanometers at a sampling length between 20 and 100 micrometers, and wherein a thickness of the front sheet is between 100 micrometers and 1 millimeter and a thickness of the back sheet is between 100 micrometers and 2 millimeters.

16. A high incidence angle retroreflective article as recited inclaim 14, wherein the parallel channels are configured at a spacing distance L and a depth D which are bounded by a relation

$\frac{L}{D}<\frac{2}{\sqrt{n^2-1}}$

in which n is a refractive index of the optically transmissive material.

17. A high incidence angle retroreflective article as recited inclaim 14, wherein each of the first and second pairs of opposing parallel walls extend perpendicular to a surface of the front sheet.

18. A high incidence angle retroreflective article as recited inclaim 14, further comprising a prism layer formed in a surface of the front sheet, wherein the prism layer comprises a plurality of asymmetric triangular prisms each having a first sloped face and a second sloped face forming a different dihedral angle with respect to a plane of the front sheet than the first sloped face, wherein the thin and flexible sheet-form structure is configured to be operatively positioned such that the first sloped face is facing an expected light source direction.

19. A high incidence angle retroreflective article as recited inclaim 14, wherein a third one of the plurality of retroreflective areas comprises a cube-corner or full-cube retroreflective material.

20. A high incidence angle retroreflective article as recited inclaim 14, wherein a third one of the plurality of retroreflective areas comprises retroreflective beads.

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