Patent Publication Number: US-7591094-B2

Title: Perforated multi-layer optical film luminaire

Description:
TECHNICAL FIELD 
   This application pertains to transmissive light reflectors formed of highly reflective multi-layer optical film. Such reflectors can be used for luminance compensation in light boxes, to redirect light rays such that the rays are emitted with high luminance in a preferred direction. Such reflectors can also be used to produce high dynamic range static images having luminance values which vary as a selected function of position on the image. 
   BACKGROUND 
   Variable transmissivity light reflectors are well known prior art devices. Some light rays which are incident upon a variable transmissivity light reflector are partially transmitted through the reflector, some of the incident rays are reflected by the reflector and the remaining rays are absorbed by the reflector. The reflector&#39;s partially transmissive characteristic is not uniform, but varies as a function of the position at which the light rays are incident upon the reflector. In the simplest case, the reflector&#39;s transmissivity characteristic may be determined by just two values, one high and one low. For example, the high value may correspond to maximal transmission of incident light rays through the reflector (the “on” state) and the low value may correspond to minimal transmission of incident light rays through the reflector (the “off” state). The light emitting surface of a luminaire can be formed by providing a selected pattern of such on and off state reflector segments at predefined positions on the light emitting surface, with the pattern forming a simple image, such as letters for a sign. In more sophisticated cases the reflector&#39;s transmissivity characteristic may vary continuously as a function of position on the reflector, or may be a continuously varying half-tone pattern—in which case a grey scale photographic quality image can be produced on the luminaire&#39;s light emitting surface. 
   The two basic applications for such variable transmissivity light reflectors are luminance compensation, and production of high dynamic range static images. Luminance compensation generally involves redirection of light rays such that the rays are emitted in a preferred direction and with luminance values which vary as a selected function of position on a light emitting surface. For example, Whitehead U.S. Pat. No. 5,243,506 entitled “High Aspect Ratio Light Emitter Having High Uniformity and Directionality” employs luminance compensation to vary the degree of transmissivity of a light guide as a selected function of position to control the distribution of light emitted by the guide so as to achieve substantially uniform emission of light rays from the guide in a selected direction or within a selected angular range. Without such luminance compensation, the light guide would tend to emit light rays in a relatively nonuniform, nondirectional fashion, rendering the guide unsuitable for use in devices such as linear navigational beacons, which preferably emit maximum light intensity in a substantially horizontal direction; certain backlit liquid crystal displays, which preferably emit light only within a desired range of viewing angles; and certain vehicle signal lights, which preferably emit maximum light intensity only in desired directions. 
   To illustrate the luminance compensation problem,  FIG. 1  depicts a typical prior art light box  10  of the type used in advertising signs. The interior of light box  10  contains and is illuminated by a plurality of fluorescent tubes  12 , only two of which are shown. Light box  10 &#39;s inside rearward surface  14  and inside side surfaces  16 ,  18  are coated or lined with a reflective material such as white paint or reflective film, it being understood that the best available prior art materials have intrinsic reflectance values of about 90%. 
   Light box  10 &#39;s light emitting image display surface  20  has a variable transmissivity characteristic which varies as a function of position over light emitting surface  20 . The particular variable transmissivity characteristic is selected to suit the image to be displayed on the outside of light emitting surface  20 . That characteristic may be produced in a manner well known to persons skilled in the art, for example as explained in Whitehead U.S. Pat. Nos. 6,024,462 and 6,079,844 which are both titled “High Efficiency High Intensity Backlighting of Graphic Displays.” For example, light emitting surface  20  may incorporate a perforated reflective material—it again being understood that the best available prior art materials have intrinsic reflectance values no greater than about 90%. 
   The width W of light box  10  (i.e. the displacement between rearward surface  14  and light emitting image display surface  20 ) must not be less than a predetermined minimum value—typically, the ratio of the width W of box  10  compared to the centre-to-centre spacing S between adjacent fluorescent tubes  12 , where W/S is of order 1. Otherwise, an unacceptably large fraction of the light rays emitted by each fluorescent tube  12  will illuminate only a relatively small region  22  of light emitting surface  20  immediately adjacent the particular fluorescent tube. Due to the relatively low intrinsic reflectance value of the material incorporated in light emitting surface  20 , an unacceptably large fraction of the light rays which illuminate regions  22  are absorbed by light emitting surface  20  and “lost.” That is, such “lost” rays are neither transmitted through light emitting surface  20  to illuminate the displayed image, nor are they reflected by light emitting surface  20  back toward rearward surface  14  for further reflection and eventual transmission through some other region on light emitting surface  20 . 
   Regions  22  typically overlap portions of the image to be displayed on light emitting surface  20 . The variable transmissivity characteristic of light emitting surface  20  is accordingly selected to permit an appropriate fraction of light rays incident upon regions  22  to escape through light emitting surface  20  to illuminate the image. But the aforementioned loss of light rays due to absorption leaves insufficient light to be reflected for eventual transmission through some other region on light emitting surface  20 . Such other regions are accordingly not illuminated to the same extent as regions  22 . Consequently, observers perceive regions  22  as over-illuminated bright spots, which is undesirable. One prior art solution to this problem is to increase the width W of light box  10  to broaden regions  22  as shown in  FIG. 2  and thereby reduce the perceptibility of bright spots on light emitting surface  20 . However this unavoidably increases the size of light box  10 , which is undesirable. Another prior art solution to the foregoing problem is to adjust the variable transmissivity characteristic of light emitting surface  20  to reduce the light transmission capability of light emitting surface  20  in each of regions  22 , while making corresponding adjustments to the variable transmissivity characteristic of light emitting surface  20  outside regions  22 . Such adjustment involves a cumbersome, time-consuming, iterative trial and error technique requiring a custom solution for every different light box (and for every different high dynamic range image). This application addresses the foregoing problem. 
   This application also discloses display of high dynamic range images. Dynamic range is the ratio of intensity of the highest and lowest luminance parts of a scene. For example, the image projected by a video projection system may have a maximum dynamic range of 300:1. This relatively low dynamic range is due to the relatively limited range of luminance values which can be reproduced by a typical video projection system. By contrast, the human visual system is capable of recognizing features in scenes which have very high dynamic ranges. For example, a person can look into the shadows of an unlit garage on a brightly sunlit day and see details of objects in the shadows, even though the luminance in adjacent sunlit areas may be tens of thousands of times greater than the luminance in the shadow parts of the scene. 
   There are many high dynamic range image situations which the human eye can perceive well, but which cannot be effectively displayed due to the dynamic range limitations of conventional image display systems. Examples include most situations where sources of light are in the field of view, such as sunset scenes, scenes containing highly reflective (“shiny”) surfaces, or night scenes containing illuminated neon signs, lamps, etc. The ability to display a larger dynamic range of luminance values would facilitate production of more visually effective graphic images, such as scenes of the aforementioned type which contain sources of light. This would in turn have value both aesthetically and in more effective advertising. However, to display a realistic rendering of a scene of the foregoing type can require a display having a dynamic range in excess of 1000:1. In this specification, the term “high dynamic range” means dynamic ranges of 800:1 or more. 
   The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     Exemplary embodiments are illustrated in referenced figures of the drawings. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than restrictive. 
       FIG. 1  is a schematic top cross-sectional view (not to scale) of a prior art light box. 
       FIG. 2  shows (not to scale) the width of the  FIG. 1  light box increased to reduce the perceptibility of undesirable bright spots. 
       FIG. 3  is a schematic top cross-sectional view (not to scale) of a light box in a luminance compensation context. 
       FIG. 4  depicts (not to scale) an enlarged fragmented portion of the  FIG. 3  light box. 
       FIG. 5A  graphically depicts a Monte Carlo ray tracing simulation of luminance distribution over the light emitting surface of a single light bulb prior art light box schematically depicted below the graph.  FIG. 5B  graphically depicts a Monte Carlo ray tracing simulation of luminance distribution over the light emitting surface of an improved single light bulb light box as schematically depicted below the graph. In both graphs luminance is plotted as a function of horizontal position on the surface of the light box. 
       FIG. 6  is a schematic top cross-sectional view (not to scale) of a light box in a high dynamic range image display context. 
   

   DESCRIPTION 
   Throughout the following description, specific details are set forth in order to provide a more thorough understanding of what is disclosed. However, what is disclosed may be practiced without these particulars. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the disclosure. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive, sense. 
   Vikuti™ Enhanced Specular Reflector (ESR) multi-layer optical film (available from 3M Electronic Display Lighting, Optical Systems Division, St. Paul, Minn.) is preferably used as the reflector material in a variable transmissivity reflector. Such film has an intrinsic reflectance value of about 99%, meaning that about 99% of all light rays incident upon the film are reflected. Prior art variable transmissivity reflectors are typically formed using materials having intrinsic reflectance values no greater than about 90%. Although maximal benefit is attained by utilizing a multi-layer optical film having an intrinsic reflectance value of about 99% or greater, persons skilled in the art will understand that significant benefits can be attained by utilizing a multi-layer optical film having an intrinsic reflectance value of about 98% or greater, with lesser—albeit acceptable in some applications—benefits being attainable by utilizing a multi-layer optical film having an intrinsic reflectance value greater than about 95%. 
   Luminance Compensation 
   One embodiment facilitates luminance compensation of light boxes like those depicted in  FIGS. 1 and 2 .  FIG. 3  depicts such a light box  30  having a light emitting surface  32  having an intrinsic reflectance value greater than 95% and preferably about 99% or greater. This can for example be achieved by forming light emitting surface  32  of the Vikuti™ ESR multi-layer optical film mentioned above. A large plurality of perforations  34  are provided through light emitting surface  32 , to give light emitting surface  32  a desired macroscopically non-varying extrinsic reflectance-reducing transmissivity characteristic as explained below. The size of and positional distribution of perforations  34  is greatly exaggerated in  FIG. 1 . In practice, each perforation  34  has a diameter of about 0.5 mm and the perforations are macroscopically positioned with uniform density per unit area on light emitting surface  32  to impart the desired macroscopically non-varying transmissivity characteristic to light emitting surface  32  in a manner well known to persons skilled in the art, as aforesaid. 
   The interior of light box  30  contains and is illuminated by a plurality of fluorescent tubes  36 , only two of which are shown in  FIG. 3 . Light box  30 &#39;s inside rearward surface  38  and inside side surfaces  40 ,  42  are formed of or lined with a material (e.g. the Vikuti™ ESR multi-layer optical film mentioned above) having an intrinsic reflectance value greater than 95% and preferably about 99% or greater. The width W of light box  30  can be less than would normally be tolerable. More particularly, the ratio W/S of the width W of light box  30  compared to the centre-to-centre spacing S between adjacent fluorescent tubes  36 , can be of order 0.1—a 10-fold reduction in comparison to the  FIG. 1  prior art structure. 
   Forming light emitting surface  32  of multi-layer optical film achieves more efficient utilization of light rays emitted by fluorescent tubes  36 . Moreover, because multi-layer optical film can reflect light rays many times before the rays are absorbed and lost, light emitting surface  32  may have a non-varying transmissivity characteristic. That is, the transmissivity characteristic may simply be a macroscopically constant, low light transmission value at all points on the surface of light emitting surface  32 , without causing an unacceptable loss in efficiency. 
   For example, if the size and positional distribution of perforations  34  are selected such that 10% of the light rays emitted by fluorescent tubes  36  are transmitted directly through perforations  34  without reflection (as in the case of ray  44  shown in  FIG. 4 ), the high reflectance of light emitting surface  32  ensures that substantially all of the remaining 90% of light rays will eventually be transmitted through perforations  34  after an average of about 20 reflections per light ray (as schematically illustrated by rays  46 ,  48  and  50  shown in  FIG. 4 ). Because that remaining 90% of light rays undergo many reflections before being transmitted through a randomly encountered one of perforations  34 , the net effect is that the light rays are transmitted more uniformly through all points on the surface of light emitting surface  32  than would otherwise be the case. 
   Light box luminance compensation utilizing prior art reflective materials requires cumbersome, time-consuming, iterative trial and error techniques which must be customized for each light box in order to compensate for light absorption losses by imparting a variable transmissivity characteristic to the reflective material. The need for such compensation can be avoided—instead of utilizing a reflector with a variable transmissivity characteristic, one may employ a reflective material having a macroscopically non-varying extrinsic reflectance-reducing transmissivity characteristic as aforesaid. For example, a suitable reflector can be constructed by perforating multi-layer optical film to give the film a macroscopically constant, low light transmission value—a very significant advantage over the prior art. 
     FIGS. 5A and 5B  respectively schematically depict Monte Carlo ray tracing simulations of a single light bulb thin prior art light box ( FIG. 5A ), and an improved light box ( FIG. 5B ). The relatively uniform luminance of the  FIG. 5B  embodiment is made apparent by the relatively flat plot of luminance values. The graphical portion of  FIG. 5A  depicts a slight dip in the luminance values directly above the fluorescent tube. This is due to the high reflectance of the multi-layer optical film. In most cases, especially at points on the light emitting surface which are close to the fluorescent tube, the luminance perceived by an observer is a composite of (1) luminance due to light rays which are transmitted directly from the fluorescent tube through perforations  34  without reflection; and (2) luminance due to reflection of the tube&#39;s image in the multi-layer optical film. However, if the light box is viewed from directly above, as illustrated in  FIG. 5A , the luminance contribution of light rays due to reflection of the fluorescent tube&#39;s image is largely obscured by the tube itself. This results in the slight dip in luminance intensity shown in  FIG. 5A . 
   It is not essential to perforate multi-layer optical film to permit light to escape through the film in order to achieve luminance compensation as described above. Other techniques can be used to allow light to controllably escape through the film. One approach is to optically couple a diffusive material to both sides of the multi-layer optical film to controllably enable some light to escape through film, as disclosed in Liu et al U.S. Pat. No. 6,208,466 issued 27 Mar. 2001. As one example, a half-tone or dot pattern of diffusive white ink can be printed on the film to control the amount of light transmitted through the film. Another approach is to “damage” the film in selected regions by disrupting the film&#39;s light reflecting capability and imparting a light transmissive capability to the film in such regions, e.g. by thermally degrading the film in such regions, or by using a laser beam to render the film substantially transparent in such regions, without perforating the film. 
   High Dynamic Range Image Display 
   A second embodiment facilitates production of high dynamic range static images. The second embodiment also utilizes multi-layer optical film having an intrinsic reflectance value greater than 95% (preferably about 99% or greater) and having a predefined variable transmissivity characteristic, corresponding to a predefined static image such as an advertisement which is to be displayed by mounting a transparent sheet  60  ( FIG. 6 ) bearing the image on light box  62  and operating light box  62  to back light the image. 
   Light box  62  has a light emitting surface  64  having a first portion corresponding to a substantial area of light emitting surface  64 , and a second portion corresponding to the remaining area of light emitting surface  64 , excluding the first portion. Neither the first portion nor the second portion need be a contiguous segment of light emitting surface  64 ; each portion may be a plurality of non-contiguous segments of light emitting surface  64 . The first portion of light emitting surface  64  is formed of multi-layer optical film having an intrinsic reflectance value greater than 95% and preferably about 99% or greater. The first portion of light emitting surface  64  also has a first extrinsic reflectance-reducing characteristic (e.g. perforations) giving the first portion a first light transmissivity characteristic of less than 5%, the first transmissivity characteristic being macroscopically invariant as a function of position over the first portion. 
   The second portion of light emitting surface  64  has a second extrinsic reflectance-reducing characteristic giving the second portion a second light transmissivity characteristic of greater than 25%. For example, a large plurality of perforations  66  can be provided through the second portion of light emitting surface  64 , to give the second portion the desired second light transmissivity characteristic of greater than 25%. The size and positional distribution of perforations  66  is greatly exaggerated in  FIG. 6 . In practice, each perforation  66  may have a diameter of about 0.5 mm. However, the diameter of perforations  66  and their density per unit area on the second portion of light emitting, surface  64  can be selectably varied, in a manner well known to persons skilled in the art, to allow more or less light to escape through selected regions of the second portion of light emitting surface  64  so that brighter regions of image  60  will be illuminated more than darker regions of image  60 , thus imparting the desired overall transmissivity characteristic to light emitting surface  64 . 
   The interior of light box  62  contains and is illuminated by a plurality of fluorescent tubes  68 , only two of which are shown in  FIG. 6 . That is, the inward side of light emitting surface  64  is backlit. Light box  62 &#39;s inside rearward surface  70  and inside side surfaces  72 ,  74  are lined with multi-layer optical film having an intrinsic reflectance value greater than 95% and preferably about 99% or greater. 
   The variable transmissivity characteristic of light emitting surface  64  corresponds to sheet  60 , which bears a static image. Sheet  60  extends substantially parallel to and in close proximity to the outward side of light emitting surface  64 . The image consists of one or more normal luminance display regions and one or more high luminance display regions. Each normal luminance display region has the same size and shape as a corresponding segment of the first portion of light emitting surface  64 . The normal luminance display regions have a third transmissivity characteristic which varies as a selected function of a desired normal luminance characteristic of the image. Each high luminance display region has the same size and shape as a corresponding segment of the second portion of light emitting surface  64 . The high luminance display regions have a fourth transmissivity characteristic which varies as a selected function of a desired high luminance characteristic of the image. The third and fourth transmissivity characteristics of image-bearing sheet  60  are selected such that, in combination with the first and second transmissivity characteristics of light emitting surface  64 , the resultant mathematical product of reflectances yields a net reflectance as a function of position corresponding to a selected high dynamic range image. Accordingly, the first, second, third and fourth light transmissivity characteristics together impart the desired high dynamic range to the image when the inward side of light emitting surface  64  is backlit. 
   Those portions of sheet  60  bearing high luminance display regions of the image (e.g. brighter parts of the image which are to be displayed at increased luminance) are more highly perforated than portions of sheet  60  bearing normal luminance display regions of the image which are to be displayed at reduced luminance (e.g. darker parts of the image). Alternatively, one may selectably remove those portions of the film which bear the high luminance display regions of the image in order to maximize the luminance of certain image highlights corresponding to those regions. The previously mentioned techniques can also be used to allow light to controllably escape through the film, without perforating the film. That is, one may optically couple a diffusive material to both sides of the multi-layer optical film to controllably enable some light to escape through film, as disclosed in Liu et al U.S. Pat. No. 6,208,466 issued 27 Mar. 2001; or, “damage” the film in selected regions by disrupting the film&#39;s light reflecting capability and imparting a light transmissive capability to the film in such regions. 
   The highly reflective multi-layer optical film “recycles” light rays which would otherwise be lost due to absorption by a prior art reflective material having a lower intrinsic reflectance value than the preferred multi-layer optical film. Specifically, the high reflectance of light emitting surface  64  ensures that most light rays emitted by fluorescent tubes  68  which are not transmitted through perforations  66  (or which do not escape through the film in accordance with some other technique) are reflected within light box  62  and eventually transmitted through perforations  66  after an average of about 20 reflections per light ray. This is especially advantageous in the display of high dynamic range images, since in most such images only a very small amount of the image is at full brightness. High light reflectance within light box  62  makes it possible to achieve much higher brightness illumination of the image (due to low loss multiple reflections of light rays) than would otherwise be the case. 
   In summary, high dynamic range images can be produced in either of two distinctly different ways. The first method uses a variably transmissive multi-layer optical film, in which regions corresponding to the bright regions of the image are more transmissive and regions corresponding to the dark regions of the image are less transmissive. The desired variable transmissivity characteristic can be achieved by either varying the size of the light transmissive perforations, or varying the size of the light transmissive pattern components (e.g. diffusive white ink dots), as long as the individual perforations or pattern components are invisible at reasonable viewing distances; and/or by varying the density of the light transmissive perforations or pattern components. When such a variably transmissive multi-layer optical film layer is combined with the image, the result is a high dynamic range image. The second method combines a uniformly transmissive multi-layer optical film with the image. To achieve high dynamic range, the film can be entirely removed in selected regions in order to maximize the luminance of image highlights corresponding to those regions. 
   The above-described luminance compensation technique can also be applied to the display of high dynamic range static images to reduce the width W of light box  62 , making it possible for light box  62  to be thinner than would other wise be the case, improving the practicality of light box  62  in image display applications. 
   Variably transmissive multi-layer optical film suitable for use with either the luminance compensation or high dynamic range image display embodiments described above can be fabricated in various ways. As one example, the film itself can be modified to degrade its light reflecting capability and enhance its light transmitting capability. In principle this is easily done since it is difficult in practice to fabricate multi-layer optical film with suitably high reflectance. It is less challenging, in practice, to fabricate a film having a lower reflectance characteristic and a selected transmittance characteristic, although it can be difficult to achieve uniform transmittance as a function of wavelength, especially for all viewing angles. As another example, highly reflective multi-layer optical film can be perforated as aforesaid. In principle the perforations can be so small that they are imperceptible to an observer when the film is viewed from a reasonable distance (e.g. distances typical for observing signs) or viewed through a diffuser applied over the film or over the image. Spatial techniques can also be used to vary the film&#39;s light transmitting capability, e.g. by applying a positionally varying half tone pattern to the film, with the pattern varying in proportion to the desired level of light transmission at each position on the image. Another approach is to employ a film having a non-zero, but low light transmittance characteristic (say 5%), and perforate only those portions of the film corresponding to high brightness regions of the image. Automated cutting devices are readily available in the sign industry and are easily adapted to such perforation. 
   As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible without departing from the spirit or scope of this disclosure. Accordingly, the scope of the disclosure is to be construed in accordance with the substance defined by the following claims.