Abstract:
An optical structure placeable between a backlight array of point light sources and a planar display. The structure distributes light emitted by the point light sources to uniformly illuminate the plane of the display, without introducing significant viewing parallax. The emitted light is partially collimated within a preferred angular viewing range, maximizing the display&#39;s luminance when viewed from the normal direction. The structure is highly reflective, such that a substantial portion of any non-emitted light rays are internally reflected by the structure, increasing the likelihood that those rays will be subsequently emitted by the structure.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 13/116,337 filed 26 May 2011, which is a continuation of U.S. patent application Ser. No. 11/831,915 filed 31 Jul. 2007 now issued as U.S. Pat. No. 7,973,878, which is a continuation of U.S. patent application Ser. No. 11/572,812 accorded the filing date of 26 Jan. 2007 now issued as U.S. Pat. No. 7,583,331, which is the United States National Stage of International Application No. PCT/CA2005/001111 filed 15 Jul. 2005, which claims the benefit of the filing date of U.S. Provisional Patent Application No. 60/591,087 filed on 27 Jul. 2004. For the purposes of the United States, this application claims the benefit under 35 U.S.C. §119 of U.S. Patent Application No. 60/591,087 filed on 27 Jul. 2004. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention pertains to displays of the type which have an array of light sources serving as a backlight. Light from the light sources is distributed to achieve suitably uniform spatial and angular illumination of the display, while maintaining high luminance in the display&#39;s normal viewing direction. 
       BACKGROUND 
       [0003]    This invention pertains to backlights for displays which have a transmission-type light modulator illuminated by a backlight. Examples of such displays include some liquid crystal displays (LCDs) as well as high dynamic range displays of the type disclosed in international patent publication WO 02/069030 published 6 Sep. 2002 and in international patent publication WO 03/077013 published 18 Sep. 2003, both of which are incorporated by reference herein. 
         [0004]    High dynamic range displays like those disclosed in the above publications incorporate a light source layer (which may be called a “backlight”) and a display layer that includes a light modulator. The backlight is controlled to produce a light pattern that represents a comparatively low-resolution version of an image to be displayed. The low-resolution image is modulated by the display layer to provide a comparatively high resolution image for perception by an observer. 
         [0005]    The backlight typically comprises an array of point type actively modulated light sources, such as light emitting diodes (LEDs). The display layer, which is positioned and aligned in front of the backlight, may be a liquid crystal display (LCD) panel or the like. Maintenance of a relatively small separation distance between the two layers allows light emitted by adjacent light sources of the backlight to merge smoothly into one another such that each pixel of the high resolution image is illuminated. Suitable image compensation techniques may be applied to remove undesirable image blurring artifacts. 
         [0006]    In many planar illumination applications (e.g. not only in planar displays as mentioned above, but also in some general illumination situations) it is desirable to uniformly illuminate (i.e. backlight) a plane. Multiple LEDs arranged in an array can be used in such applications since they provide a robust, low-power alternative to incandescent light sources. However, LEDs provide only point source illumination, not uniform planar illumination. It is consequently necessary to somehow distribute the light emitted by LEDs of a LED array so as to uniformly illuminate the plane. 
         [0007]    In display applications it is also desirable to avoid parallax (apparent changes in the direction of an object, due to changes in the observer&#39;s position which correspond to different lines of sight to the object) between each LED and the illuminated display area directly in front of the LED. Otherwise, an observer will perceive changes in that area if the area is viewed from different angles, which is undesirable. 
         [0008]    The parallax problem has prevented attainment of uniform planar illumination in situations where each point type light source interacts in some manner with the display area directly in front of the light source, as is the case for LED/LCD type high dynamic range displays in which each LED corresponds to a specific pixel or cluster of pixels on the LCD display. In such displays each LED should primarily illuminate the LCD pixels directly in front of the LED. This illumination characteristic should remain substantially invariant as the observer&#39;s viewing angle changes. 
         [0009]    It is also desirable that the light emitted by the backlight be partially collimated within a preferred angular viewing range, namely within about 25° of the display&#39;s normal direction in order to maximize the display&#39;s luminance when it is viewed from the normal direction. It is additionally desirable that the optical structure as a whole (i.e. anything between the light source layer and the display layer) be reasonably reflective, in order to maximize the efficiency of the reflective polarizers incorporated in state-of-the-art LCD displays and thereby minimize light loss due to polarization. 
       SUMMARY OF THE INVENTION 
       [0010]    This invention provides displays that include an optical structure between a backlight array of light sources and a display layer. The light sources may be point light sources such as light emitting diodes (LEDs). The structure distributes light emitted by the point light sources. The invention also provides optical structures that may be used to distribute light from arrays of point sources and related methods. 
         [0011]    Various aspects of the invention and features of embodiments of the invention are described below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The appended drawings illustrate non-limiting example applications of the invention. 
           [0013]      FIG. 1  is a greatly enlarged not-to-scale cross-sectional side elevation view of a fragmented portion of a planar light distribution structure. 
           [0014]      FIG. 2  is a greatly enlarged not-to-scale cross-sectional side elevation view of a fragmented portion of another planar light distribution structure having an additional diffuser. 
       
    
    
     DESCRIPTION 
       [0015]    Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention 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 invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense. 
         [0016]      FIG. 1  depicts a layered planar light distribution structure  10 . Some light sources  12 , which may be LEDs, of a display backlight  13 , and a display panel  14 , which may comprise an LCD panel or other light modulator, are shown schematically in  FIG. 1 . Light distribution structure  10  incorporates a rear reflector  16 , a light-diffusing volume  18 , an optional reflective polarizer  20 , and an optional angularly selective light transmitter  22 . 
         [0017]    Rear reflector  16  has an array of transparent regions  24  at locations corresponding to LEDs  12  of the backlight array. Transparent regions  12  may comprise apertures or windows that are substantially transparent to at least some light emitted by LEDs  12 . One transparent region  24  is provided for each LED  12 . Regions  24  are sized and aligned to match the size and alignment of LEDs  12 . For example, LEDs  12  may be arranged in a rectangular array, a hexagonal array, a square array, or the like, and regions  24  may be arranged in a pattern that matches the arrangement of LEDs  12 . Regions  24  may also be shaped to match LEDs  12 . 
         [0018]    In the illustrated embodiment, LEDs  12  lack lenses. Such lensless LEDs emit light approximately in a Lambertian pattern (i.e. the emitted light has an intensity that varies with viewing angle in such a way that the intensity is directly proportional to the cosine of the viewing angle). In alternative embodiments, LEDs or other light sources may have lenses or may be otherwise constructed to emit light in a non-Lambertian manner. Each region  24  transmits light emitted by the corresponding, immediately adjacent LED  12  into diffusing volume  18 . 
         [0019]    LEDs  12  may be outside diffusing volume  18 , as shown. In the alternative, LEDs  12  could extend through regions  24 , which may be apertures, and project slightly into diffusing volume  18 . 
         [0020]    The face  26  of rear reflector  16  that faces into diffusing volume  18  is highly reflective. Face  26  is preferably at least partially specularly reflective (i.e. the angle of reflection substantially equals the angle of incidence, in contrast to a diffuse reflector) and may be substantially entirely specularly reflecting. Perforated “radiant mirror film” available from 3M Specialty Film and Media Products Division, St. Paul, Minn. is one example of a material that can be used to form rear reflector  16 . 
         [0021]    The thickness dimension  28  of light diffusing volume  18  (which may be an air gap in the simplest case) is preferably minimized, while retaining sufficient thickness that light rays passing from rear reflector  16  into diffusing volume  18  are asymmetrically diffused (i.e. light rays are scattered in many directions) before the rays pass through reflective polarizer  20 . Consequently, any directional characteristic of light rays which enter diffusing volume  18  is essentially absent from light rays which exit diffusing volume  18 . 
         [0022]    Thickness dimension  28  can be reduced in cases where diffusing volume  18  has an anisotropic scattering coefficient such that light rays which are substantially parallel to normal viewing direction  30  are more intensely scattered than light rays which are substantially perpendicular to normal viewing direction  30 . Such anisotropic scattering can be achieved by placing, within diffusing volume  18 , multiple layers of a thin, weakly light-scattering sheets  19  (See  FIG. 2 , not shown in  FIG. 1 ). Partial reflection of light at surfaces of sheets  19  causes substantial scattering of light traveling in a direction substantially parallel to normal viewing direction  30 . Sheets  19  may be made of a suitable transparent polymer material, for example. Anisotropic scattering could also be caused by providing within diffusing volume  18  a transparent medium such as a suitable resin or gel doped with white particles (e.g. particles of paint pigment or the like) or other diffusely-reflecting particles. 
         [0023]    Optional reflective polarizer  20  (which has a polarization characteristic matched to that of reflective polarizers incorporated in LCD display panel  14 ) reflects rearwardly into diffusing volume  18  light rays having polarization characteristics which are unmatched to the polarization characteristics of polarizers incorporated in LCD display panel  14 . Such unsuitably polarized light rays undergo further diffusion within diffusing volume  18  and are again reflected (“recycled”) by rear reflector  16  toward optional reflective polarizer  20 . 
         [0024]    Diffusion within diffusing volume  18  randomizes the polarization characteristics of recycled light rays such that some of the recycled rays are eventually able to pass through reflective polarizer  20  toward LCD display panel  14 . Any remaining unsuitably polarized light rays are again recycled as aforesaid by reflective polarizer  20 , diffusing volume  18 , and rear reflector  16  until the polarization characteristics of the recycled rays matches that of polarizer  20  so that the recycled rays can pass through reflective polarizer  20  toward LCD display panel  14 . 
         [0025]    Optional angularly selective light transmitter  22 , may be formed by crossing, at 90° to each other, the microreplicated prism structures on two parallel sheets of Vikuiti™ Brightness Enhancement Film available from 3M Specialty Film and Media Products Division, St. Paul, Minn. Light transmitter  22  selectively transmits partially collimated light rays toward LCD display panel  14  in a direction substantially parallel to normal viewing direction  30 , while rearwardly reflecting a substantial portion of any non-emitted light rays back toward reflective polarizer  20  and rear reflector  16  so that the non-emitted rays may be further reflected (“recycled”) for subsequent emission through light transmitter  22 . 
         [0026]    Layered planar light distribution structure  10  is highly efficient in the sense that it is characterized by low light absorption losses. If rear reflector  16 , any material within diffusing volume  18 , reflective polarizer  20 , if present, and angularly selective light transmitter  22 , if present, are made of materials that do not substantially absorb the light emitted by LEDs  12 , almost all light rays emitted by LEDs  12  into structure  10  can eventually be emitted through structure  10  toward LCD display panel  14 . Unwanted image artifacts are significantly reduced and, in some cases, substantially eliminated, due to the structure&#39;s highly diffuse character. 
         [0027]    For clarity and conciseness various elements which can be provided by those skilled in the art are not described in detail herein. For example, a display incorporating an optical structure as described herein would include suitable driving circuits to cause LEDs  12  or other light emitters to emit light. Such circuitry may optionally permit the brightness of LEDs  12  or other light sources to be individually modulated. Any suitable driving circuits may be used including those known to those of skill in the art. Further, a display typically has suitable driving circuits for operating individual pixels in a display panel to modulate light according to image data corresponding to an image to be displayed on the display. Suitable display panel driving circuits are also known to those skilled in the field of this invention. Consequently, the driving circuitry for LEDs  12 , the driving circuitry for display panel  14  and other well-understood elements such as power supplies and the like are not described in detail herein. 
         [0028]    Where a component (e.g. a member, part, assembly, sheet, collimator, reflector, etc.) is referred to above, unless otherwise indicated, reference to that component (including a reference to a “means”) should be interpreted as including as equivalents of that component any component which performs the function of the described component (i.e., that is functionally equivalent), including components which are not structurally equivalent to the disclosed structure which performs the function in the illustrated exemplary embodiments of the invention. 
         [0029]    As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. For example:
       either or both of reflective polarizer  20  and angularly selective light transmitter  22  may be omitted—satisfactory results can be obtained by providing only rear reflector  16  and light diffusing volume  18  in light distribution structure  10 . It is however preferable to include reflective polarizer  20  in order to increase the light output capability of structure  10 , since unsuitably polarized light rays are otherwise lost. It is also preferable to include angularly selective light transmitter  22  in order to increase the brightness of light emitted by structure  10 —albeit at the expense of a reduced viewing angle, since light transmitter  22  partially collimates light which passes through it. This can be offset by providing an additional diffuser  32  as shown in  FIG. 2  between light transmitter  22  and LCD display panel  14 . Additional diffuser  32  increases the viewing angle by laterally diffusing light rays which are narrowly diffused by passage through light transmitter  22 .   When reflective polarizer  20  and angularly selective light transmitter  22  are both present they may be arranged in either order.   It is not necessary that rear reflector  16  be perfectly flat. Rear reflector  16  could be bumpy on a small scale (i.e. have a surface structure). The faces of diffusing volume  18  should be generally parallel to one another so that light will be emitted substantially uniformly from diffusing volume  18 .       
 
         [0033]    While a number of example aspects and embodiments have been discussed above, those of skill in the art will recognize certain modifications, permutations, additions and sub-combinations thereof. It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions and sub-combinations as are within their true scope.