US008740397B2
`
`(12) United States Patent
`Vasylyev
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 8,740,397 B2
`Jun. 3, 2014
`
`(54) OPTICAL COVER EMPLOYING
`MCROSTRUCTURED SURFACES
`(75) inventor styletorovich Vayyev Elk Grove
`(US)
`(73) Assignee: SVV Technology Innovations, Inc.,
`Sacramento, CA (US)
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 155 days.
`
`(*) Notice:
`
`(21) Appl. No.: 13/351,800
`(22) Filed:
`Jan. 17, 2012
`
`(65)
`
`Prior Publication Data
`US 2012/0182615A1
`Jul. 19, 2012
`
`Related U.S. Application Data
`(60) Provisional application No. 61/461.522, filed on Jan.
`18, 2011.
`
`(2006.01)
`
`(51) Int. Cl.
`GO2B5/24
`(52) U.S. Cl.
`USPC .......................................................... 359/530
`(58) Field of Classification Search
`CPC ....................................................... GO2B 5A124
`USPC .................................................. 359/529,530
`See application file for complete search history.
`
`
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`5.975,706 A * 1 1/1999 Nakayama .................... 359,530
`6,064,452 A * 5/2000 Ogino ............................. 349/57
`6,274,860 B1
`8/2001 Rosenberg
`6,333,458 B1
`12/2001 Forrest et al.
`6,440,769 B2
`8/2002 Peumans et al.
`7,672,549 B2
`3/2010 Ghosh et al.
`2006.
`R 1938 8. te et al.
`2008/0264483 A1 10, 2008 Keshner et al.
`2009, O126792 A1
`5, 2009 Gruhlike et al.
`2010, 0186798 A1
`7/2010 Tormen et al.
`2010/0278480 A1 1 1/2010 Vasylyev
`2011/0226332 A1
`9, 2011 Ford et al.
`2012/0012741 A1
`1/2012 Vasylyev
`* cited by examiner
`y
`Primary Examiner — Euncha Cherry
`57
`ABSTRACT
`(
`A light trapping optical cover employing an optically trans
`parent layer is described. The transparent layer has at least
`one corrugated Surface formed by a plurality of isosceles
`right-angle prismatic corrugations configured to internally
`retroreflect light into the transparent layer. The corrugated
`surface also includes optical windows configured for input
`p
`9.
`p
`ting or outputting light to or from the transparent layer. The
`optical cover may further employ a focusing array of light
`collectors being pairwise associated with the respective opti
`cal windows.
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`19 Claims, 14 Drawing Sheets
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`US 8,740,397 B2
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`1.
`OPTICAL COVER EMPLOYING
`MCROSTRUCTURED SURFACES
`
`CROSS-REFERENCE TO RELATED
`APPLICATIONS
`
`This application claims priority from U.S. provisional
`application Ser. No. 61/461.522 filed on Jan. 18, 2011, incor
`porated herein by reference in its entirety.
`
`STATEMENT REGARDING FEDERALLY
`SPONSORED RESEARCH ORDEVELOPMENT
`
`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 any
`one 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.S 1.14.
`
`BACKGROUND OF THE INVENTION
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`1. Field of the Invention
`The present invention relates to a device and method for
`enhancing the light trapping in light harvesting devices. Par
`ticularly, the present invention relates to collecting light from
`a large Surface area of the light harvesting device comprising
`a light absorbing material and trapping the light within the
`device so as to increase the optical path through the light
`absorbing material and improve the useful light absorption.
`More particularly, the present invention relates to enhancing
`the light trapping in photovoltaic Solar panels, light detectors,
`day lighting systems, bioreactors, water light-treatment reac
`tors, and the like. The present invention also relates to illumi
`nation devices, particularly to light emitting panels and con
`duits.
`2. Description of Background Art
`Many light harvesting devices employ a light-absorbing
`active layer that has at least apartial transparency with respect
`to the incident light or absorbs more weakly in certain wave
`lengths than in the others. Conventionally, the absorption in
`Such devices can be improved by increasing the thickness of
`the active layer. However, this results in the increased system
`dimensions, material consumption, weight and cost. Alterna
`tively, light trapping approaches are well known in which the
`light path is altered within the device by micro-texturing one
`or more device surfaces. While this allows to somewhat
`increase the light path and thus improve absorption compared
`to a non-textured device, a significant portion of the light still
`escapes from the device without being fully absorbed. It is
`therefore an object of this invention to provide an improved
`optical structure that can be used in conjunction with light
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`harvesting devices and that can provide efficient light trap
`ping with minimal energy loss.
`The present invention solves the above problems by pro
`viding a transparent optical cover structure having one or
`more micro-structured surfaces that allow for trapping the
`incident light within the light harvesting device by means of
`at least TIR and cause the multiple passage of the trapped
`light through the active layer thus improving the light absorb
`tion and device efficiency at the minimum consumption of
`active layer's material.
`Many light emitting devices employing panel-like struc
`tures, such as lighting panels or backlights designed to dis
`tribute light along the Surface of the panel and emit light from
`one or more broad Surfaces of the panel, are limited to con
`ducting light propagating at relatively high TIR angles with
`respect to a normal to the panel Surface. Furthermore, many
`Such light emitting devices employ light extracting features
`that impair the optical transparency of the device or/or
`involve additional fabrication steps or materials, such as
`selective Surface metallization or using, adding reflectors
`which increases system complexity and cost.
`The present invention solves these problems by providing
`a transparent optical cover structure having one or more
`micro-structured Surfaces that allow for light propagation
`along the panel in a greater angular range and provide for an
`efficient light distribution and extraction mechanism.
`Other objects and advantages of this invention will be
`apparent to those skilled in the art from the following disclo
`SUC.
`
`BRIEF SUMMARY OF THE INVENTION
`
`The present invention solves a number of light distribution
`and/or harvesting problems within a compact optical cover
`utilizing an efficient light trapping mechanism. An optically
`transparent layer is provided which can be associated with an
`opposing reflective Surface and forma waveguiding structure.
`The transparent layer employs a corrugated Surface with ret
`roreflective surface corrugations for confining light below the
`Surface and further employs optical windows for inputting or
`outputting light to or from the layer. The optical cover may
`optionally include a collector array for collecting light onto
`the optical windows or collimating light emanated from the
`optical windows. Various light harvesting or light emitting
`devices may be associated with the optical cover of this inven
`tion.
`In at least one embodiment, the present invention describes
`an optical cover which traps light by means of retroreflection
`from isosceles right-angle Surface corrugations and propa
`gates light along its prevailing plane by means of at least a
`total internal reflection (TIR).
`The optical cover includes a layer of optically transparent
`material having a broad corrugated Surface and an opposing
`Surface extending generally parallel to the corrugated Surface.
`The corrugated Surface employs a plurality of Surface corru
`gations which may beformed by right-angle isosceles prisms
`having a common longitudinal axis extending parallel to a
`reference line in the Surface plane. Each Surface corrugation
`is configured to retroreflect light propagating in the transpar
`ent layer within an acceptance angle with respect to a Surface
`normal. The corrugated Surface further employs a plurality of
`optical windows being surface portions that are either free of
`the Surface corrugations or where the corrugated relief is
`Suppressed.
`The optical cover can operate in response to light received
`on the optical windows and injected into the transparent layer.
`At least a substantial portion of light received by the apertures
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`of optical windows is trapped underneath the corrugated Sur
`face by retroreflection from the surface corrugations. The
`trapped light can propagate along the prevailing plane of the
`layer by bouncing between the corrugated Surface and any
`opposing reflective surface that may be positioned below the 5
`layer. When a light harvesting device is provided between the
`corrugated surface and the reflective surface, the useful
`absorption of light by the device may be improved. Also,
`when a suitable light emitting device is provided between the
`corrugated surface and the reflective surface, light distribu
`tion and collimation may be improved.
`In at least one implementation, each optical window has a
`Surface being generally parallel to the prevailing plane of the
`transparent layer. In at least one implementation, the optical
`windows are arranged into parallel strips extending generally
`perpendicular to the longitudinal axis of the Surface corruga
`tions. In at least one implementation, each of the optical
`windows includes one or more refractive faces inclined at an
`angle with respect to the prevailing plane at least in a cross- 20
`section perpendicular to the longitudinal axis of the Surface
`corrugations.
`In alternative implementations, the optical windows may
`include various surface relief features which can selected
`from the group of elements consisting of cavities, prismatic 25
`grooves, blind holes, through holes, undercuts, notches,
`extensions, Surface discontinuities, discontinuities in said
`layer, Surface texture, and Surface corrugations. In a further
`alternative implementation, the optical windows may com
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`prise cavities having a V-shape in a cross-section.
`In at least one implementation, the optical cover may fur
`ther comprise a plurality of light collectors disposed in energy
`exchange relationship with the optical windows. In at least
`one implementation, the optical cover may further comprise a
`lens array having a focal plane disposed in an immediate
`proximity of the corrugated Surface. In at least one implemen
`tation, the lens array has a shape in a longitudinal section
`selected from the group of elements consisting of elongated,
`cylindrical, square, rectangular and hexagonal.
`In various implementations, the optical cover may be asso
`ciated with other devices or surfaces. In at least one imple
`mentation, the optical cover may further comprise one or
`more light harvesting devices disposed on an opposing side of
`the transparent layer with respect to the corrugated Surface. In 45
`at least one implementation, each of the light harvesting
`devices is selected from the group of elements consisting of
`one or more photovoltaic cells, radiation detectors, light
`absorbers, photo-chemical reactors and photo-bioreactors. In
`at least one implementation, the optical cover further com- 50
`prises one or more light sources disposed below the prevailing
`plane of the transparent layer with respect to the corrugated
`Surface. In at least one implementation, the optical cover
`further comprises a reflective surfaces disposed below the
`prevailing plane of the transparent layer with respect to the 55
`corrugated Surface. In at least one implementation, the reflec
`tive surface comprises isosceles Surface corrugations extend
`ing parallel to the longitudinal axis of the corrugations of the
`corrugated Surface. In at least one implementation, the reflec
`tive surface is a mirrored Surface.
`In at least one implementation, the optical cover has a form
`of a flexible sheet or film.
`In at least one embodiment, the present invention describes
`an optical article comprising a layer of optically transparent
`material. The optically transparent layer has at least one broad 65
`corrugated Surface formed by right-angle isosceles corruga
`tions. The corrugations have retroreflective properties at least
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`in one plane and include one or more openings configured for
`unimpeded communication of light into or from the transpar
`ent layer.
`The present invention provides a number of beneficial ele
`ments which can be implemented either separately or in any
`desired combination without departing from the present
`teachings.
`An element of the invention is an apparatus for collecting
`light over a given area where such light is traveling in a
`generally transversal direction with respect to the light col
`lection area.
`Another element of the invention is an apparatus for dis
`tributing light over a given area and emitting it along a trans
`versal direction with respect to the prevailing plane of the
`light distribution area.
`Another element of the invention is the inclusion of an
`optically transparent layer having at least one corrugated
`surface configured for retroreflection of at least some light
`propagating in the layer.
`Another element of the invention is the inclusion of open
`ings or optical windows in the corrugations that form the
`corrugated Surface.
`Another element of the invention is the use of light collect
`ing elements that can either collect and focus incident light
`onto the respective openings or optical windows or, con
`versely, collimate light emanated by the respective openings
`or optical windows.
`Another element of the invention is the use of various
`profiles for the openings or optical windows that allow for
`more efficient light coupling to or from the transparent layer.
`Another element of the invention is the use of an additional
`reflective surface that opposes the corrugated surface and
`providing waveguiding function to the optical cover.
`Another element of the invention is an optical cover con
`figured with an attached optically responsive device (e.g.,
`photovoltaic cell or photo reactor) or a light emitting device
`(e.g., light emitting diode or fluorescent lamp).
`Further 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.
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`BRIEF DESCRIPTION OF THE SEVERAL
`VIEWS OF THE DRAWING(S)
`
`The invention will be more fully understood by reference
`to the following drawings which are for illustrative purposes
`only:
`FIG. 1 is a schematic perspective view of an optical cover
`according to at least one embodiment of the present inven
`tion;
`FIG. 2 is a schematic perspective view of an optical cover
`showing an alternative orientation of linear optical windows
`with respect to Surface corrugations, according to at least one
`embodiment of the present invention;
`FIG. 3 is a schematic perspective view of an optical cover
`comprising prismatic Surface corrugations and optical win
`dows having square or rectangular apertures, according to at
`least one embodiment of the present invention;
`FIG. 4 is a schematic perspective view of an optical cover
`comprising prismatic Surface corrugations and optical win
`dows having round apertures, according to at least one
`embodiment of the present invention;
`FIG. 5 is a schematic close-up view of an exemplary con
`figuration of the square-aperture optical window, according
`to at least one embodiment of the present invention;
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`FIG. 6 illustrates, in a schematic cross-section, an optical
`cover portion and raytracing of exemplary light rays propa
`gating through an optical window in a corrugated Surface,
`according to at least one embodiment of the present inven
`tion;
`FIG. 7 is a schematic view of an optical cover portion
`including a square-aperture optical window comprising a
`prismatic cavity, according to at least one embodiment of the
`present invention;
`FIG. 8 is a schematic cross-sectional view of optical win
`dow shown in FIG.7, according to at least one embodiment of
`the present invention;
`FIG.9 is a schematic view illustrating a further example of
`an optical window comprising a rectangular (in a cross-sec
`tion) extension above a light receiving Surface, according to at
`least one embodiment of the present invention;
`FIG. 10 is a schematic view illustrating a yet further
`example of an optical window comprising a convex surface,
`according to at least one embodiment of the present inven
`tion;
`FIG. 11 is a schematic view illustrating a yet further
`example of an optical window comprising a cavity having a
`rectangular shape in a cross-section, according to at least one
`embodiment of the present invention;
`FIG. 12 is a schematic view illustrating a yet further
`example of an optical window comprising a concave surface,
`according to at least one embodiment of the present inven
`tion;
`FIG. 13 is a schematic cross-sectional view illustrating a
`yet further example of an optical window comprising micro
`structured Surface, according to at least one embodiment of
`the present invention:
`FIG. 14 is a schematic cross-sectional view and raytracing
`of an optical cover portion further showing a lens element,
`according to at least one embodiment of the present inven
`tion;
`FIG. 15 is a schematic cross-sectional view illustrating an
`exemplary method of making a optical window, according to
`at least one embodiment of the present invention;
`FIG. 16 is a schematic cross-sectional view showing a
`optical window formed by optical coupling a lens element to
`an optical cover, according to at least one embodiment of the
`present invention;
`FIG. 17 is a schematic cross-sectional view illustrating
`light input into an optical cover through a corrugated Surface,
`according to at least one embodiment of the present inven
`tion;
`FIG. 18 is a schematic cross-sectional view illustrating
`light input into an optical cover through an optical window,
`according to at least one embodiment of the present inven
`tion;
`FIG. 19 is a schematic cross-sectional view illustrating
`light trapping in an optical cover by using Surface corruga
`tions, according to at least one embodiment of the present
`invention;
`FIG. 20 is a further illustration of light trapping in an
`optical cover, according to at least one embodiment of the
`present invention;
`FIG. 21 is a schematic cross-sectional view of an optical
`cover employing a light harvesting device, according to at
`least one embodiment of the present invention;
`FIG. 22 is a schematic perspective view of a rectangular
`lens array employing cylindrical (linear-focus) lenses,
`according to at least one embodiment of the present inven
`tion;
`FIG. 23 is a schematic perspective view of a rectangular
`lens array employing square-shaped point-focus lenses, in
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`accordance with at least one embodiment of the present
`invention, according to at least one embodiment of the present
`invention;
`FIG. 24 is a schematic perspective view of a rectangular
`lens array employing hexagon-shaped point-focus lenses,
`according to at least one embodiment of the present inven
`tion;
`FIG.25 is a schematic view, in a cross-section perpendicu
`lar to the cross-section shown in FIG. 21, of an optical cover
`employing a light harvesting device, according to at least one
`embodiment of the present invention;
`FIG. 26 is a schematic cross-sectional view of an optical
`cover used in conjunction with a light harvesting device com
`prising multiple light absorbing elements, according to at
`least one embodiment of the present invention;
`FIG. 27 is a schematic cross-sectional view of an optical
`coverportion employing opposing sloped surfaces, according
`to at least one embodiment of the present invention;
`FIG. 28 is schematic expanded view of an optical cover in
`an alternative waveguide configuration, according to at least
`one embodiment of the present invention;
`FIG. 29 is a schematic expanded view of a further embodi
`ment of the present invention illustrating light distribution
`and collimation function of an optical cover;
`FIG. 30 is a schematic view of an optical cover in a sheet
`roll form, according to at least one embodiment of the present
`invention;
`
`DETAILED DESCRIPTION OF THE INVENTION
`
`Referring more specifically to the drawings, for illustrative
`purposes the present invention is embodied in the apparatus
`generally shown in the preceding figures. It will be appreci
`ated that the apparatus may vary as to configuration and as to
`details of the parts without departing from the basic concepts
`as disclosed herein. Furthermore, elements represented in one
`embodiment as taught herein are applicable without limita
`tion to other embodiments taught herein, and in combination
`with those embodiments and what is known in the art.
`A wide range of applications exist for the present invention
`in relation to the collection of electromagnetic radiant energy,
`Such as light, in a broad spectrum or any Suitable spectral
`bands or domains. Therefore, for the sake of simplicity of
`expression, without limiting generality of this invention, the
`term “light will be used herein although the general terms
`'electromagnetic energy', 'electromagnetic radiation'.
`“radiant energy' or exemplary terms like “visible light'.
`“infrared light', or “ultraviolet light” would also be appropri
`ate.
`FIG. 1 illustrates the present invention and shows an
`embodiment of a light trapping optical cover 2. Optical cover
`2 comprises a layer 8 of essentially transparent refractive
`material confined between a broad surface 10 and an oppos
`ing broad Surface 12 that extends generally parallel to Surface
`10. Both surfaces 10 and 12 are broadly extending longitudi
`nally and laterally so that the thickness of transparent layer 8
`is Substantially smaller compared to its other two dimensions.
`Surface 12 is essentially smooth and transparent and is
`configured for a good optical transmission in either direction.
`Surface 10 is also essentially transparent and configured for
`an unimpeded light passage in at least predefined directions.
`Particularly, surface 10 is configured for a generally unim
`peded passage of light impinging onto Surface 10 from the
`outside of layer 10. Surface 10 is further configured to reflect
`rays impinging onto Surface 10 from the inside of layer 8 and
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`propagating at Sufficiently low incidence angles with respect
`to a Surface normal in at least one plane that is transversal to
`a prevailing plane of layer 8.
`According to a preferred embodiment, Surface 10 com
`prises a plurality of longitudinal isosceles corrugations 20
`extending parallel to each other along Surface 10. Each cor
`rugation 20 is shaped as a triangular prism corner refractive
`reflector having two symmetrical faces disposed at approxi
`mately 90° with respect to each other and at approximately
`45° with respect to a normal to surface 10. The material of
`transparent layer 8 should exceed V2–1.414, in which case
`corrugations 20 can act as retroreflectors at least for some rays
`propagating in layer 8.
`Surface 10 further comprises a plurality of optical windows
`14 which represent surface portions having different light
`bending properties compared to the rest of the corrugated
`surface. Particularly, windows 14 are preferably configured to
`admit light into layer 8 through their apertures without
`increasing the propagation angle with respect to a Surface
`normal at least in a plane which is perpendicular to the lon
`gitudinal axis of corrugations 20. Furthermore, optical win
`dows 14 are preferably configured to further limit light devia
`tion from a normal to surface 10 in the above plane in order to
`maximize the acceptance angle of retroreflection by corruga
`tions 20.
`It will be appreciated by those skilled in the art that corru
`gated surface 10 will generally bend light incident into layer
`8 to a higher off-normal angle in the plane perpendicular to
`the longitudinal axis of corrugations 20 when compared, for
`example, to any Smooth surface portion which is void of any
`Such corrugations. Therefore, one convenient method of lim
`iting the off-normal angle of light entering layer 8 through
`windows 14 is providing a different type of surface reliefthan
`the relief associated with the corrugated portion(s) of surface
`10 and which has lower or no surface slopes in the plane
`transversal to the corrugations. Particularly, it is preferred that
`the surface profile of each optical windows 14 at least in a
`cross-section perpendicular to the longitudinal axis of corru
`gations 20 is parallel to the prevailing plane or layer 8. It will
`be understood that, due to the parallelism of surfaces 10 and
`12, the prevailing plane of layer 8 is parallel to each of the
`Surfaces.
`In the embodiment illustrated in FIG. 1, windows 14 are
`exemplified by openings in corrugations 20 where the Surface
`of the openings is represented by Smooth portions of Surface
`10 which are void of corrugations 20 so that each window 14
`generally has a Surface which is parallel to the prevailing
`plane of layer 8 in either cross-section. Each optical window
`14 has an elongated rectangular aperture which longitudinal
`axis is extending perpendicular to corrugations 20.
`A smooth surface portion of layer 8 may be characterized
`by a critical angle (p
`of a total internal reflection (TIR) with
`respect to light striking the surface from the inside of layer 8.
`The critical TIR angle (p
`may be found from the following
`expression: parcsin (n/nsin 90° arcsin (n/n), where
`n and n are the refractive indices of the material of layer 8
`and the outside medium, respectively. In an exemplary case of
`the interface between glass with the reflective index n of
`about 1.51 and air with n of about 1, p. is approximately
`equal to 41.47. Any rays internally striking a smooth Surface
`portion at incidence angles lower than (p
`will thus exit from
`layer 8 without internal reflection.
`In contrast, corrugations 20 can provide retroreflection of
`light internally striking surface 10 at sufficiently low inci
`dence angles with respect to a surface normal. It will further
`be appreciated that, corrugations 20 will retro-reflect light by
`means of a total internal reflection (TIR) from the respective
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`Case 6:20-cv-00139-ADA Document 1-3 Filed 02/21/20 Page 19 of 26
`
`US 8,740,397 B2
`
`10
`
`15
`
`8
`faces when the incidence ray is within a certain acceptance
`angle from a surface normal. The acceptance angle varies
`depending on the orientation of the incidence ray with respect
`to the plane of surface 10 and to the longitudinal axis of
`corrugations 20.
`An advantage of employing Surface corrugations 20 is that,
`in order the lossless TIR to occur at surface 10, the light
`propagating within layer 8 need not be restricted to incidence
`angles greater than the critical TIR angle which may charac
`terize the optical interface of surface 10 but may also include
`near-normal or even normal incidence rays which will still be
`internally reflected back into layer 8. However, the angle by
`which any ray deviates from a normal to surface 10 in a
`cross-section perpendicular to the longitudinal axis of corru
`gations 20 must be less thana predetermined maximum angle
`0
`to prevent escaping of the light from layer 8 through
`surface 10. It will be appreciated that the light rays may
`deviate from the Surface normal by any angle in a cross
`section that is parallel to the longitudinal axis of corrugations
`20.
`It can be shown that, for right-angle isosceles corrugations
`20 having a prismatic shape and refractive-reflective facets
`inclined at a 45° angle to the base of the respective prisms, the
`maximum angle 0
`can be found from the following rela
`tionship: 0,
`45°-p. If n is the refractive index of layer
`8 and the Surrounding medium is air, then
`
`1
`6 = 45- arcsin.)
`in
`
`which in case of acrylic (PMMA) material gives approxi
`mately 3° and about 6 for polycarbonate (PC). Referring
`further to a cross-section perpendicular to the longitudinal
`axis of corrugations 20 and to an exemplary case when optical
`windows 14 of FIG. 1 having planar and smooth surfaces are
`used for inputting light into layer 8, it can be shown that the 3°
`and about 6' propagation angles within layer 8 correspond to
`the outside incidence angles of approximately 4° and 9° for
`PMMA and PC, respectively. It will be appreciated that the
`minimum acceptance angle in this cross-section will be
`higher for a higher refractive index of the material of layer 8.
`The openings that form individual optical windows 14 have
`a smooth Surface extending parallel to the prevailing plane of
`layer 8 and surface 10 which allows the incident light to pass
`through the windows in either direction. Particularly, each
`window 14 allows light to enter layer 8 and become trapped
`underneath surface 10 by means of TIR from corrugations 20.
`It should be understood that the elongated-shape optical
`windows 14 are not limited to the perpendicular arrangement
`with respect to corrugations 20 and may be disposed at any
`other suitable angle. By way of example, FIG.2 shows optical
`windows 14 extending parallel to corrugations 20.
`It should also be understood that optical windows 14 may
`have any Suitable shapes, dimensions and distribution pattern
`other than those illustrated in FIG. 1 and FIG. 2. By way of
`example and not limitations, optical windows 14 may have
`any two-dimensional shape and may be distributed over Sur
`face 10 in an ordered two-dimensional array of rows and
`columns having either a constant or variable pitch.
`FIG. 3 illustrates an alternative configuration of optical
`cover 2 where optical windows 14 are formed by square
`shape openings incorrugations 20. The spacing between indi
`vidual windows 14 may b