Abstract:
Apparatus for illuminating a feature on a transparent light transmitting substrate or window: the substrate is capable of trapping light within itself and propagating the light along the substrate with total internal reflection. A light source is connected by an optical arrangement positioned with respect to one of the surfaces of the substrate for directing light into the substrate at an incident angle selected for propagation of light along the substrate. An extraction feature at a location on the surface of the substrate is configured to illuminate internal reflection at the feature, so that the feature becomes illuminated. Arrangements for the shape, orientation and adjustment of the optic, for configurations of the feature, for coupling the optic and also for coupling the feature to the substrate and particularly its surface are disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 61/201,524, filed on Dec. 11, 2008, incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention concerns illuminating a transparent substance, such as existing storefront window pane and particularly storefront panes. It is desirable to illuminate in a cost-effective manner so as to attract attention and try to promote incremental sales. 
     Light (UV, visible and/or IR) from a source (e.g., LED, laser diode, optical fiber, fluorescent or photoluminescent materials) is optically coupled into a transparent substrate (e.g., storefront window) via toroidal or circular ( FIGS. 1 ,  1 A,  1 B,  2 ,  4 ,  12 ) prism couplers. These couplers can be refractive ( FIGS. 1 ,  1 A,  1 B), reflective ( FIG. 4 ), or some combination thereof, and comprise a refractive medium (e.g. oil, gel, water, adhesive, etc) between the optic (e.g. circular prism) and the substrate (e.g. window) in order to non-invasively introduce light into the substrate at angles that cannot be introduced via air coupling. Use of such prism couplers to the face of a transparent substrate is one element of the invention disclosed herein. The light is then trapped within the substrate or window, which is akin to light trapped within an optical fiber i.e., via total internal reflection, TIR. Definitions of TIR, the ‘critical angle’ and the ‘evanescent wave’ appear in U.S. Pat. No. 5,959,777, column 2, line 56-Col. 3, line 8, and ‘optical contact’ in Col. 1, lines 55-59 in the same patent, all incorporated by reference. 
     Light can then be extracted from the surface of the substrate or window by defeating TIR via techniques known in the art. Examples of such techniques would be the application of scattering/fluorescing inks or the lamination of surface or bulk diffusing films to portions of the window surface 
     2. Prior Art 
     Traditional edge lighting of a transparent substrate introduces light into the substrate along at least one edge thereof, and light is trapped in the substrate and propagates along the substrate as the light is reflected off the surfaces on the inside of the substrate (see, e.g., U.S. Pat. No. 6,036,328). 
       FIG. 5  hereof illustrates a prior art transparent substrate or light guide which is illuminated by edge lighting, particularly an uncollimated source (LED in this example) is coupled into the edge face of a light guide, S 1 , separated by an air gap (n 1 =1.0). 
       FIG. 5  shows equations used in analysis of a traditional edge-lighting approach for a light guide surrounded by air (n 3 ˜1), receiving light into its entrance surface S 1  from an air-coupled (n 1 ˜1) LED having a lambertian angular distribution. Note that  FIG. 5  shows one plane through the window. The light from the LED actually is emitted in a hemisphere comprising rays R 1  that would be in and out of the plane of the illustrated flat panel light guide or pane, i.e., out of the page hereof. In the analyses described herein, all calculations are for rays within the plane of the paper. One of skill in the art can determine the effects at other angles out of the plane of the paper. 
       FIG. 6  illustrates analysis results for a light guide of  FIG. 5  having a refractive index, n 2 , of 1.51. Note that the light rays (for all angles) reflect off the sidewall surfaces S 2  and S 4  via total internal reflection (TIR). However, all angles also exit surface S 3 , opposite entrance surface S 1 . Note also that the incident angles (relative to axis AX 2 ) at surface S 2 , identified as (90−θ2) are restricted to angles between 48.53 and 90 degrees. Further, the critical angle at surface S 2 , (90−θ2)=asin(n 3 /n 2 )=41.47 degrees, and its complement is (90−41.47)=48.53 degrees. Therefore, for the geometry shown in  FIG. 5 , it is physically impossible for rays to strike surface S 2 , relative to axis AX 2 , between the critical angle and its complement. 
     Since the rays from the LED, R 1 , span the entire gamut of ray angles, −90°≦θ1≦90° into S 1  (see column  2  in  FIG. 6 ; only positive angles are shown), one can see that for a rectangular slab light guide (i.e., a mall window), light introduced at one end face, S 1 , will leak out the other end face, S 3 , unless a reflective material is affixed on or adjacent to S 3 . 
     An alternate edge lighting approach would be to optically couple the LED to the edge face, S 1 , instead of using an air gap. That would introduce all angles within the light guide, and so rays R 2  would extend −90°≦θ2≦90°, and so light within a range of angles would leak out of edge faces S 2  and S 4 . For example, for n 2 =1.51, rays R 2  would leak out of face S 2  for θ2&gt;48.53°; i.e. the incident angle at S 2  relative to axis AX 2 , (90−θ2)≦the critical angle, or 41.47°). So, to prevent leakage, reflective tape would then be placed on portions of S 2  and S 4  starting at point intersecting with face S 1 , wherein the length of the tape progressively gets longer as the LED moves along edge S 1  further away from the intersection with S 2  and S 4 , respectively. 
     One then might consider partially collimating the LED before optically coupling to the edge face, S 1 . First, a typical lens would lose its ability to collimate if its curved output face were optically coupled to the edge face S 1  (unless the refractive index of the lens was very high in order to maintain the appropriate Δn). A non-imaging approach might work, with the exit face optically coupled to the edge face, S 1 , and the collimation set θ2&lt;48.53°, which would preclude leakage out of face S 2 , although not out of S 3  unless the incident angle at S 3  relative to axis AX 7 , θ2, is greater than the critical angle or 41.47°. Note that since S 2  and S 3  are planar surfaces at the angle of incidence, the angles discussed above work the same when reflected about the axis being considered (these can be considered as negative angles). Further, for the ordinary case of a rectangular slab (window), there is a relationship between the critical angle at S 2  and that at S 3 . For example, at S 2 , there will be TIR for angles (90−θ2)&gt;41.47°, which can be rewritten (90−41.47°)&gt;θ2, and thus θ2&lt;48.53°. At S 3 , TIR will occur if θ2&gt;41.47°. Therefore, combining both constraints, TIR can occur at both S 2  and S 3  if 41.47°&lt;θ2&lt;48.53°; i.e. for θ2 between the critical angle at S 2  and its complement. However, as stated previously, this range of angles is not possible for that shown in  FIG. 5 . Finally, note that non-imaging optics (when used with a multi-lumen LED) would have a length that may interfere with existing structures in retrofit applications, and might require additional spacing between adjacent mall glass windows in new installations (or require wider window frames for a given size exterior window). The length of the optic would scale with smaller LEDs (see general discussion of non-imaging optics in U.S. Pat. No. 5,971,551 noting that a collimator is but a concentrator in reverse), but then many more LEDs would be required to achieve the same number of lumens, all else being equal. 
     Second, the natural compression of angles once inside the light guide, in the edge lighted air-gap approach, −41.47°≦θ2≦41.47° (for n 2 =1.51, typical for glass) limits the spread of light through from the point of entry at face S 1  (see e.g.,  FIG. 17F ). Compare this with the omnidirectional spread of the instant invention shown in  FIG. 17B . Therefore, for the edge lighted air gap approach, in order to fill the window with light, one would need a substantial number of LEDs along edge face S 1 . Further, since the span across the window can be rather large (say 4-6 feet for mall glass), then due to losses (absorption, scatter, etc) over that span, one would need to fill the opposing edge face, S 3  with LEDs as well in order to provided reasonably uniform light across the window (see e.g.,  FIG. 17I ). 
     By use of the prism coupling approach hereof, light can be injected at precisely the location(s) on the window where it is needed since it is not restricted to a location at the edges of the window. 
     Light in the window or light guide may be caused to escape the light guide, rather than being internally reflected. The escape may be caused by some treatment at selected locations on a surface of the light guide. Typical ways used to cause escape may include a film applied to a surface at selected locations, e.g., described in U.S. 2007/0279554 and U.S. Pat. No. 6,171,681, e.g., cling film and electret film, described in U.S. 2004/0043221 (the &#39;221, referenced below). See U.S. Pat. No. 5,319,491, col. 2, line 40-col. 3, line 26, incorporated by reference, which describes additional approaches for coupling light out of a substrate. 
     For example, indicia on a cling film may be caused to glow, e.g., by a fluorescent substance applied to them when the film is directly mounted to the light guide and the indicia are illuminated by a light source optically coupled to an appropriate optic. Optical coupling occurs between the light guide and the cling film, even without need for an interposed coupling medium between them. 
     DESCRIPTION OF THE EMBODIMENTS 
     The following is a description of some embodiments. It is not intended to limit the scope hereof, as other embodiments and arrangements may also achieve the same objective and operate and perform in a similar manner. 
     As shown in  FIGS. 1 and 7 , a side-emitting light emitting diode (see e.g., U.S. Pat. No. 6,598,998, filed May 2001, and commercially available e.g., Lumileds P/N LXHL-DW03) is fitted with a special optic which is in the general shape of a thick countersunk flat washer made from clear acrylic or polycarbonate. That assembly is affixed to the face (not to an edge) of a window pane, e.g., via double stick tape such as 3M Scotch brand clear mounting tape, P/N 4010T. This arrangement, which is known as ‘prism-coupling’, (see, e.g., U.S. Pat. No. 4,545,642) forces light to travel within the window pane as if it were a light guide. Light is thereby trapped within the window pane via total internal reflection. This allows light to be coupled into the face of storefront window (or other transparent substrate), causing the window to act like a light guide and enabling it to perform many of the applications known to use such guides. In the inventor&#39;s opinion, the arrangement allows easy coupling to existing, installed windows, whereas traditional edge lighting techniques would require access to the edges of the window, which in many instances would be a costly endeavor. One of skill in the art of light guides would normally have employed edge coupling, not prism coupling; see, e.g., U.S. Pat. No. 6,036,328 (traditional edge lit) and U.S. Pat. No. 6,679,621 (LED with side-emitting lens into clearance holes within a substrate). 
     LEDs are available from a number of suppliers, including Lumileds, Cree, Seoul Semiconductor, Osram, Nichia, etc. Some of these manufacture LEDs in the UV and violet wavelengths; e.g., Cree UV LEDs, P/N XR7090UVV-L100-0001. The advantage of using UV is that it can excite a larger number of fluorescent colors than could a blue LED (because UV wavelengths are shorter). There are safety concerns when using UV LEDs re: damage to the human eye/skin. The data shows that wavelengths greater than 320 nm are desirable from a safety perspective. By carefully considering the excitation spectra of the fluorescent material before selecting the LED wavelength, one can minimize the safety risk by choosing LEDs with longer wavelengths. Of course other factors need to be considered, such as efficacy, cost, supplier availability, etc, in order to achieve the appropriate balance of safety margin, price and performance. 
     The LED surface temperature will present a thermal shock to the window and so the coupling method between the LED and the window should introduce a sufficient thermal break to avoid window damage. One such method may be the use of a thick layer of high temperature coupling grease, keeping in mind any modified ray distributions from the LED when passing through the grease, such as that due to bubbles in the grease and the expanded area of the grease compared to the LED. Another may be to interpose a layer of shock-resistant glass (e.g., borosilicate) whose optical properties are consistent with the application (e.g., wavelength transmittance, low haze, etc.). In both instances, the heat from the LED is preferentially conducted away from the window (to a thermally bonded heatsink), thereby lowering the surface temperature of the LED and minimizing the thermal shock to the window, especially important when the window has thermally stabilized at a low temperature (e.g., uninsulated window pane during a cold winter day) where a thermal shock (from the concentrated heat load of the LED) might cause the window to crack. 
     Light can be extracted from any point or area or the window by applying to the window something that scatters light that impinges on it. For example, as shown in  FIG. 13 , one may write on the window with a fluorescent marker (see extraction feature EF 1 ), such as those used with illuminated menu boards in restaurants (e.g., P/N WO2000MKR4 from International Patterns) or affix a window film (see WF 1 ) with computer-printed graphics (with the appropriate light scattering ink or diffusing feature, EF 2 , such as matte finished Scotch™ tape) or simply abrading the surface (EF 4 ), or even procure windows that have scattering features embedded therein (EF 3 ). The Detailed Description hereof shows other techniques. Some other known techniques are described in the Background section hereof. 
     In addition to windows, it is contemplated that the invention can be used with signboards, tabletops, mailboxes, glass doors, toys, etc., that is any object which acts as a light guide. A table is set forth below indicating standard rural mailbox dimensions for various sizes and series of rural mailboxes, the letters representing the dimensions corresponding to the letters appearing in  FIG. 20B-1 . 
                                                                                                   A   B   C   D   E                                    E11 Series   Standard (T1) Size   8.75   6.75   19.00   5.38   10.60   in       E16 Series   Large (T2) Size   10.88   8.50   20.25   6.63   13.35   in       ST20 Series    Jumbo (T3) Size    15.00   11.50   23.50   9.25   18.06   in           Standard (T1) Size   222.25   171.45   482.60   136.53   269.31   mm           Large (T2) Size   276.23   215.90   514.35   168.28   339.13   mm           Jumbo (T3) Size   381.00   292.10   596.90   234.95   458.83   mm                      FIG. 20B-2 , which shows a flattened view of a T 2  size mailbox, indicates that the mailbox shown includes two 4.25″×20.25″ seasonal/personalized decal areas  72 , two PT15-150 flexible solar cells connected in parallel  74 , a window decal  76 , a laminated extraction film  78 , a reflector film  80 , and an LED/prism  82 . The window decal can be non-coupling (e.g., cling-type or low index coupled). The window decal can also be coupled (e.g., back-side water coupled with front-side scattering ink, or back-side coupled via selectively placed adhesive dots with front-side scattering ink). Both the laminated extraction film and the reflector film can optionally be provided with a coupled decal. It should be noted that the front-side scattering ink can also work in sunlight; the ink can be a combination of white, fluorescent, and colored materials. Areal density of the ink and/or the coupling materials can be distributed to ensure uniform illumination, (both in sunlight and when the LED is illuminated). Any suitable ray trace program, such as Lighttools from Optical Research Associates, can assist in the design. Such computer assisted designs can provide a marketing edge in the sale of decals. It should also be noted that the curvature of the mailbox will cause rays from the LED/prism to alter their angular properties, and, thus, should be considered in an overall design of the optical approach (refractive indices, prism dimensions, LED collimations, etc.) which can be optimized by any suitable ray trace program. The LEDs can be integrated within the glass clamps that support the window or signboard (e.g., from CR Laurence and NovaDisplay, respectively), or some other feature (e.g., the support for a rural mailbox flag). Alternatively, they can be supported by a window decal (similar to the half-baseballs in the fake broken-window decals, and optically coupled using e.g., water). Window decals can also be laminated.
 
     Some or all of the extraction features can also scatter light and/or glow in response to external light sources such as the sun, a spotlight, etc such that the LEDs might be used only at night, or might be illuminated continuously and/or pulsed as an accent. 
     Different wavelength LEDs may be used (UV, visible, IR) and their intensity may can be varied to attract attention within a single window, or may be coordinated between an array of windows e.g., mall storefronts or the facade of a high-rise or be otherwise disposed on the window(s). 
     The LEDs can be flashed on-and-off in order to act, in part or in full, as a communication medium via extraction patterns on the window. For example, in a mall-setting, one store&#39;s window can communicate with another&#39;s, thereby providing wireless links to coordinate special effects, further enhancing the retrofit opportunities. As is well known in the art, the LEDs can be flashed at high frequencies unperceivable by the human eye. 
     As more extracting features are placed across a window, more LEDs coupled at one location to the surface of the substrate or coupled at several locations across the substrate surface may be necessary to maintain a consistent brightness level, all other things being equal. 
     In order for the light from the LED to travel across the span of the window, the window material must be very clear and have high light transmission. Altuglas International&#39;s Plexiglas MC, an acrylic sheet, rated highest for visible light transmission (T vis ) among available windows (glazing), followed by low-iron glasses such as PPG&#39;s Starphire and Pilkington&#39;s Optiwhite. For optimal transmittance of UV light in a plastic sheet, Spartech&#39;s Solacryl SUVT is suitable. In addition to transmittance, other properties must be carefully considered, such as haze, surface finish, impact resistance, and cost. For example, a mailbox was fabricated (by California Quality Plastics Inc.) from a high impact resistant plastic (Acrylite Plus), but the transmittance was lower than one fabricated from Acrylite GP. 
     Commercially available acrylic sheets (e.g., CYRO Industries&#39; ACRYLITE EndLighten, Altuglas&#39; Elit II, and Lucite International&#39;s Perspex Prismex) that are designed to be edge-lighted from a fluorescent lamp, and are known to already have small extraction features precisely distributed across their surface(s) (e.g., via an array of printed dots on their surface; see, e.g., U.S. Pat. No. 4,937,709, filed August-1989) provide uniform illumination. Such a dot pattern may be optimized for use with the instant invention. In fact, there are large-bed inkjet printers that can dispense white ink and it is contemplated that other materials (e.g., micron-sized glass beads and/or fluorescent inks in a UV curable adhesive that matches the refractive index of the window) can be dispensed, silk-screened, etc., and that would cause light to exit the window in a predetermined pattern. 
     The LEDs require a suitable source of electrical power as is known in the art and these range from simple (continuous DC current) to sophisticated (computer-controls for arrays of LEDs). 
     The LED power source may be located a distance from the LED. It has been found that microphone cable (manufactured by Mogami) provides excellent flexibility and can handle the typical current (e.g., 700 mA) of a string of high brightness LEDs. Should the LED be placed towards the center of a window, it may be desirable to use very thin wires (LEDs typically require 700 milliamps or less) or transparent conductors (e.g., vacuum deposited layer of Indium Tin Oxide (ITO) on optically clear film) to carry the current to the LED without being obtrusive. 
     High brightness LEDs typically consume between 1 and 5 watts, and due to their small size require heat sinking. Suitable heatsinks and thermal-grade adhesives &amp; greases can be found from hobbyist parts distributors. One suitable heatsink used was P/N G15275. These heatsinks can also be anodized in different colors and patterns. The LED&#39;s anode and cathode leads can be electrically isolated from the heatsink via polyimide tape. 
     Another approach to thermal management is to use the heatsinking capacity of the end-device. For example, the LED can be thermally bonded to the inside of the glass clamps previously mentioned. CR Laurence P/N Z412BN is cast zinc, and its thermal efficiency vs. aluminum is discussed in “Efficiency and Cost Tradeoffs between Aluminum and Zinc-Aluminum Die Cast Heatsinks”. LEDs have also been thermally bonded to aluminum L-shaped extrusions (available from Home Depot) and fabricated into the shape similar to the abovementioned glass clamp. Another embodiment used the twisted (and flexible) aluminum shafts used to support the ‘lollipop’ style driveway markers (also available from Home Depot). The LED was thermally bonded to one end of the shaft, and the other end was hung from a suction cup attached to a window. It is contemplated that the shaft can be bent into decorative shapes while the heat is transferred along the length of the shaft and dissipated into the air (the shaft advantageously has a cross-shaped cross-section, adding additional surface area thereby enhancing heat dissipation). Heat pipes can also be used. 
     The optic is placed around the side emitting LED and directs the light into the substrate. As previously mentioned, the initial embodiment was formed in the shape of a thick flat washer with a countersunk interior hole, but made from clear acrylic and polycarbonate (see  FIG. 1 ). Additional embodiments are shown in  FIGS. 2 and 4 . 
     Various types of coupling media for filling any gap between the optic that delivers light to the substrate and the substrate have been successfully utilized, including tap water, hair-gel, food-grade grease, Vaseline petroleum jelly, double-stick tape, acrylic based sealant, mineral oil and microscope coupling fluid. Some coupling media, like 3M Scotch-brand double stick tape, can also support the weight of the LED/heatsink assembly. Greases and sealants are especially useful when used with glass clamps as they do not run-off like mineral oil and tap water. The key aspects of this media are its refractive index and its clarity. The index must be high enough so that light can exit the optic and enter the window at the appropriate angles. High clarity ensures high coupling efficiency. Note that other properties of the media also should be considered, such as thickness (vis-à-vis clarity and its effect on the path of light rays), thermal insulation, compliance to accommodate mismatches between the coefficient of thermal expansion between the optic and the window or substrate, adhesion over environmental conditions, ease of removal during servicing or repositioning, and stability under prolonged exposure from ultraviolet light (either from the LED or sunlight). Further, the overall design approach should avoid contributing to excessive stress or strain on the window pane to minimize the risk of breakage (during normal operation, and considering, for example, someone pulling on the device after it is optically coupled to the window). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows orthogonal views of a toroidal prism, and its location in an end-application that utilizes a side-emitting LED. 
         FIG. 1A  is an enlarged elevation view of the side-emitting LED shown in  FIG. 1 , which is a Luxeon III side-emitting LED. 
         FIG. 1B  is a graph of a typical spatial radiation pattern of the side-emitting LED shown in  FIG. 1A . 
         FIG. 2  shows a circular/toroidal prism specifically designed for use with a LED having a lambertian angular distribution. 
         FIG. 3  shows the x-y coordinates of the curvature of the optical surface of the prism in  FIG. 2 . 
         FIG. 3A  shows an exemplary slab diverter approach to focus the omnidirectional light from the toroidal prism with a  90  degree arc as highlighted in the Figure. 
         FIG. 4  shows an alternate embodiment, whereby a LED is placed on one side of a window, and a reflective optic is aligned and optically coupled to the opposite side. 
         FIG. 5  shows equations used in analysis of traditional edge-lighting approach for a light guide, receiving light into its entrance surface S 1  from a LED having a lambertian angular distribution. 
         FIG. 6  shows analysis results for light guide of  FIG. 5  having a refractive index of 1.51. Note that the light rays for all angles reflect off the sidewalls S 2  and S 4  via total internal reflection (TIR). However, all angles also exit surface S 3 , opposite entrance surface S 1 . Note also that the incident angles at surface S 2 , identified as (90−θ2) are restricted to angles between 48.53 and 90 degrees. As discussed previously, for the geometry shown in  FIG. 5 , it is physically impossible for rays to strike surface S 2  between the critical angle, asin(n 3 /n 2 )=41.47 degrees, and its complement, (90−41.47)=48.53 degrees. This physical limitation does not exist for the inventive approach shown in  FIG. 7 , discussed directly below. 
         FIG. 7  shows equations used in analysis of prism-coupled approach for a light guide, receiving light into its entrance surface S 1  from a side-emitting LED having, such as those available from Lumileds. Note that the angular distribution chart from Lumileds shows a small proportion of the light exiting the LED in the vertical direction (corresponding to 0 degrees on the chart), and the amount gets progressively higher as the angle gets closer to 90 degrees on either side of the vertical axis. Therefore, ψ=0 degrees on the drawing corresponds to +/−90 degrees on the Lumileds chart, and is so-annotated. 
         FIG. 7A  shows a Luxeon III side emitting LED angular distribution. 
         FIGS. 8A and 8B  show analysis results for approach in  FIG. 7  having a refractive index of 1.51, a prism refractive index of 1.49, and a prism angle, α, of 7 degrees.  FIG. 8B  continues the table shown in  FIG. 8A  from the bottom of the table shown in  FIG. 8A . Note that the light not only TIRs off of side S 2  for ψ=0 to +90 degrees (see column for θ3), but much of the higher intensity light also TIRs off of side S 3 , opposite entrance surface S 1 , for ψ=0 to +20 degrees. This is because incident angles at surface S 2 , identified as (90−θ2) can go as low as 42.09 degrees for ψ≧0 in the configuration shown in  FIG. 7  (as opposed to 48.53 degrees in  FIG. 6 ), and thus there exists a range of incident angles, (90−θ2), at S 2  between the critical angle, 41.47 degrees, and its complement, 48.53 degrees. The table shows that rays can TIR off of both surface S 2  and S 3 , allowing light to propagate through a rectangular slab light guide (with polished faces for maximum efficiency) until the light has been extracted or dissipated. This is both significant and unexpected, and is further exemplified in  FIGS. 9-11 , wherein TIR is achieved at both S 2  and S 3  when the incident angle at S 2  is between the critical angle and its complement. Finally note that in  FIG. 8 , the last column in the table identifies that the extreme angles of ψ (25 to 90 degrees) will leak out of surface S 3  (and S 1 ), thereby requiring, e.g. a specular reflector on surface S 3  (and S 1 ) if these rays must be preserved with the light guide. 
         FIGS. 9A and 9B  show analysis results for the approach in  FIG. 7  for a prism angle, α, of 14 degrees.  FIG. 9B  continues the table shown in  FIG. 9A  from the bottom of the table shown in  FIG. 9A . Note the differences in angles of ψ by which TIR can be achieved at surfaces S 2  and S 3  (and their opposing faces due to symmetry) when compared to other angles of α. 
         FIGS. 10A and 10B  show analysis results for the approach in  FIG. 7  for a prism angle, α, of 21 degrees.  FIG. 10B  continues the table shown in  FIG. 10A  from the bottom of the table shown in  FIG. 10A . Note the differences in angles of ψ by which TIR can be achieved at surfaces S 2  and S 3  (and their opposing faces due to symmetry) when compared to other angles of α. 
         FIGS. 11A and 11B  show analysis results for the approach in  FIG. 7  for a prism angle, α, of 28 degrees.  FIG. 11B  continues the table shown in  FIG. 11A  from the bottom of the table shown in  FIG. 11A . Note the differences in angles of ψ by which TIR can be achieved at surfaces S 2  and S 3  (and their opposing faces due to symmetry) when compared to other angles of α. 
         FIG. 12A  shows an alternate embodiment using a collimated LED with an auxiliary ‘diverter’ used to transform the angles into appropriate angles for use by the prism. The term ‘diverter’ connotes a functional representation, wherein the actual optical element can be reflective (as shown), refractive, diffractive, etc. 
         FIG. 12B  shows essentially the same arrangements as in  FIG. 12A , except that the ray propagation is reversed, with the prism used to collect light from the light guide and funnel into a photodiode. 
         FIG. 12C  shows the use of a non-imaging optic (shown as a formed sheet metal reflector, but can also be refractive) to collimate ambient light (provided via an optical fiber) to be used as the source instead of a LED (or laser diode, etc). The Figure also shows the use of laminated glass window, consisting of two sheets of ¼″ glass (such as low-iron Starphire from PPG) bonded via a layer of poly vinyl butyral (PVB). The refractive index of PVB must be considered in determining whether light will traverse the PVB layer, or reflect via TIR. 
         FIG. 13  demonstrates various methods by which rays can be extracted from the window or light guide. The first extraction feature, EF 1 , could be the ink from a fluorescent marker. EF 2  is a lens-like diffusing element attached to window film WF 1 , coupled to the window via adhesive film, AF 1 . EF 3  is a scattering particle or a void with the bulk of the light guide. Some scattered rays will exit the window like ray RF, while others will TIR, like ray RG. EF 4  is a surface divot that could be achieved via etching, sandblasting, or other methods used to remove material from the surface of the window. 
         FIGS. 13A-13F  show and describe various extraction mechanisms that provide indicia illuminated by the window (via an optically coupled inner layer) and optionally indicia on an optically isolated (to a predefined degree, not necessarily 100% isolated) outer layer. 
       In addition to the use of cling vinyl films and electret films, other methods separately or in combination, are contemplated by which the light guide and inner/outer layers of an extraction mechanism can be held in a predetermined spatial relationship (static or relative translation/rotation), such as mechanical fastening, chemical bonding, and forces induced by gravity, magnetism, surface tension, suction, etc. Such methods can be distributed across the surfaces of the layers in periodic/aperiodic fashion. Optical losses may be considered as certain methods will induce haze, absorption or otherwise seemingly lossy attributes. Note that in some instances, what might seem to be a lossy attribute actually imparts a desired effect, aesthetic or otherwise. 
       It is also contemplated that when there are inner and outer layers, they can be optically isolated (at one or more points) such as a dual pane thermal window, or by such use of air-spaced (or other low refractive index) structures as demonstrated in microstructured adhesives, bubble wrap, corrugated plastic sheets, thermal formed plastic sheets, transparent insulation such as honeycomb structures, placement of an intervening mesh or netting, spacer beads/rods/fibers, printed spacers, foams, adhesive dots, or simply ink dots dispensed using the same ink and inkjet printer as used for the inner/outer layers ensuring a cured dot height sufficient for optical isolation. 
       The inner layer may be optically coupled to the window (light guide) via an adhesive (continuous layer or distributed over preselected areas), or simply as a function of its smooth surface (cf. the &#39;221 referenced above) and static cling. Similarly, the inner layer can be optically isolated from the window (light guide) in selected areas by a low index material (e.g., TEFLON) and/or by physical separation to avoid evanescent coupling (e.g., via surface roughness, printed dots, mechanical deformation, etc.). 
       Light redirecting optical films are known (see e.g., U.S. Pat. No. 7,090,389). It is contemplated that these structures, or derivatives thereof, can be optically coupled to the surface of a window in order to redirect light within the window to guide it more towards an area of interest. 
         FIGS. 13E and 13F  show that exemplary mosquito netting can act as an excellent optical isolation layer since the netting has large voids and since the fibers tend to be rounded, only making point contact with the inner layer film, thereby also minimizing the potential for optical contact. The netting is typically polyester. Further, a daytime non-illuminated image may be imparted to the netting if desired, such as via dye sublimation printing, known for its compatibility with polyester . 
         FIG. 13F  shows a window cling optically coupled to a window. 
         FIG. 14  shows equations used in analysis of prism-coupled approach for a light guide, similar to that shown in  FIG. 7 , except a coupling material, CM 1 , of refractive index n 6 , is included in the analysis. 
         FIGS. 15A-1  and  15 A- 2  show the influence on the refractive indices of a first material (e.g., prism) and a second material (e.g., coupling material) to determine whether light rays of a given incidence angle at the boundary between the materials will TIR. Steps  1  through  3  are described on the Figures.  FIG. 15A-2  continues the table shown in  FIG. 15A-1  from the bottom of the table shown in  FIG. 15A-1 . 
         FIGS. 15B-1  and  15 B- 2  provide an analysis of the relative angles when a vinyl window film is deployed in the stackup arrangement shown in  FIG. 15B-3 . Note that the analysis of water coupling is referenced in  FIG. 15A-2  (step  4 ).  FIG. 15B-2  continues the table shown in  FIG. 15B-1  from the bottom of the table shown in  FIG. 15B-1 . 
         FIG. 16  shows detailed cross-sectional view of a mall window application, with prisms and LEDs integrated into window clamps. 
         FIG. 17A  shows a front view of a mall window in  FIG. 16 , also showing a LED driver, with a wireless communications link to a remote PC, which can be used to control the intensity vs. time profile of the LEDs. In one exemplary embodiment, the profile is coordinated between a plurality of windows (and other sources of light and sound). 
         FIGS. 17B-17E  show front views of four adjacent mall windows, each with a unique number of LEDs illuminated using prism coupling. 
         FIGS. 17F-17I  show front views of four adjacent mall windows, each with a unique number of LEDs illuminated using traditional edge coupling. 
         FIGS. 18A-1 ,  18 A- 2 ,  18 A- 3 ,  18 A- 4 ,  18 B- 1 ,  18 B- 2 ,  18 B- 3 ,  18 B- 4 ,  18 C- 1 ,  18 C- 2 ,  18 C- 3 ,  18 C- 4 ,  18 D- 1 ,  18 D- 2 ,  18 D- 3 ,  18 D- 4 ,  18 E- 1 ,  18 E- 2 ,  18 E- 3 ,  18 E- 4 ,  18 F- 1 , and  18 F- 2  show x/y coordinates (and intermediate calculations) of the surface profile shown in  FIG. 3 . Each of  FIGS. 18A-2 ,  18 B- 2 ,  18 C- 2 ,  18 D- 2 ,  18 E- 2 , and  18 F- 2  continues the table shown in  FIGS. 18A-1 ,  18 B- 1 ,  18 C- 1 ,  18 D- 1 ,  18 E- 1 , and  18 F- 1 , respectively, from the right side of the table shown in  FIGS. 18A-1 ,  18 B- 1 ,  18 C- 1 ,  18 D- 1 ,  18 E- 1 , and  18 F- 1 , respectively. Each of  FIGS. 18A-3 ,  18 B- 3 ,  18 C- 3 ,  18 D- 3 , and  18 E- 3  continues the table shown in  FIGS. 18A-1 ,  18 B- 1 ,  18 C- 1 ,  18 D- 1 , and  18 E- 1 , respectively, from the bottom of the table shown in  FIGS. 18A-1 ,  18 B- 1 ,  18 C- 1 ,  18 D- 1 , and  18 E- 1 , respectively. Each of  FIGS. 18A-4 ,  18 B- 4 ,  18 C- 4 ,  18 D- 4 , and  18 E- 4  continues the table shown in  FIGS. 18A-2 ,  18 B- 2 ,  18 C- 2 ,  18 D- 2 , and  18 E- 2 , respectively, from the bottom of the table shown in  FIGS. 18A-2 ,  18 B- 2 ,  18 C- 2 ,  18 D- 2 , and  18 E- 2 , respectively, and each of  FIGS. 18A-4 ,  18 B- 4 ,  18 C- 4 ,  18 D- 4 , and  18 E- 4  continues the table shown in  FIGS. 18A-3 ,  18 B- 3 ,  18 C- 3 ,  18 D- 3 , and  18 E- 3 , respectively, from the right side of the table shown in  FIGS. 18A-3 ,  18 B- 3 ,  18 C- 3 ,  18 D- 3 , and  18 E- 3 , respectively. Note that other profiles than that shown in  FIG. 3  are contemplated, and are influenced by the material properties discussed herein, and tradeoffs imposed by design constraints (e.g., size vs. coupling efficiency, etc). 
         FIGS. 19A and 19B  show objects illuminated with the instant invention, detailing a variety of indoor and outdoor applications. Note that the tic-tac-toe annotations on each object indicate where the light is extracted. Note that the mailbox is a formed sheet of a transparent polymer. 
         FIGS. 20A-1  and  20 A- 2  show an exemplary flexible solar-cell and an exemplary solar-cell based light controller, respectively, for use in a remote mailbox application further detailed in  FIGS. 20B-1  and  20 B- 2 . 
         FIG. 20B-1  shows a rural mailbox with representative dimensions shown thereon.  FIG. 20B-2  is a large size mailbox shell in a flattened condition in order to detail the position of various elements as detailed in the drawing. 
         FIG. 21A  shows the use of a commercially available secondary lens for LEDs that converts a lambertian distribution into a side-emitting distribution.  FIG. 21  B shows enlarged perspective views from the bottom and the top of the commercially available secondary lens for LEDs.  FIG. 21C  shows a plot of the full width at half maximum (FWHM) divergence versus side emitter rotation angle (azimuthal) for the commercially available secondary lens for LEDs shown in  FIGS. 21A and 21B .  FIG. 21D  shows the luminous intensity in Candela (Cd) at a side emitter rotation angle of zero degrees azimuth for the commercially available secondary lens for LEDs shown in  FIGS. 21A and 21B . The minimum luminous intensity shown in  FIG. 21D  is 1.592 Cd, the maximum luminous intensity shown in  FIG. 21D  is 33.4 Cd, the average luminous intensity is 6.55 Cd, and the 
       FWHM in  FIG. 21D  is 8.95 degrees.  FIG. 22A  shows the use of the commercially available secondary lens for LEDs that converts a lambertian distribution into a side-emitting distribution for use by a toroidal prism.  FIG. 22B  is an enlarged view of the LED and its mounting shown in  FIG. 22A . 
         FIG. 23A  shows a prior art diffusing film comprising resin particles in a binder. As the resin dries, it forms an undulating surface, following the contours of the bead-like particles. 
         FIG. 23B  provides a first order optical analysis of the effects of the diffusing film in  FIG. 23A  when receiving light at an angle of δ (relative to the horizontal axis shown) via a window (not shown). 
         FIGS. 24A and 24B  show the relative intensity out of a Lumileds lambertian and side emitting LED, respectively. Portions of these intensity curves were modeled in Excel as shown in  FIG. 24C . 
         FIGS. 25A-1  and  25 A- 2  detail the angle, φ, that light leaves the diffusing screen in  FIG. 24B  as a function of the incident angle, δ, and the slope of the diffusing surface, γ, as detailed in  FIG. 23B . Note that the incident angle, δ (third column of  FIGS. 25A-1  and  25 A- 2 ) maps to (90−θ2) in  FIGS. 5 and 7 , which maps to θ1 and ψ (second column of  FIGS. 25B-1  and  25 B- 2 , respectively) coming from the LEDs in those figures, respectively. Also shown in the first column of  FIGS. 25A-1  and  25 A- 2  is the relative intensity of the LEDs in  FIGS. 5 and 7 , respectively, as a function of angles θ1 and ψ, respectively. Note that for certain combinations in the table (denoted by ##TIR##), the light striking the undulating surface of the diffuser cannot exit, and is reflected back into the diffuser via TIR. The ultimate fate of these rays can be determined by any suitable ray trace program as is known in the art. 
         FIGS. 25B-1  and  25 B- 2  are similar to  FIGS. 25A-1  and  25 A- 2 , respectively, except that they provide additional granularity around those conditions causing TIR at the undulating surface of the diffusing film. 
         FIG. 26A  demonstrates the intensity of those rays exiting the diffuser film for a traditional edge light approach, with each successive curve indicating a different slope, γ, at the exit surface of the diffuser. Slope values are annotated for γ values of 7.1 degrees, 7.9 degrees, etc. 
         FIG. 26B  demonstrates the intensity of those rays exiting the diffuser film for the prism coupling approach, with each successive curve indicating a different slope, γ, at the exit surface of the diffuser. Slope values are annotated for γ values of −39.1 degrees, −32.5 degrees, etc. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     Referring to  FIG. 1 , a window or other transparent substrate  10  is to have light from an LED  20 , which is a semiconductor that emits light in response to electrical stimulation transmitted into the transparent substrate at angles related to the material of the substrate so that the light is “trapped” within the substrate as a result of total internal reflection (i.e., TIR) off the inside of the exterior surfaces of the substrate. Propagation of the light continues within the transparent substrate until it has been fully absorbed/extracted. 
     In a preferred form, an LED  20  preferably of the side-emitting type is positioned on a support and heat sink  22  near to a side surface of the substrate  10  to deliver the light that is to enter the substrate. Additional sources of light might include a fluorescent material or a photoluminescent material, which are conventionally stimulated to emit light. 
     In order to cause the light from the LED to enter the substrate (i.e., window pane), an optical coupling arrangement  30  is provided. As shown, it comprises what has been termed a toroidal prism due to its central through-hole and circular shape. 
       FIG. 2  shows a circular/toroidal prism specifically designed for use with a LED having a lambertian angular distribution 
       FIG. 3  shows the x-y coordinates of the curvature of the optical surface of the prism in  FIG. 2   
       FIGS. 18A-1 ,  18 A- 2 ,  18 A- 3 ,  18 A- 4 ,  18 B- 1 ,  18 B- 2 ,  18 B- 3 ,  18 B- 4 ,  18 C- 1 ,  18 C- 2 ,  18 C- 3 ,  18 C- 4 ,  18 D- 1 ,  18 D- 2 ,  18 D- 3 ,  18 D- 4 ,  18 E- 1 ,  18 E- 2 ,  18 E- 3 ,  18 E- 4 ,  18 F- 1 , and  18 F- 2  show x/y coordinates (and intermediate calculations) of the surface profile shown in  FIG. 3 . 
     As not all LED manufacturers offer side emitting LEDs, and secondary side emitting lenses may not be acceptable for a variety of reasons, a custom optic like the cross-section shown in  FIG. 2  can be constructed for use with any LED, obviating the need for a side emitting LED and separate toroidal prism. In  FIG. 2 , the optic (termed a circular prism because there is no central through-hole) has been designed to accept any LED and bend the rays at the required angles into the window. In this particular embodiment, rays from the LED were analyzed to minimize Fresnel reflections, account for draft angles to allow for molding, maintain a compact size, etc. 
       FIG. 1  demonstrates a one-zone planar input surface; i.e., a constant surface tangent bounded by the top and bottom surfaces that form discontinuous surface tangents. 
       FIG. 2  demonstrates three zones, each have a smoothly changing surface tangent and each bounded by a discontinuity in the surface tangent. 
     The particular design embodiment in  FIG. 2  has been subdivided in three regions or zones—0°˜20°, 20°˜60°, and 60°-90°, with representative rays shown as A, B, and C, respectively. Note that the reflections within this circular prism for rays A and C have been accommodated via TIR, although reflective coatings can be employed as desired. Note that the circular prism needs to be coupled to the window with coupling media. 
       FIG. 3  shows the profile as calculated in Excel, with the detailed coordinates and interim calculations shown in  FIGS. 18A-1 ,  18 A- 2 ,  18 A- 3 ,  18 A- 4 ,  18 B- 1 ,  18 B- 2 ,  18 B- 3 ,  18 B- 4 ,  18 C- 1 ,  18 C- 2 ,  18 C- 3 ,  18 C- 4 ,  18 D- 1 ,  18 D- 2 ,  18 D- 3 ,  18 D- 4 ,  18 E- 1 ,  18 E- 2 ,  18 E- 3 ,  18 E- 4 ,  18 F- 1 , and  18 F- 2 . 
     The prism arrangement has, as its entire surface at  36  facing the LED, a series of radius curves of the prism, and the radius curves are selected by one of skill in the art to bend the light emitted by the LED and entering the transparent substrate  10  at angles, for example as in  FIG. 3 , for the light to remain trapped within the substrate between the internal surfaces. This is shown schematically in  FIG. 2 . Light from the LED entering the substrate at too large an angle with respect to the surface of the transparent substrate would pass through the substrate rather than being trapped within. It may be blocked by an opaque mask over that part of the substrate. 
       FIG. 3A  shows an exemplary slab diverter approach to focus the omnidirectional light from the toroidal prism with a 90 degree arc as highlighted in the Figure. When placed on a surface of a window at each corner of the window or each corner of rectangular indicia within the window, this type of diverter optic concentrates the light to the areas where it is needed. Other arc angles can be selected to obtain a desired effect. While  FIG. 3A  shows a parabolic diverter for use with certain rays, other forms are contemplated, such as linear, multifaceted, elliptical, circular, CPC (compound parabolic concentrator, a generic reference herein to non-imaging optics), or some combination thereof. 
     The diverter can be constructed so that it can be rotated when optically coupled through the window (e.g., via optical grease or oil, similar to an optical slipring) or alternatively, coupled via an air gap or lens (array) from a rotating diverter through a secondary optic (e.g., circular prism) that is affixed and optically coupled to the window. This allows the light distribution within the glass to be optimized for the desired aesthetic effects; e.g., uniform illumination of new indicia that are optically coupled to a window, where the extraction of light through prior indicia is markedly different than the new indicia. Note that instead of a single side-emitting LED with a mechanically rotating optic, an array of (semi)collimated LEDs can be substituted herein for temporally directing the beam through a surface coupled optic. 
     The attachment to the window can be engineered to allow the diverter to rotate in order to adjust the direction of light within the window to best suit an application. For example, a clear vinyl can be coupled first to the window, and then coupled to the diverter via optical grease to allow movement without losing the optical coupling. Further, faceted structures can be added to the toroidal prism (the one shaped like a thick, countersunk flat washer). For example, the outer circumference can comprise facets over certain portions, redirecting light as appropriate. The outer circumference can also be coated or surrounded by a specular reflector. Certain portions of the top surface can comprise prismatic-like features to redirect light as needed. These features can be subtractive-from and/or additive-to to the top surface of the toroidal prism. The top can also be de-coupled (optically) in select locations to allow reflections from the opposing side to propagate into the window without bouncing back into the diverter. 
     The side-emitting optic itself can be partially surrounded by a specular reflector, intercepting certain rays and redirecting them to another canted prism input face. This could be of use if an application uses a thin window, and the light is to be concentrated within a certain angular zone. 
     The light that passes straight up from the LED  20  through the substrate  10  at a too large angle to be trapped inside the substrate may be used in the substrate by positioning a coupling arrangement in the form of a reflective optic, of the type shown in  FIG. 4 , for example filled with a coupling medium, at the other surface of the substrate from the LED. The optic  50  is shaped so that the optic will reflect impinging light back into the substrate and at an angle where that light would then be trapped within the substrate as well. 
       FIG. 4  shows an alternate embodiment, whereby a LED is placed on one side of a window, and a reflective optic is aligned to the opposite side. 
     In order to show breadth of the invention, the approach in  FIG. 4  optically couples an LED chip on one face of a window, and optically couples a reflective optic on the opposing face. The coupled LED, as previously discussed, allows all angles to exist within the window. Those angles that would naturally TIR from the opposing face of the window are not intercepted by the reflective optic. Those angles that would pass through the window naturally are redirected by the optically coupled reflective optic into angles that TIR between the opposing faces. No effort has been made to optimize the angles to maximize the opportunity for TIR at the end faces of the window (not shown). Such a design is contemplated and can be fashioned by one of skill in the art. 
     An advantage to this approach is that the reflective optic naturally blocks all light that would leak through the opposing face (note the additional light blocking layer above the LED in  FIG. 7 ). 
       FIG. 12A  shows an alternate embodiment using a collimated LED with an auxiliary ‘diverter’ used to transform the angles into appropriate angles for use by the prism. The term ‘diverter’ connotes a functional representation, wherein the actual optical element can be reflective (as shown), refractive, diffractive, etc. 
     An advantage of this approach is that widely available collimated LEDs (and laser diodes) can be used, and a reflective diverter can be employed to change the beam&#39;s direction to that of a side-emitting LED. A reflective diverter also acts to block any light from leaking through opposing face, S 3 . 
       FIG. 12B  shows essentially the same arrangements as in  FIG. 12A , except that the ray propagation is reversed, with the prism used to collect light from the light guide and funnel into a photodiode. 
     As previously mentioned, non-imaging concentrators can be used in reverse as collimators. The same is true here, where the circular prism is used in the reverse sense to collect light trapped within the window. Note that the other toroidal and circular prism couplers can be used in the sensor mode as well. It is also contemplated that combination source/sensors can be deployed (e.g., via beamsplitters). For example, light from an external communications source can be coupled into the window via a holographic optical element (HOE) as taught in U.S. Pat. No. 6,724,508, and sensed by the instant invention via toroidal/circular prism couplers. 
       FIG. 12C  shows the use of a non-imaging optic (shown as formed sheet metal reflector, but can also be refractive) to collimate ambient light to be used as the source instead of a LED (or laser diode, etc). The figure also shows the use of laminated glass window, consisting of two sheets of ¼″ glass (such as low-iron Starphire from PPG) bonded via a layer of poly vinyl butyral (PVB). The refractive index of PVB must be considered in determining whether light will traverse the PVB layer, or reflect via TIR. 
     As shown in  FIG. 12C , remote sources of light such as sunlight, room light, an optical communications signal, light from an optical fiber, etc. can be coupled into the window. Other optical approaches are contemplated, either as a substitute or complement to the collimating non-imaging optic shown in  FIG. 12C . 
       FIG. 4  illustrates the optic  50  at the substrate without the arrangement  30 . They may be used as separate alternatives or may be used in combination at a particular installation. In the latter situation, the light passing across the substrate is not masked, but is permitted to impinge on the optic  50 . 
       FIG. 7  shows equations used in analysis of prism-coupled approach for a light guide, receiving light into its entrance surface S 1  from a side-emitting LED having, such as those available from Lumileds. Note that the angular distribution chart from Lumileds shows a small proportion of the light exiting the LED in the vertical direction (corresponding to 0 degrees on the chart), and the amount gets progressively higher as the angle gets closer to 90 degrees on either side of the vertical axis. Therefore, ψ=0 degrees on the drawing corresponds to +/−90 degrees on the Lumileds chart, and is so-annotated. 
       FIG. 14  shows equations used in analysis of a prism-coupled approach for a light guide, similar to that shown in  FIG. 7 , except that a coupling material, CM 1 , of refractive index n 6 , is included in the analysis. 
       FIGS. 15A-1  and  15 A- 2  show an analysis of the influence on the refractive indices of a first material (e.g., prism) and a second material (e.g., coupling material) to determine whether light rays of a given incidence angle at the boundary between the materials will TIR. Steps  1  through  3  are described on the Figures. 
     FIG.  FIGS. 15A-1  and  15 A- 2  detail an exemplary approach in the choice of appropriate coupling media for an acrylic prism (n=1.49) trying to pass angles up to 75 degrees off-normal through the coupling media, and thus requiring a minimum refractive index of 1.44 for the coupling media (e.g., acrylic PSA, n=1.47, P/N ARClear 8154 from Adhesives Research. 
     An interesting application is the case of a vinyl window decal that is water-coupled to a window. Water has a refractive index of about 1.37 at 300 nm, down to 1.33 at 670 nm. Clear vinyl window cling is a PVC material, and PVC has a refractive index of 1.54. As shown in  FIGS. 15B-1  and  15 B- 2 , the angle within the clear vinyl, θ2, from an acrylic prism (α=7°), and coupling media of the same index, is &lt;50° (to ensure TIR at both S 2  and S 3 ). As shown in callout box # 4  in  FIG. 15A-2 , the minimum index for the coupling media between the vinyl and the window glass must be about 1.18, and therefore water satisfies the constraint (for the wavelengths for which 1.54 is the index of vinyl). This non-limiting example demonstrates that water can couple light from a clear vinyl window cling into window glass when a prism is coupled into the vinyl as shown in the stackup in  FIG. 15B-3 . A significant advantage of the window cling approach is the simplicity of installation (and removal), allowing use of the invention by both novice and professional installers. 
     The toroidal prism, TP 1 , shown in  FIG. 7  has smooth surfaces, and is in the general shape of a thick flat washer with a countersunk central through-hole. The side emitting LED provides a semi-collimated beam about a plane orthogonal to the vertical axis through the center of the LED as shown in the graph of  FIG. 7A  (based on a Luxeon LED). Note from the graph that there is some residual light at all other angles, some of which will not TIR and thus leak through the window. Note in  FIG. 7  that directly above the top side of the LED there is a reference to a blocking film. 
     As stated previously, for a window with n=1.51, light will TIR from both faces S 2  and S 3  if −48.53°&lt;θ2&lt;−41.47° and 41.47°&lt;θ2&lt;48.53° (where the negative angles indicate those angles reflected about the axis normal to S 2  or S 3 ). This is detailed in  FIGS. 8A and 8B , which are based on a configuration of a toroidal prism like that in  FIG. 7  with α=7° and 0°≦ψ≦20°. Note that the designer must be mindful of Fresnel reflections of ray R 1 A relative to prism face S 6  as the incident angles can be close to grazing where the Fresnel reflections become substantial. 
     Note also that this type of side emitting LED is not offered by every LED manufacturer. There are secondary lenses, e.g., P/N 10267 available from Carclo Technical Plastics, 111 Buckingham Avenue, Slough, Berkshire SL1 4PF England / 600 Depot Street, Latrobe, PA 15650 USA, telephone UK: 0044 (0) 1753 575011, Telephone USA: 00 (1) 724 539 6982; 84 shown in  FIGS. 21A and 21B . Note, however, the size is much larger than that of the integrated lens in the Lumileds part, and so it may be more difficult to deploy, either from a mechanical packaging perspective or from the optical principle of etendue (describes the fundamental limit governing the amount of light that can be coupled from a given source into a system; see for example, US 2008/0212328). So, a window of a given thickness and refractive index can only accept a certain amount of light via prism coupling based upon the etendue of the optical system (LED +optics).  FIG. 21A  shows side-emitting optic  84  assembled on luxeon LED  86 , a clear acrylic or polycarbonate sheet  90  being positioned on both sides of the assembly of the side-emitting optic  84  and the luxeon LED  86 . The average FWHM of the side-emitting secondary optic  84  is 8.4 degrees, the minimum FWHM is 7.45 degrees, and the maximum FWHM is 9.3 degrees. The efficiency of the side-emitting secondary optic  84  is 87% and Cd/lm is 38/46 =0.8 @ 350mA (based on an average of eight readings).  FIG. 22A  shows the commercially available secondary lens  84  for LEDs that converts a lambertian distribution from LED  98  into a side-emitting distribution for use by a toroidal prism  92 . The commercially available secondary lens  84  is bounded by a specular reflector  94 , the toroidal prism  92 , and air  88  between the specular reflector  94  and the toroidal prism  92 , the toroidal prism  92  having an underside angle  96  with the horizontal of 7 degrees. Air  88  is also present below the specular reflector  94 . As shown in  FIG. 22B , LED  98  is mounted on a FR 4  board  100  and aluminum board  102 . 
       FIGS. 7  though  11 B demonstrate the effects of the prism angle, α, on the angles of ψ for which TIR can be achieved at both S 2  and S 3 . 
     In  FIGS. 8A and 8B , α=7°, and assuming the LED emission is contained within 0°≦ψ≦20°, TIR is achieved, quite unexpectedly, at both surfaces S 2  and S 3 . This provides a significant boost in efficiency, as TIR is effectively lossless. It thus precludes light leakage at the edges of the window (if that is a requirement as opposed to an effect that is desirable for a given application). 
     In  FIGS. 9A and 9B ,  10 A and  10 B, and  11 A and  11 B, α is set to 14°, 21°, and 28°, respectively. The tables show, amongst other things, the tradeoffs between leakage and reflection at S 2  and S 3  depending upon the range of angles from the LED, ψ. 
     Finally, other factors can be optimized via ray trace programs, such as Fresnel reflections, skew rays, etc. Further, it is contemplated that the prism surfaces can deploy faceted geometric features, diffractive features, etc, in order to direct the beams into the window in one or more preferred directions, increase coupling efficiency into the window, or optimize any other price/performance target. It is also contemplated that an element can be placed on the opposite side of the window from the prism in order to block light (can be air gapped), redirect the beam (optically coupled), or become illuminated by any light leakage for effect or other predetermined purpose. Examples appear in  FIG. 4 . Note also that the prism angle, α, can be varied within the same part in order to optimize performance for a given application. Also, a portion of the prism can be replaced with a reflective optical feature to redirect light in one or more preferred directions. An example of such as system is taught in U.S. Pat. No. 6,565,235. For example, a reflector can surround 270° of a side-emitting LED, redirecting incident light towards the opposing side so that light side-emits around only a 90° swath. This would be useful in an application where the prism is coupled near the corner of a window, directing the light within the window through a 90° sweep, from rays parallel to one edge of the window to those parallel to the adjacent (orthogonal) edge (in the case of a rectangular window). In other applications, a collimated LED can be prism-coupled to direct light along a preferred narrow path within the window. In fact, an array of LEDs can be so-coupled and arranged along a line, in a radial pattern, or any other configuration to achieve the price/performance so desired. Such an array (UV LEDs for fluorescent films, visible LEDs for diffusing films, and combinations thereof) can be time-sequenced (intensity and/or color) to achieve interesting visual effects. 
       FIGS. 8A and 8B  show analysis results for the approach in  FIG. 7  having a refractive index of 1.51, a prism refractive index of 1.49, and a prism angle, α, of 7 degrees. Note that the light not only TIRs off of side S 2  for all values of ψ (see column for θ3), but significantly, and quite unexpectedly, light also TIRs off of side S 3 , opposite entrance surface S 1 , for ψ=0 to +20 degrees. This is because incident angles at surface S 2 , identified as (90−θ2) can go as low as 42.09 degrees (as opposed to 48.53 degrees in  FIG. 6 ). For those rays that can TIR off of both surface S 2  and S 3 , light will propagate through a rectangular slab light guide (with polished faces to avoid TIR-defeating scattering sites) until the light has been extracted or dissipated. 
       FIGS. 9A and 9B  show analysis results for the approach in  FIG. 7  for prism angle, α, of 14 degrees. Note the differences in angles of ψ by which TIR can be achieved at surfaces S 2  and S 3  (and their opposing faces due to symmetry) when compared to other angles of α. 
       FIGS. 10A and 10B  show analysis results for the approach in  FIG. 7  for prism angle, α, of 21 degrees. Note the differences in angles of ψ by which TIR can be achieved at surfaces S 2  and S 3  (and their opposing faces due to symmetry) when compared to other angles of α. 
       FIGS. 11A and 11B  show analysis results for the approach in  FIG. 7  for prism angle, α, of 28 degrees. Note the differences in angles of ψ by which TIR can be achieved at surfaces S 2  and S 3  (and their opposing faces due to symmetry) when compared to other angles of α. 
     In order for light to pass through the prism and into the window, the prism must be optically coupled to the window surface via a coupling medium  54  therebetween. Examples of optical coupling media are provided above. The coupling medium fills any minimal gap between the opposing surfaces of the prism or other optic and the substrate so that all light emerging from the prism or other optic will pass through that medium to the substrate. 
     The light is transmitted along the transparent substrate and does not emerge and is not visible outside the substrate. The purpose of this invention is to enable illumination of locations  60  on the substrate, to create letters or images or the like (i.e. indicia). At locations where the transparent substrate should have a visible illuminated area, something is placed on or performed on the substrate as at  60  to scatter light that impinges upon it from inside the substrate. As noted above, this may comprise dots printed on the substrate at locations to create an image, glass beads, fluorescent inks, roughened area of the surface, etc., whatever would cause light to scatter and exit the substrate by defeating TIR. This enables the substrate or window to be used for providing information, decoration, etc. by characters, figures, etc. that appear to be illuminated at the transparent substrate. This is an esthetically interesting and pleasing way of providing information or decoration. 
       FIG. 13  demonstrates various methods by which rays can be extracted from the window or light guide. The first extraction feature, EF 1 , could be the ink from a fluorescent marker. EF 2  is a lens-like element attached to window film WF 1 , coupled to the window via adhesive film, AF 1 . WF 1  can also be a grazing incidence hologram as taught in U.S. Pat. No. 5,710,645. EF 3  is a scattering particle or a void with the bulk of the light guide. Some scattered rays will exit the window like ray RF, while others will TIR, like ray RG. EF 4  is a surface divot that could be achieved via etching, sandblasting, or other methods used to remove material from the surface of the window. 
       FIG. 16  is a detailed cross-sectional view of a mall window with prisms and LEDs integrated into window clamps. 
     Note from this figure that optical coupling into the compliant foam within the window clamps should be minimized to avoid defeating TIR and causing absorption into the foam. This foam is used to better distribute the clamping pressure to the glass, avoiding any excess pressure on the glass that might lead to fracture. Also note that heat from the LEDs need to be considered, and can be conducted away through the window clamp and into the support tube. The transient and steady state temperatures can be predicted by suitable thermal analysis programs such as ANSYS Thermal Analysis System, and the general analytic approaches are taught in “Cooling Techniques for Electronic Equipment, 2nd Edition”, D. Steinberg, ISBN 0471524514. 
     An optically coupled window film shown in  FIG. 16  is subdivided in pieces. This represents, as a non-limiting example, computer cut window decals that are optically coupled to the window. These decals preferably comprise a scattering property such that light can be extracted from the window at selected points/areas, such as the lines of text, or a graphic image. The gaps between the film pieces assist in ensuring uniformity of the illuminated image as the light is not quickly extracted from one end of the decal without enough left to reach the center. 
       FIG. 17A  is a front view of a mall window as in  FIG. 16 , also showing a LED driver, with a wireless communications link to a remote PC, which can be used to control the intensity vs. time profile of the LEDs. In one exemplary embodiment, the profile is coordinated between a plurality of windows (and other sources of light and sound). 
       FIGS. 17B-17E  are front views of four adjacent mall windows using circular/toroidal prism coupling, each window with a unique number of LEDs illuminated. 
       FIGS. 17F-17I  are front views of four adjacent mall windows using traditional edge lighting, each window with a unique number of LEDs illuminated. Note that the angles shown within the windows are limited to −41.47°&lt;θ2&lt;41.47° in accordance with what&#39;s shown in  FIG. 6  and discussed previously relative to  FIGS. 5 and 6 . The effects of this limited angular extent within the window is clearly seen when comparing the distribution of rays between  FIG. 17E  and  FIG. 17I . The lack of uniformity in  FIG. 17I  suggests that additional LEDs are required to distribute the LED flux more evenly across the span of the glass. Further, since edge lighting by definition is confined to sources around the edge, as the window span gets wider, losses within the window (absorption, scatter, etc) makes it more difficult to get enough light to reach the opposing edge in order that beams from opposing LEDs to overlap to ensure uniform flux within the window across the span. 
     In contrast, compare the overlap in beams between  FIG. 17C  and  FIG. 17G , and the void in  FIG. 17G  between the illuminated LEDs. The edgelit approach is thus very sensitive to both the height and width of the window span. To compensate, additional LEDs can be placed along the edge, assuming that the light can travel across the span with sufficient intensity. The prism approach, conversely, can be placed anywhere on the window span—near the edge, in the middle, etc., minimizing the number of LEDs needed, and therefore minimizing both the installation cost, and the cost of energy supplied to the LEDs. 
     The general direction of the rays in  FIGS. 17F-17I  go from left-to-right and right-to-left, with no rays within the window above or below 45° from the horizontal (prior to extraction). This limited angular diversity ultimately translates into a limited diversity of rays upon extraction using commonly available (and low cost) diffusing type films. Conversely, the angular diversity of rays within the approach shown in  FIGS. 17B-17E  provide a more widely viewable indicia when extracted via the same diffusing film. 
     As an example, consider a diffusing film of the prior art as shown in  FIG. 23A  based on  FIG. 1  of Kimura et al., U.S. Pat. No. 6,771,335. This film comprises resin particles  104  dispersed in a resin binder  106  with a top surface  108 , both having similar refractive indices. The diffusing properties mainly result from the undulated surface profile since it is assumed, based on Kimura et al., that an incident ray of light is not deflected by the resin particles  104 , (Kimura et al. provides in column 2, lines 11-13, that, “urther, in the light diffusion sheet of the present invention, the difference between refractive indices of the binder resin and the resin particles is preferably 0.05 or less.”). In  FIG. 23B , there is a magnified section of  FIG. 23A  along with the geometric implications following Snell&#39;s Law. It shows that for a ray traveling within the plane of the window, striking the diffusing surface (e.g., n1 110 being equal to 1.51, the same as the window, for simplicity) at an angle, δ 112, it will be redirected into the air (n2 114 being equal to 1.0) at an angle, relative to the surface normal of the window, of φ 116 being equal to γ 118 plus β 120 which is equal to {γ+asin[(n1/n2)sin(90-δ-γ)]}, where γ 118 is the slope of the diffusing surface relative to the plane of the window. 
     The light coupled into the window can also be made to vary in time using a timing device connected to the LED, for example, by an adjustable shade on the optic or rotating the optic for example and over different areas of a window. 
     Another variable to consider is the intensity of the rays.  FIG. 24A  shows the intensity distribution of light rays exiting a lambertian LED (Lumileds Luxeon III). The highest intensity is at 0° (perpendicular to the LED die), and it then falls off in a˜cosine distribution. For this example, the distribution from 0° through 90° has been approximated by that indicated in  FIG. 24C , which is identified as θ1 in  FIG. 5 . Similarly, the intensity distribution of light rays exiting from a side emitting LED (also Lumileds Luxeon III), is shown in  FIG. 24B . As with the lambertian LED, 0° represents the angle perpendicular to the LED die. In this case, the peaks are near ±80° (hence the term side-emitter), and the distribution from −90° to 0° has been approximated by that indicated in  FIG. 24C , except the angular displacement has been shifted to start at 0° to be consistent with ψ in  FIG. 7A . 
     Looking back to  FIG. 23B , δ represents (90−θ2) in both  FIG. 5  and  FIG. 7  (angle within the window, relative to axis AX 2 , at which the light ray is incident on surface S 3 , upon which the diffuser is affixed). 
       FIGS. 25A-1  and  25 A- 2  detail both the angle, φ (columns  5 ,  7 ,  9 , . . .  21 ,  23 ), and intensity of the rays exiting into air (n 1 =1.0) from the diffuser (n=1.51) as a function of the incident angle, δ (column  3 ), and the slope, γ (columns  4 ,  6 ,  8 , . . .  20 ,  22 ), of the diffuser exit surface. 
       FIG. 25A-1  is the solution for the lambertian LED with an air gap to an edgelit window (i.e.,  FIG. 5 ), and  FIG. 25A-2  is the prism solution utilizing a side-emitting LED (i.e.,  FIG. 7 ). 
     Column  1  of  FIGS. 25A-1  and  25 A- 2  indicates the normalized intensity profiles, shown in  FIG. 24C , for both the lambertian and side-emitting LEDs, respectively. Recall that the angle δ in column  3  of  FIGS. 25A-1  and  25 A- 2  represents the angle of the LED shown in  FIG. 24C  after propagating into the refractive media of the window; i.e., (90−θ2) in both  FIGS. 5 and 7 . Particularly noteworthy is the larger angles available with the prism approach as evidenced by Column  3 . 
       FIG. 25B-1  shows that rays from the edgelit approach require a minimum 7.9° surface slope, γ, of the diffuser before light begins to emerge within 90° of normal.  FIG. 25B-2  shows that rays from the prism approach require a minimum −39.1° surface slope, γ, of the diffuser before light begins to emerge. 
     Note that the ##TIR## entries indicate that for the specified incident ray angle, δ, in column  3 , the ray cannot exit the diffuser film at the specified slope, γ, of the diffuser (it will reflect via total internal reflection). 
     The data in  FIGS. 25A-1 ,  25 A- 2 ,  25 B- 1 , and  25 B- 2  has been plotted in  FIG. 26A  for the edgelit approach and  FIG. 26B  for the prism approach. It is especially noteworthy, and quite unexpected, that the exit angles out of the diffuser for the prism approach are closer to the surface normal than that of traditional edge lighting approach. In fact, this off-normal direction has required the use of an additional prismatic film in order to straighten-out the exiting rays (Cf. U.S. Pat. No. 5,126,882). 
     Note that in  FIG. 26A  there is substantial energy outside of φ=90°. Angles beyond 90° will be redirected back towards the plane of window surface. Depending upon the system, the rays may travel through the film and re-emerge either out of the film again, out the opposing side of the window, or back into the collection of rays that TIR through the window. Conversely, in  FIG. 26B , no energy is outside of φ=90°, which was quite unexpected. 
     Therefore, with the prism coupling approach, whether circular, toroidal, or other geometric form, the light introduced into the window will be tilted closer to the normal of the window than is possible with edge lighting, thus able to exit via a simple diffusing element (e.g., a translucent window graphic film) closer to normal. 
     As a practical example, consider an illuminated graphic on a mall storefront window located on the second floor of a three floor mall having a center atrium. It is desirable to have the graphic viewable from shops on all three floors; However,  FIGS. 17I and 26A  demonstrate that the edgelit approach has less angular diversity within the window than the prism approach shown in  FIGS. 17E and 26B . 
     Other uses for the invention besides in a window display might be in an easel, a posted restaurant menu board, a table top, architectural windows, mailboxes, etc., wherever there may be a transparent substrate, an LED and the appropriate coupling arrangement to bring the light from the LED through the prism into the window or substrate. 
     Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.