Abstract:
An illumination device comprising in order from bottom to top (a) a solid transparent light transmission plate having a bottom and top surface, and (b) a light extraction unit having a light entry and a light extraction surface separated from the transmission plate by one or more optical interfaces, said light transmission plate comprising at least one cavity/light source unit comprising a cavity containing a light source and air or other material of refractive index less than the light transmission plate, wherein the light plate includes a surface for restricting the angle at which at least some of the light from the light source leaves the cavity, wherein the light extraction unit includes (1) a light polarizer (2) a light-scatterer, or (3) a light redirector.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention generally relates to a display backlight illumination assembly for producing both non-polarized and nominally polarized light and more particularly relates to an assembly that is direct lit rather than edge lit. 
       BACKGROUND OF THE INVENTION 
       [0002]    As the performance and cost of liquid crystal displays (LCDs) improve, LCDs are becoming the primary display choice in applications ranging from cell phones to large screen televisions. Since liquid crystals are not self-luminous, they require a backlight assembly for illumination. Such backlights are commonly located directly beneath an LC panel assembly, wherein the LC panel assembly serves to modulate the light emitted by the backlight, creating the image observed by the end-user. To create a visually appealing display, the backlight is required to provide a planar source of light that has a uniform spatial brightness. Further, it is desired that the backlight assembly provide this uniform spatial light distribution in an efficient manner, so that display energy requirements and heat generation may be minimized. 
         [0003]    For transmissive LCDs, there are generally two types of backlights, edge-lit and direct-lit. The edge-lit type typically comprises a lightguide, a light source partially surrounded by a reflector and positioned adjacent to at least one edge of the lightguide, and a combination of optical structures such as scattering dots, scattering particles and reflecting and refracting surfaces that couple light out of the guide and redirect it into preferred directions towards the LC panel. 
         [0004]    Unlike edge-lit backlights that have one or more light sources located around the side edges of the backlight, direct-lit backlights have their one or more light sources located directly under the backlight. Direct-lit backlights replace the lightguide plates found in edge-lit backlights with one or more light diffusing plates and films that direct light into the desired view angle. Further, these plates and films scatter the light so to produce a sufficiently uniform distribution of light emitted towards the LC panel. Replacement of the lightguide plate by the diffuser plates and additional layers typically produces direct-lit displays that are thicker than edge-lit units. The thinness of edge-lit backlights is enabled, in part, through the use of lightguide plates. Lightguide plates guide light from the one or more light sources located along the lightguide edges, and emit the light via scattering or light emitting features. Lightguides conduct light through total internal reflection, and emit light via scattering, refraction, or both. Such multiple reflections and scattering events mean that much of the light emitted towards the LC panel travels a distance at least several times greater than the physical thickness of the edge-lit backlight. These longer propagation distances allow light to mix and uniformize. In contrast, direct-lit backlights do not typically include lightguides. Light propagation and mixing typically do not occur in a horizontal direction, but in the thickness direction of the LC panel. This means that a thicker display is typically required in direct-lit constructions so to achieve an acceptable degree of spatial uniformity of brightness. 
         [0005]    Light emitted by the edge-lit and direct-lit backlights is typically randomly polarized. When this randomly polarized light is incident upon the bottom polarizer of a typical LC panel, approximately half of the light is lost through absorption. This loss due to absorption can be reduced significantly by polarizing the light prior to its impinging the bottom polarizer of the LC panel. This can be done today through the use of a reflective polarizer, inserted between the LC panel and the backlight. Examples of such reflective polarizers are described in U.S. Pat. No. 6,590,707 B1 and WO2000065385 A1 and comprise additional components that are added to an LC display in order to increase the overall energy efficiency. 
         [0006]    One way to overcome the inherent disadvantages of typical direct-lit backlight is by redirecting the light from each of the one or more sources in such a manner that it appears to be coming from the edge of the backlight, and launching this light into a lightguide disposed between the light sources and LC panel. WO2004/027466A1, U.S. Pat. No. 7,068,910 and WO2004/027467A1 present a direct-lit backlight design wherein light redirection is accomplished by an additional structure inserted between the light sources and the lightguide. Further, a diffuse reflector is inserted around the one or more light sources, so to recycle the polarization reflected in the TIR interaction. This configuration is disadvantaged relative to more common edge-lit backlight constructions in that it adds substantially to the overall thickness of the display. Additionally, the nature of the diffuse reflector does not lend itself to either improved efficiency or uniformity. 
         [0007]    U.S. Pat. No. 6,808,279 (Greiner) discloses a lighting device with an optical waveguide that has a light emission surface and a plurality of channels each with a least one substantially linear light source. 
         [0008]    While these known approaches generally provide illumination schemes for direct-lit liquid crystal display application they have certain shortcomings. The former solution while addressing uniformity makes less efficient use of the light and tends to be less compact. The latter approach does address both uniformity and compactness; however light extraction and redirection are overlooked. Thus, there remains a need in the field of backlights in direct-lit LC displays, for backlighting systems that are capable of achieving high spatial uniformity of brightness and compact thickness. In addition, there is a need for such backlights to overcome the absorptive losses that are common when randomly polarized light impinges the bottom polarizer of a typical LC panel. 
       SUMMARY OF THE INVENTION 
       [0009]    The invention provides an illumination device comprising in order from bottom to top (a) a solid transparent light transmission plate having a bottom top surface, and (b) a light extraction unit having a light entry and a light extraction surface separated from the transmission plate by one or more optical interfaces, said light transmission plate comprising at least one cavity/light source unit comprising a cavity containing a light source and air or other material of refractive index less than the light transmission plate, wherein the light plate includes a surface for restricting the angle at which at least some of the light from the light source leaves the cavity, wherein the light extraction unit includes (1) a light polarizer (2) a light-scatterer, or (3) a light redirector. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter of the present invention, it is believed that the invention will be better understood from the following description when taken in conjunction with the accompanying drawings, wherein: 
           [0011]      FIG. 1  is an exploded schematic cross-sectional view of a traditional direct-lit backlight. 
           [0012]      FIG. 2  is a schematic cross-sectional view of a first embodiment of the backlight unit using a backlight assembly according to the present invention. 
           [0013]      FIG. 3  is a schematic cross-sectional view of a second embodiment of the backlight unit using smaller bottom reflector according to the present invention. 
           [0014]      FIG. 4  is a schematic cross-sectional view of a third embodiment of the backlight unit using trapezoidal cavity for light source according to the present invention. 
           [0015]      FIG. 5  is a schematic cross-sectional view of a fourth embodiment of the backlight unit having light source wrapped around by reflectors according to the present invention. 
           [0016]      FIG. 6  is a schematic cross-sectional view of a fifth embodiment of the backlight unit that light source reflector is skewed according to the present invention. 
           [0017]      FIG. 7  is a schematic cross-sectional view of a first embodiment of a light extraction layer with microstructures comprises a micro structured anisotropic layer and an isotropic layer. 
           [0018]      FIG. 8  is a schematic cross-sectional view of a second embodiment of the light extraction layer with microstructures comprises a micro structured anisotropic layer and an isotropic layer. 
           [0019]      FIG. 9  is a schematic cross-sectional view of a third embodiment of a light extraction layer comprising of a scattering layer. 
           [0020]      FIG. 10  is a schematic cross-sectional view of a fourth embodiment of a light extraction layer comprising of a plurality of light extraction features distributed along optical interface. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    The invention is summarized above. 
         [0022]    The invention disclosed herein can reduce the overall thickness of a direct-lit backlight assembly while producing a sufficiently uniform spatial distribution of light that is directed to the LC panel of the display. In addition, light emitted from the backlight assembly may be preferentially polarized so to reduce the overall light absorption by the bottom polarizer of the LC panel assembly. 
         [0023]    The present applications thus describe display backlight illumination assemblies for producing non-polarized and nominally polarized light and more particularly relates to assemblies that are direct lit. 
         [0024]    The following definitions apply to the description herein: 
         [0025]    Optic axis refers to the direction in which propagating light does not see birefringence. 
         [0026]    Light transmission plate refers to the solid transparent plate in which light enters at angles that induce total internal reflection. 
         [0027]      FIG. 1  illustrates the structure and components of a traditional direct-lit backlight such as those presently used in LCD televisions. The traditional backlight includes an array of light sources  35 , typically cold cathode fluorescent lamps and a shaped reflector  210  located behind the array of light sources  35  for directing light towards the viewer. A thick diffuser plate  220  is usually placed over the array of light sources  35  to diffuse light from the individual sources. One or more additional diffuser films or plates  230  are located above the thick diffuser plate  220  to enhance the spatial uniformity of the backlight brightness as well as to increase the overall on-axis gain by directing light preferentially in the forward direction, towards the viewer. One of the diffuser films or plates  230  can be replaced by one or more enhancement films  231  having light redirecting features that refract light into a reduced cone angle towards the viewer. Light outside this cone angle is “recycled”, reflected back into the backlight where it propagates until it is reflected or scattered through the backlight system, towards the viewer. This traditional backlight can include a reflective polarizer film  232  located between the one or more backlight diffusers or plates  230  and enhancement films  231  and the LC panel  240 . The reflective polarizer film  232  causes one polarization state to be transmitted towards the LC panel  240 , with the orthogonal state reflected and “recycled” in the backlight. The polarization state transmitted towards the LC panel  240  is preferentially aligned with the pass axis of the bottom polarizer  241  of the LC panel  240 . The recycled orthogonal polarization state is scattered or reflected in the backlight so to regenerate some amount of light in the one desired polarization state; this amount of light in the one desired polarization state is transmitted by the backlight assembly towards the LC panel  240 . The process is repeated with the reflected light in the backlight, with additional amounts of the one desired polarization state emitted towards the LC panel. The use of the reflective polarizer film  232  is the enhanced efficiency of light utilization in the LC display, and the reduction of the amount of light absorbed by the bottom polarizer  241  of the LC panel  240 . An example is given in US2005/0135117 for direct-lit backlights. 
         [0028]      FIG. 2  shows a first embodiment of the backlight unit using a backlight assembly according to the present invention. The illumination device  1  comprises a light transmission plate  5 , a back reflector  50  located on the bottom surface  13  of the light transmission plate  5  and an unspecified light extraction unit  20  which forms an optical interface  15  with the light transmission plate  5  and has a light extraction surface  25 . Within the light transmission plate  5  there are a multiplicity of cavity/light source units  40   a  each of which are comprised of a light source  35 , a front reflector/absorber  45   a , light input surface  10  and an air filled cavity  30 . In addition the illumination device  1  has substantially parallel end reflectors  55  which may be diffuse or reflective. 
         [0029]    The thickness of the light transmission plate is adjusted to the amount of light and uniformity required for a particular application as well as the number and sizes of the sources to meet the requirements. Conveniently the thickness can range between 1 mm and 50 mm. More appropriate would be thicknesses in the range 2 mm to 20 mm. Good performance has been found in the neighborhood of 5 mm to 15 mm thickness. 
         [0030]    The illumination device  1  according to the present invention operates as follows. Cavity/light source units  40   a  located within the light transmission plate  5  in combination with the front reflector/absorber  45   a  and the back reflector  50  direct the light emitted from light source  35  to light input surfaces  10  whereby it enters the light transmission plate  5  in a range of angles which eventually impinge upon the optical interface  15  at angles equal to or greater than the angle for total internal reflection for an air interface. The unspecified light extraction unit  20  located at this interface redirects this light, causing it to exit the illumination device  1  through the light extraction surface  25 . In addition spacing, size and distribution of the cavity/light source units  40   a  can be varied to better control the spatial uniformity of the light leaving the light extraction surface  25 . Typically the light entering the lightguide ranges from approximately plus or minus 36 degrees to plus or minus 45 degrees from the light input surface  10  normal. Preferably light enters the lightguide plate within approximately plus or minus 38 to 43 degrees of the light input surface  10  normal. 
         [0031]    In another embodiment, illustrated in  FIG. 3 , the cavity/light source units  40   b  again consists of a front reflector/absorber  45   b , an air filled cavity  30  and light input surfaces  10 . However, in contrast to the first embodiment, the back reflector is replaced with individual rear reflectors  60   a . This can be done to reduce the backlight unit manufacturing cost. In a third embodiment, illustrated in  FIG. 4 , the cavity/light source units  40   c  with front reflectors/absorber  45   c  are configured to have slanted light input surfaces  10   a . It is the purpose of the slanted input surfaces  10   a  to redirect the range of angles entering the light transmission plate  5  toward a direction, which is consistent with both the acceptance and redirection properties of the unspecified light extraction unit  20 . 
         [0032]    Another embodiment is illustrated in  FIG. 5 . In this case, the cavity/light source units  40   d  consist of light sources  35  that have integrated front reflectors  75  and rear reflectors  70 . These light sources  35  are located within air filled cavities  30 . Again light enters the light transmission plate  5  through light input surfaces  10 . This is a way of simplifying the backlight unit manufacturing process. 
         [0033]    In a fifth embodiment, illustrated in  FIG. 6 , the cavity/light source units  40   e  are configured with curved rear reflectors  80 . The curved rear reflector  80  provides additional control over both the direction and angular range of light entering the light transmission plate  5  through the light input surfaces  10 . This shaping and redirection of the light distribution when combined with the properties of the unspecified light extracting unit  20  will determine the distribution and direction of the light leaving the light extraction surface  25 . 
         [0034]      FIG. 7  shows another embodiment of the direct-lit backlight according to the present invention. Therein the light extraction unit with microstructures  85   a  comprises a micro structured anisotropic layer  95   a  and an isotropic layer  90   a . The optical interface  15  is formed between the isotropic layer  90   a  and the anisotropic layer  95   a . In this embodiment, light of a particular polarization, e.g. s-polarized light is selectively redirected within the light extraction layer with microstructures  85   a  for passage through the light extraction surface  25 . The other polarization, e.g. p-polarization, is totally internally reflected within the light extraction layer with microstructures  85   a  and returned to the light transmission plate  5  for recycling. This behavior is illustrated for the s-polarization by ray  102   a  and for the p-polarization by ray  103   a . For this embodiment, light transmission plate  5  and the isotropic layer  90   a  have nearly equal refractive indices for both polarizations, which is less than the extraordinary index of anisotropic layer  95   a  and nearly equal to its ordinary index. The optical axis of the anisotropic layer  95   a  is oriented perpendicular to the plane of the  FIG. 7  which is along the x-axis. Consequently upon entering the light transmission plate  5  after passage through the light input surface  10 , ray  102   a  enters the anisotropic layer  95   a  encountering microstructures  100  at microstructure interfaces  101   a . There the s-polarized light ray  102   a  is totally internally reflected and is directed toward light extraction surface  25  where it exits the illumination device  1 . Conversely upon entering light transmission plate  5 , the p-polarized light of ray  103   a  encounters nearly equal refractive indices along its path to the light extraction surface  25  where it will be totally internally reflected and thereby returned toward the light transmission plate  5  for recycling. Additionally the microstructures  100  can be varied in position and shape or their number per unit area can be adjusted to further improve the uniformity of the light output. 
         [0035]    Another embodiment of the illumination unit according to the present invention is shown in  FIG. 8 . Here again the light extraction layer with microstructures  85   b  comprises a micro structured anisotropic layer  95   b  and an isotropic layer  90   b . However in this embodiment, light passing through optical interface  15  first encounters the anisotropic layer  95   b  before passing through isotropic layer  90   b . As illustrated by s-polarized light ray  102   b , light interaction with the microstructures  100  at the microstructure interface  101   b  causes refraction of the s-polarization toward the light extraction surface  25 . The p-polarized light ray  103   b , again passes through material having essentially the same refractive indices and is totally internally reflected at the light extraction surface  25 . 
         [0036]    Another embodiment shown in  FIG. 9 , a light extraction unit with light scattering material  86  rather than the micro structured anisotropic layers shown in  FIGS. 7 and 8  accomplishes light extraction. The light extraction unit with light scattering material  86  comprises a continuous polymeric material  110  a scattering material or voids  115 . The refractive indices of the continuous polymeric material  110  substantially matches the refractive indices of both the isotropic pressure sensitive adhesive (PSA)  105  and the light transmission plate  5 . However there is a mismatch between the indices of the polymeric layer  110  and the scattering material or void  115 . As a result light incident on light extraction unit with light scattering material  86  that further impinges the scattering material or voids  115 , such as light rays  116   a  and  116   b  will be redirected with some of the scattered light reaching the light extraction layer  25  where it will exit the illumination unit  1 . Light that reaches the light extraction surface  25  at angles smaller than the angle for total internal reflection will be transmitted by the illumination. Light that reaches the extraction surface  25  at angles greater than the angle for total internal reflection will be returned toward the light transmission plate  5  for recycling. 
         [0037]      FIG. 10  shows yet another embodiment of the illumination unit according to the present invention. The light extraction unit with microstructures  85   c  consists of a plurality of light extraction features  125  distributed along optical interface  15 . Each of the features has a refractive index that is substantially equal to that of the light transmission plate  5 . Voids  120  between features have a lower refractive index than that of the light transmission plate  5 . Consequently a light rays  140   a  and  140   d , entering light transmission plate  5  through light input surface  10  and is incident upon the light extraction feature  125  will be reflected by void surface  130  toward light extraction surface  25  as a result of total internal reflection. Conversely, light ray  140   c  entering light transmission plate  5  is incident upon optical interface  15  between light extraction features  125 . There the light ray  140   c  is totally internally reflected as a result of the lower index void  120  regions and sent back to the light transmission plate  5  for another chance to be extracted via light extracting feature  125  without any light loss. Light ray  140   b  passes through light transmission plate  5  and the cavity/light source unit  40   a  in the direction of the end reflectors  55 . Light reflected from the end reflectors  55  will be redirected back into the light transmission plate  5  for light extraction. 
       EXAMPLES 
     Comparative Example of Traditional Direct-Lit Backlight 
       [0038]    As previously discussed, a traditional direct-lit backlight includes an array of light sources  35 , a shaped reflector  210  located behind the light sources with a thick diffuser plate  220  placed over the array but at a distance to improve the spatial uniformity of the backlight brightness. Any additional diffusing films that are added for uniformity improvement capability or hiding power are generally much thinner than the typical thick diffuser plate  220 , which has a thickness on the order of two millimeters. As such, for comparative purposes the thickness of the traditional direct-lit backlight is considered as the distance from the reflector surrounding the light source array  35  to the top of the thick plate diffuser. With this as a basis a sampling of traditional systems indicates that thicknesses can range from 19 to 24 millimeters. 
         [0039]    For the system presented in WO2004027466 and U.S. Pat. No. 7,068,910, this backlight thickness is the distance from the diffuse reflector to the light output unit. The embodiments contained herein can used to arrive at an estimate of this backlight thickness. The lightguide plate, which acts as a light buffer typically, has a thickness on the order of 11 millimeters. In this system design, the light sources are located external to the lightguide plate, which adds to the overall thickness. Adding the diffuse reflector results in a backlight thickness on the order of at least 16 millimeters. Consequently this backlight configuration has a thickness within the range similar to that of the traditional system if not larger. 
       EXAMPLE 
     One Implementation of Inventive Backlight 
       [0040]    For the polarizing backlight unit  FIG. 7  described herein the thickness is considered as the distance from the back reflector  50  to the light extraction surface  25 , respectively, as the inventive systems do not require a thick plate diffusing plate. It has been found through simulations that embodiments described herein that employ cavities within the light transmission plate  5  to contain one or more light sources  35  can have a backlight thicknesses on the order of 11 millimeters, significantly smaller than the thickness of the comparative traditional backlights. 
         [0041]    The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. The patents and other publications referred to herein are incorporated by reference. 
       Parts List 
       [0000]    
       
           1 —Illumination device 
           5 —Light transmission plate 
           10 —Light input surface 
           10   a —Slanted light input surfaces 
           13 —Bottom surface 
           15 —Optical interface 
           20 —Unspecified light extraction unit 
           25 —Light extraction surface 
           30 —Air filled cavity 
           35 —Light source 
           40   a ,  40   b ,  40   c ,  40   d ,  40   e —Cavity/light source unit 
           45   a ,  45   b ,  45   c ,  45   e —Front reflector/absorber 
           50 —Back reflector/absorber 
           55 —End reflector 
           60   a ,  60   b —Rear reflector 
           70 —Rear light reflector 
           75 —Front light reflector 
           80 —Curved rear reflector 
           85   a ,  85   b ,  85   c —Light extraction unit with microstructures 
           86 —Light extraction unit with scattering material 
           90   a ,  90   b —Isotropic layer 
           95   a ,  95   b —Anisotropic layer 
           100 —Microstructures 
           101   a ,  101   b —Microstructure interface 
           102   a ,  102   b —S-polarized light ray 
           103   a ,  103   b —P-polarized light ray 
           105 —Isotropic pressure sensitive adhesive (PSA) 
           110 —Polymeric layer 
           115 —Scattering material or voids 
           116   a ,  116   b —Light ray 
           120 —Voids 
           125 —Light extraction feature 
           130 —Void surface 
           135 —Substrate 
           140   a ,  140   b ,  140   c ,  149   d —Light ray 
           210 —Shaped reflector 
           220 —Thick diffuser plate 
           230 —Diffuser film or plate 
           231 —Enhancement film 
           232 —Reflective polarizer film 
           240 —LC panel 
           241 —Bottom polarizer