Abstract:
A lens for use in a lighting arrangement, such as an LCD backlight, includes a light guiding region and a lens surface. The light guiding region is shaped to focus light from a light source toward a region of the lens located on its periphery. The lens surface has reflection regions for reflecting light from the light source back into the lens and transmission regions for transmitting light from the light source outside the lens. A characteristic of the transmissions regions, such as width, varies as a function of the location of the light source so as to create substantially uniform average light distribution for areas of the lens positioned at various distances with respect to the light source.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a backlight for a liquid crystal display (LCD), and more particularly, to a lens configuration for increased light distribution uniformity in an LCD backlight. 
   2. Description of the Related Art 
   The active parts of liquid crystal display (LCD) panels, such as polarizers, liquid crystal layers, indium tin oxide electrodes, supports, etc., are relatively thin. However, standard designs for direct or edge-lit backlight assemblies for LCDs are bulky, and need to be custom designed for each LCD panel size. In fact, most of the physical volume of a typical/conventional backlit LCD panel is occupied by the backlight assembly itself. 
   The reason for the bulkiness of the backlight assembly is the space required for light originating at a few discrete points in the assembly, to be thoroughly randomized, or diffused, so that the liquid crystal (LC) layer is illuminated by a uniformly distributed flux of light. Due to surface size constraints, only a small number of LEDs can be used for backlighting systems. The LEDs need to illuminate an entire LCD. A bulky diffuser is thus required to make the intensity of the light from the small number of LEDs uniform across the LC screen. 
   The number of LEDs used for backlighting LCD systems may be increased. However, a larger number of LEDs leads to higher cost, increased energy consumption, and poses reliability problems. Moreover, regardless how many LEDs are used, each LED creates a highly non-uniform local lighting profile, as the intensity of light generated by an LED decreases as a function of distance from the LED. 
   Disclosed embodiments of this application address these and other issues by utilizing lens that are configured to increase uniformity of light distribution in an LCD backlight arrangement. 
   SUMMARY OF THE INVENTION 
   In one aspect, the present invention is directed to a lens for use in a lighting arrangement. The lens comprises: a light guiding region, which is shaped to direct light from a light source toward a region of the lens located on the periphery of the lens; and a lens surface, the lens surface having reflection regions for reflecting light from the light source back into the lens and transmission regions for transmitting light from the light source outside the lens. A characteristic of the transmissions regions varies as a function of the location of the light source so as to create substantially uniform average light distribution for areas of the lens positioned at various distances with respect to the light source. 
   According to another aspect, the present invention is an LCD backlight comprising a plurality of lenses arranged in a pattern. Each lens includes: a light guiding region, which is shaped to direct light from a light source toward a region of the lens located on the periphery of the lens; and a lens surface, the lens surface having reflection regions for reflecting light from the light source back into the lens and transmission regions for transmitting light from the light source outside the lens. A characteristic of the transmission regions varies as a function of location of the light source so as to create substantially uniform average light distribution for areas of the lens positioned at various distances with respect to the light source. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further aspects and advantages of the present invention will become apparent upon reading the following detailed description in conjunction with the accompanying drawings. These drawings do not limit the scope of the present invention. In these drawings, similar elements are referred to using similar reference numbers, wherein: 
       FIGS. 1A and 1B  illustrate the configuration of a typical liquid crystal display (LCD) device; 
       FIG. 2  illustrates a direct backlight illumination arrangement for typical backlit LCD devices; 
       FIG. 3  illustrates the light distribution of a pinpoint source in a rectangular tile used in typical backlit LCD devices; 
       FIG. 4A  illustrates a top view of a configuration for a light source cavity, for use in a lens for LCD backlights according to an embodiment of the present invention; 
       FIG. 4B  illustrates a cross-section of the light source cavity illustrated in  FIG. 4A , for use in a lens for LCD backlights according to an embodiment of the present invention; 
       FIG. 5A  illustrates a top view for a lens for LCD backlights according to an embodiment of the present invention; 
       FIG. 5B  illustrates a cross-sectional side view for the lens for LCD backlights illustrated in  FIG. 5A , according to an embodiment of the present invention; 
       FIG. 6  illustrates a rectangular lens for LCD backlights, according to an another embodiment of the present invention; and 
       FIG. 7  illustrates a tile matrix arrangement of lenses for LCD backlights, according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   Aspects of the invention are more specifically set forth in the accompanying description with reference to the appended figures.  FIGS. 1A and 1B  illustrate the configuration of a typical liquid crystal display (LCD) device with backlighting. 
   As shown in  FIG. 1A , a typical LCD device  1  includes a liquid crystal (LC) layer  20  sandwiched between two polarizing filters  30 A and  30 B (hereafter “polarizers”). The LC layer is protected by a transparent front protective sheet  10 , e.g., a glass plate. Behind the LC and polarizing layers are a light diffusing film  40  (hereafter “diffuser”), a backlight source  50 , and a reflective surface  60 . A casing or enclosure  70  is provided to hold the aforementioned layers in place.  FIG. 1B  illustrates an exploded view of the stack of LCD layers described above. The specification may collectively refer to these layers as the “LCD stack” of a backlit LCD device (including diffuser  40  and backlight source  50 ). 
   In a typical backlit LCD device (also referred to as a “transmissive” LCD device), the backlight is emitted directly from source  50  and reflected from reflective surface  60  to the diffuser  40 . The diffuser  40  diffuses this light to make the intensity or brightness more uniform across the LCD. Polarizers  30 A and  30 B are cross-polarized with respect to each other. 
   Thus, the backlight polarized by polarizer  30 B must be rotated to some extent by LC layer  20  in order to pass through polarizer  30 A. The degree to which the LC layer  20  rotates the light is dependent upon the amount of voltage applied across the various liquid crystal molecules in the LC layer  20 . For instance, a pair of electrodes (not shown) may be positioned across each LC cell to apply an appropriate voltage to “twist” the corresponding LC molecules, thereby rotating the backlight to pass through. 
     FIG. 2  illustrates a direct backlight illumination scheme for typical backlit LCD devices. In a direct backlight scheme as illustrated in  FIG. 2 , a sufficiently thick spacer box  130  is needed between the LED PCB  140  and the diffuser  40 A. The LED PCB  140  supports LEDs  52 A. The spacer box  130  and the diffuser  40 A are used to randomize the light originating at LEDs  52 A, and create a uniformly distributed flux. 
     FIG. 3  illustrates the light distribution of a pinpoint source in a rectangular tile used in typical backlit LCD devices. Important characteristics for good quality backlights are uniform light output distribution and a low profile. However, achieving a uniform light output distribution and a low profile using a discrete set of pinpoint sources, such as LEDs, is difficult. The difficulty arises from the fact that the intensity of the light generated by a pinpoint source decreases as a function of the distance from the source. As illustrated in  FIG. 3 , the intensity of the light from a pinpoint source A 1  is smaller at point A 5  than at point A 3 , as point A 5  is further out from source A 1  than point A 3 . 
   This pinpoint source behavior causes two problems. First, even if a diffuser is used, the light output is considerably stronger closer to a pinpoint source than further away from the pinpoint source. Secondly, if a backlight is built from elementary (for example, rectangular) tiles, the intensity of light at the vertices of the tiles is lower than at the mid-sides, even if perfect diffusers are used with the rectangular tiles. For example, for a tile  210  as illustrated in  FIG. 3 , with point source A 1  located at the center of the tile, the intensity of light at the tile vertices A 31 , A 33 , A 35 , and A 37 , is lower than the intensity of light at the tile mid-sides A 21 , A 23 , A 25 , and A 27 . 
     FIG. 4A  illustrates a top view of a configuration for a light source cavity  330 A, for use in a lens for LCD backlights according to an embodiment of the present invention. In  FIG. 4A , rectangular region  333  is a portion of a lens. Light source  52 C may be an LED light source, or any other type of light source that can be used for LCD or other electronic and optical applications. 
   As explained with reference to  FIG. 3 , for a tile using a center light source, the intensity of light at the vertices of the tile is typically lower than at the mid-sides. Light source cavity  330 A is designed to compensate for light intensity along the diagonals (tile vertices), so that the difference in light intensity between diagonals and mid-sides is minimized. The light source cavity  330 A illustrated in  FIG. 4A  is shaped so as to divert some of the light from the mid-side areas towards the diagonal areas. 
   A top view for the shape of light source cavity  330 A of the lens is shown in  FIG. 4A . The cross-sectional top view shows that the wall of the light source cavity  330 A forms four convex lenses interleaved with four concave lenses. The concave lenses are  305 A,  305 B,  305 C and  305 D, and the convex lenses are  311 A,  311 B,  311 C and  311 D. By shaping the light source cavity  330 A to have convex and concave parts, portions with positive and negative optical powers are obtained. The convex portions are facing the corners of the rectangular region  333 , while the concave portions are facing the centers of the sides. Convex portions make rays converge, rather than diverge. The convex lenses loosely focus the light towards the corners of the tile (towards the diagonals), while the four concave lenses diverge the light away from the mid-sides. For example, ray R 322  is directed towards a corner of the tile (a vertex at a diagonal), while ray R 321  is directed away from a mid-side. This shape for the light source cavity  330 A addresses the problem that the corners of a tile are not as bright as the tile mid-sides. Hence, the light source cavity  330 A shape in  FIG. 4A  compensates for the light intensity rotational asymmetry associated with rectangular tiles. 
     FIG. 4B  illustrates the cross-section for the light source cavity  330 A illustrated in  FIG. 4A , for use in a lens for LCD backlights according to an embodiment of the present invention. As illustrated in  FIG. 4B , the top of the light source cavity  330 A comprises a concave lens segment  351 , which spreads the light sideways, away from the center axis A 350  of the light source  52 C. For example, rays R 361  and R 363  are directed away from the center axis A 350  of the light source  52 C. Hence, the configuration of the inner surface of the light source cavity  330 A causes light from light source  52 C to spread more uniformly. Surface  352  is a convex lens, which makes rays converge. The shape of the light source cavity  330 A also depends on the characteristics of the LED light source present at the center of the light source cavity. 
     FIG. 5A  illustrates a top view of a lens  400 A for LCD backlights according to an embodiment of the present invention.  FIG. 5B  illustrates a cross-sectional side view for the lens  400 A for LCD backlights illustrated in  FIG. 5A , according to an embodiment of the present invention. The rectangular lens  400 A is a rectangular tile, whereby a plurality of tiles can be included in an LCD backlight assembly. A light source cavity  330 A, as described at  FIGS. 4A and 4B , is located in the center of lens  400 A. A light source  52 C is located at the center of light source cavity  330 A. 
   The lens  400 A improves uniformity of light distribution for light from source  52 C. The lens cross-section is profiled so that lens  400 A has sections where most of the light is bounced back into the lens  400 A, through reflection, partial reflection, or total internal reflection. These sections are the reflection areas  405  as shown in  FIG. 5A . The index of refraction of the lens material of lens  400 A may be designed to achieve total internal reflection. A high index of refraction, which is constant across the lens  400 A, may be used for this purpose. In one embodiment, the lens  400 A is made of a plastic material, which reduces lens fabrication costs. 
   Additionally, transmission channels  407  are created at positions selected to insure uniform average distribution of light. The transmission channels  407  in this embodiment are created by locally slanting the profile of the lens  400 A, so that the total internal reflection condition is no longer met. The step-down channel profile is illustrated in  FIG. 5B , where the slants  421 ,  422 ,  423 ,  424 ,  425 ,  426 ,  427 , and  428  are the light transmission channels  407  for the lens  400 A. 
   Alternating channels may be carved into an otherwise flat profile of lens  400 A. The openings of the slants (or widths) for the channels  407  are smaller close to the centrally located source  52 C, and get larger farther away from the center  52 C. In one embodiment, for d 1 , d 2 , d 3 , . . . , d n  channel lengths for n channels  407 , and t 1 , t 2 , t 3 , . . . , t n  channel widths (slants) for the same n channels  407 , equations (1), (2), . . . , (n) are satisfied: 
                     t   1       t   2       =         d   1       d   2       ⁢     ⅇ     A   ⁡     (       d   1     -     d   2       )                   Eq   .           ⁢   1                   t   2       t   3       =         d   2       d   3       ⁢     ⅇ     A   ⁡     (       d   2     -     d   3       )                   Eq   .           ⁢   2                   t   3       t   4       =         d   3       d   4       ⁢     ⅇ     A   ⁡     (       d   3     -     d   4       )                   Eq   .           ⁢   3                     t   4       t   5       =         d   4       d   5       ⁢     ⅇ     A   ⁡     (       d   4     -     d   5       )             ⁢     
     ⁢   ⋯           Eq   .           ⁢   4                   t     n   -   2         t     n   -   1         =         d     n   -   2         d     n   -   1         ⁢     ⅇ     A   ⁡     (       d     n   -   2       -     d     n   -   1         )                     Eq   .           ⁢   n     ⁢     -     ⁢   1                   t     n   -   1         t   n       =         d     n   -   1         d   n       ⁢     ⅇ     A   ⁡     (       d     n   -   1       -     d   n       )                   Eq   .           ⁢   n               
where A is a constant depending on the geometry of the lens (shape, size, number and spacing of channels, etc.). If the size of the openings (t 1 , t 2 , t 3 , . . . , t n ) increases as a function of the distance from the center  52 C (d 1 /2, d 2 /2, d 3 /2, . . . , d n /2) in this manner, then the variation of light intensity with distance from the center  52 C is compensated for. This happens because the inner openings (channels) are brighter but narrower than the outer openings (channels), hence the light intensities from inner and outer channels are equal. Although eight transmission channels are shown for lens  400 A, more or less transmission channels  407  may be used, depending on the desired average light intensity from lens  400 A.
 
   Although a diffuser may still be used, the overall backlight may be much thinner, since a typical diffuser needs to be located at a distance comparable with the spacing of the light sources, for good light uniformity. The spacing of the light sources is the distance between light source  52 C and a next adjacent light source (not shown). Since the spacing between slant opening or channels  407  is much smaller than the spacing between light sources, the diffuser may be positioned closer to the arrangement of lenses  400 . 
   To minimize the difference in light intensity between tile diagonals and tile mid-sides for rectangular tiles, some light may be diverted from the mid-side areas towards the diagonal areas of the tile by using the light source cavity  330 A described at  FIGS. 4A and 4B . By placing the light source cavity  330 A inside lens  400 A and inserting the light source  52 C in the cavity  330 A, a space saving effect is achieved. 
   In alternative embodiments of the current invention, the surface of the lens used for LCD backlights can have other geometries that improve uniformity of light distribution from a central source. For example, transmission areas on the lens surface can have other shapes besides the concentric rectangular shapes shown in  FIG. 5A . Transmission areas on the lens surface can also be created using “tap points,” or openings in the lens surface, at a variety of locations. Imperfections or other features on the lens surface can be shaped and arranged so that the ratio between the transmitted light and the internally reflected light is changed and controlled as desired. 
   In one embodiment of the present invention, lens  400  is configured so that most of the light is bounced back into the lens by mirrored (total reflection) sections  405  on the lens surface. Additionally, transmission channels  407  are created at positions selected to insure uniform average distribution of light. In this embodiment, the transmission channels  407  are created by removing mirror portions from the top surface of the lens  400 . Hence, light transmission channels  407  correspond to absence of mirror surfaces, while reflection areas  405  correspond to mirror regions. This design may be achieved by covering the lens surface with a mirror, and then carving out mirror portions to obtain transmission channels  407 . 
     FIG. 6  illustrates a rectangular lens  400 C for LCD backlights, according to another embodiment of the present invention. The lens  400 C is designed with an alternative configuration to that illustrated in  FIGS. 4A and 4B  to minimize the difference in light intensity between diagonals and mid-sides of rectangular tiles, and achieve light uniformity for a rectangular tile. As illustrated in  FIG. 6 , the lens  400 C is designed to be thinner at mid-sides (such as at points  611  and  613 ) and thicker along the diagonals, or tile corners (such as at points  622 ,  624 , and  626 ). An LED is placed in the middle of the lens  400 C. The top surface of lens  400 C may have a stepped profile, as illustrated in  FIGS. 5A ,  5 B to obtain transmission channels and total internal reflection areas. 
     FIG. 7  illustrates a tile matrix arrangement  800  of lenses for LCD backlights, according to an embodiment of the present invention. The tile matrix arrangement  800  includes lenses  400 _ 1 ,  400 _ 2 ,  400 _ 3 ,  400 _ 4 ,  400 _ 5 ,  400 _ 6 ,  400 _ 7 ,  400 _ 8 ,  400 _ 9 ,  400 _ 10 ,  400 _ 11 , and  400 _ 12 . Lenses  400 _ 1 ,  400 _ 2 ,  400 _ 3 ,  400 _ 4 ,  400 _ 5 ,  400 _ 6 ,  400 _ 7 ,  400 _ 8 ,  400 _ 9 ,  400 _ 10 ,  400 _ 11 , and  400 _ 12  may be designed according to embodiments of the present invention described above with reference to  FIGS. 5A ,  5 B, and  6 . LED sources  52 _ 1 ,  52 _ 2 ,  52 _ 3 ,  52 _ 4 ,  52 _ 5 ,  52 _ 6 ,  52 _ 7 ,  52 _ 8 ,  52 _ 9 ,  52 _ 10 ,  52 _ 11 , and  52 _ 12  are present at the center of lenses  400 _ 1 ,  400 _ 2 ,  400 _ 3 ,  400 _ 4 ,  400 _ 5 ,  400 _ 6 ,  400 _ 7 ,  400 _ 8 ,  400 _ 9 ,  400 _ 10 ,  400 _ 11 , and  400 _ 12 . LCD backlights can be built from such LED driven thin rectangular tiles. Different numbers of such tiles can be used for various size panels. A lens for LCD backlights according to this invention can have other shapes rather than squares for which a tessellated pattern is obtained using the lenses. 
   The apparatuses described in this application implement thin and stackable lenses that are configured to increase uniformity of light distribution. A single piece lens may incorporate all the light guiding surfaces required for uniform distribution of light from a pinpoint source. Multiple lenses may be embedded in a single sheet, or may be stacked, to create backlights of any size. 
   Although detailed embodiments and implementations of the present invention have been described above, it should be apparent that various modifications are possible without departing from the spirit and scope of the present invention. For example, although aspects of the present invention have been described in the context of LCD backlights, it should be realized that the principles of the present invention may be applicable to other optical and electronic systems.