Abstract:
The present disclosure is directed to backlighting systems, which include first and second lightguides, at least one light source optically connected to an edge of the first lightguide and at least one light source optically connected to an edge of the second lightguide for supplying light into their respective interiors. In the appropriate exemplary embodiments, the backlighting systems of the present disclosure include an extractor disposed at a surface of the second lightguide for diffuse extraction of light from the interior of the second lightguide. In such exemplary embodiments, at least a portion of the light supplied into the interior of the second lightguide and then diffusely extracted therefrom enters the interior of the first lightguide through a substantially optically clear surface. In some exemplary embodiments, the backlighting systems of the present disclosure include recycling enhancement structures, which may be attached to the first lightguide.

Description:
FIELD OF THE INVENTION 
   The present disclosure relates to backlighting systems, which may be advantageously used with large high-performance liquid crystal displays. More specifically, the disclosure relates to backlighting systems that include multiple lightguides, and, optionally, various recycling enhancement structures. 
   BACKGROUND OF THE INVENTION 
   Liquid crystal displays (LCDs) are widely used in electronic display devices, such as computer monitors, handheld devices and televisions. Unlike cathode ray tube (CRT) displays, LCDs do not emit light and, thus, require a separate light source for viewing images formed on such displays. Ambient light illumination is sufficient for some applications, but with most large area and high performance LCDs, ambient light causes glare and is detrimental to readability. Thus, in order to improve readability, most large area and high performance LCDs include a source of light located behind the display, which is usually referred to as a “backlight.” 
   Presently, many popular systems for backlighting LCDs include direct-lit backlights, in which multiple lamps or a single serpentine-shaped lamp are arranged behind the display in the field of view of the user, and edge-lit backlights, in which the light sources are placed along one or more edges of a lightguide located behind the display, so that the light sources are out of the field of view of the user. In order to compete with CRT displays, large LCDs displays (e.g., greater than ˜20″ or 50 cm in diagonal) must have high luminance targets, e.g., about 500 nt or more. Such high luminance targets are currently met by direct-lit backlights for LCDs. 
   The use of conventional direct-lit backlights systems, however, has caused some concerns among manufacturers of large LCDs, such as LCD televisions. One concern is a discrepancy between the intended lifetimes of LCDs, which for most LCD television purchasers may be 10 to 20 years, and the lifetimes of individual lamps in the televisions&#39; backlights, which are approximately 10,000 to 20,000 hours and usually at the lower end of this range. In particular, cold cathode fluorescent lamps (CCFLs), which are frequently used for backlighting, have varying lifetimes and aging characteristics. If one CCFL burns out in a conventional direct-lit backlight, the result will be a dark line directly across the display. In addition, the spatial color uniformity of a conventional direct-lit display suffers as each CCFL ages differently. Major LCD manufacturers and television set makers currently do not have a model for servicing LCD backlights that fail in either of these two modes. 
   Furthermore, light reaching the viewer from multiple sources in a conventional direct-lit backlight usually is not mixed as well as the light in edge-lit backlights. Nonetheless, despite this shortcoming as well as the uniformity and aging disadvantages of conventional direct-lit backlights, they are currently a popular choice for backlighting LCDs, e.g., LCD televisions, because they allow reaching luminance targets that are competitive with CRT displays. Although edge-lit-backlights would appear to be more advantageous in many respects, achieving desired levels of luminance with traditional edge-lit backlights has remained a challenge. One difficulty has been arranging a large enough number of light sources at an edge of a single lightguide to provide sufficient optical power to reach the target luminance. Other difficulties include enhancement film warping in traditional backlights, e.g., due to high thermal gradients and handling problems. 
   Thus, there remains a need in the field of backlights for large high-performance LCDs for backlighting systems that are capable of achieving high luminance targets and are more efficient. In addition, there remains a need for backlighting systems for large high-performance LCDs that overcome other shortcomings of the currently available backlights described above. 
   SUMMARY 
   These and other shortcomings of the presently known backlights for large high-performance LCDs are addressed by the inventors of the present disclosure by providing multiple-lightguide backlighting systems as disclosed and claimed herein. Such systems may be advantageously used with a variety of devices, including LCD televisions, LCD monitors, point of sale devices, and other suitable devices. In addition to allowing to achieve high output luminances, the present disclosure mitigates the risks of using variable lifetime light sources, so that burnout or aging of an individual light source would not be catastrophic to the display viewing quality. Thus, if an individual light source ages or burns out in a multiple-lightguide system according to an embodiment of the present disclosure, the effect on spatial brightness and color uniformity will be relatively insignificant due to the enhanced light mixing. 
   The present disclosure eliminates the need for a thick diffuser plate traditionally used in direct-lit backlights to hide individual sources from the viewer, thus providing additional gains in brightness. The present disclosure also eliminates the need for a structured reflector traditionally used in direct-lit backlights, resulting in cost reductions and increased ease of manufacturing. In addition, light extracted directly from the top lightguide could be allowed to exit at a wide range of angles, which would enhance off-axis viewability of the display. Moreover, the present disclosure makes possible inclusion of additional features for preventing warp and physical damage to various recycling enhancement structures that may be used in exemplary embodiments of the present disclosure. 
   Thus, the present disclosure is directed to backlighting systems, which in one exemplary embodiment include first and second lightguides, at least one light source optically connected to an edge of the first lightguide and at least one light source optically connected to an edge of the second lightguide for supplying light into their respective interiors. In some embodiments, the backlighting systems of the present disclosure include an extractor disposed at a surface of the second lightguide for diffuse extraction of light from the interior of the second lightguide. In such exemplary embodiments, at least a portion of the light supplied into the interior of the second lightguide and then diffusely extracted therefrom enters the interior of the first lightguide through a substantially optically clear surface. 
   Such backlighting systems may further include a first recycling enhancement structure disposed at a surface of the first lightguide, which may include a reflective polarizer, a reflective polarizer and a diffuser, or a reflective polarizer and a prismatic structure. The diffuser may be spatially graded. Preferably, the first recycling enhancement structure is attached to a surface of the first lightguide. Alternatively or additionally, the backlighting systems of the present disclosure may include a second recycling enhancement structure disposed between the first lightguide and the second lightguide, which may include a prismatic structure. Preferably, the second recycling enhancement structure is attached to a surface of the first lightguide. 
   Other embodiments of the backlighting systems of the present disclosure include first and second lightguides, at least one light source optically connected to an edge of the first lightguide and at least one light source optically connected to an edge of the second lightguide for supplying light into their respective interiors. Such exemplary embodiments also include a second recycling enhancement structure disposed between the first lightguide and the second lightguide. 
   The second recycling enhancement structure may include a prismatic structure. The prismatic structure preferably includes a plurality of prisms having apexes pointing generally away from the second lightguide. In the appropriate embodiments, the second recycling enhancement structure may include a first surface defining a plurality of prisms substantially symmetrical about a first horizontal axis and having apexes pointing generally away from the second lightguide and a second surface defining a plurality of prisms substantially symmetrical about a second horizontal axis and having apexes pointing generally away from the second lightguide. The first axis may be generally orthogonal to the second axis. Preferably, the second recycling enhancement structure is attached to a surface of the first lightguide. 
   The backlighting systems constructed according to the present disclosure may also include a first recycling enhancement structure disposed at a surface of the first lightguide. The first recycling enhancement structure may include a reflective polarizer, a reflective polarizer and a diffuser, or a reflective polarizer and a prismatic structure. Preferably, the first enhancement structure is attached to a surface of the first lightguide. 
   In the appropriate exemplary embodiments of the present disclosure, the first and second lightguides each comprise two opposing edges, at least one light source is optically connected to each of said edges, and the opposing edges of the first lightguide are not aligned with the opposing edges of the second lightguide. Alternatively, the first and second lightguides each comprise two adjacent edges, at least one light source is optically connected to each of said edges, and the adjacent edges of the first lightguide are not aligned with the adjacent edges of the second lightguide. 
   The present disclosure is also directed to backlighting systems, which include first and second lightguides, a plurality of light sources optically connected to a first edge of the first lightguide, a plurality of light sources optically connected to a second edge of the first lightguide, a plurality of light sources optically connected to a first edge of the second lightguide, and a plurality of light sources optically connected to a second edge of the second lightguide. In some embodiments, the backlighting systems of the present disclosure include an extractor disposed at a surface of the second lightguide for diffuse extraction of light from the interior of the second lightguide. In such exemplary embodiments, at least a portion of the light supplied into the interior of the second lightguide and then diffusely extracted therefrom enters the interior of the first lightguide through a substantially optically clear surface. Such backlighting systems may further include a first recycling enhancement structure disposed at a surface of the first lightguide, which may include a reflective polarizer, a reflective polarizer and a diffuser, or a reflective polarizer and a prismatic structure. The diffuser may be spatially graded. Preferably, the first recycling enhancement structure is attached to a surface of the first lightguide. 
   Alternatively or additionally, these exemplary embodiments may include a second recycling enhancement structure disposed between the first lightguide and the second lightguide. The second recycling enhancement structure may include a prismatic structure. The prismatic structure preferably includes a surface defining a plurality of prisms having apexes pointing generally away from the second lightguide. In the appropriate embodiments, the second recycling enhancement structure may include a first surface defining a plurality of prisms substantially symmetrical about a first horizontal axis and having apexes pointing generally away from the second lightguide and a second surface defining a plurality of prisms substantially symmetrical about a second horizontal axis and having apexes pointing generally away from the second lightguide. The first axis may be generally orthogonal to the second axis. Preferably, the second recycling enhancement structure is attached to a surface of the first lightguide. 
   Furthermore, in the appropriate embodiments of the present disclosure, at least one of the first lightguide and the second lightguide may have variable thickness, and at least one lightguide may include first and second wedge portions. The extractor disposed at a surface of the second lightguide may be spatially graded. Additionally or alternatively, substantially optically clear surface extraction structures may be disposed on the surface of the first lightguide that faces the second lightguide. The backlighting systems of the present disclosure may also include a reflector sheet disposed next to the surface of the second lightguide that faces away from the first lightguide and a diffuser sheet disposed next to the surface of the second lightguide that faces away from the first lightguide. 
   These and other aspects of the backlighting systems of the subject invention will become more readily apparent to those having ordinary skill in the art from the following detailed description together with the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that those having ordinary skill in the art to which the subject invention pertains will more readily understand how to make and use the subject invention, exemplary embodiments thereof will be described in detail below with reference to the drawings, wherein: 
       FIG. 1  is a schematic cross-sectional view of a presently available direct-lit backlight for LCD televisions; 
       FIG. 2  is a schematic cross-sectional view of an exemplary embodiment of a backlighting system according to the present disclosure; 
       FIGS. 2A and 2B  illustrate alternative ways of arranging light sources at the edges of generally rectangular lightguides; 
       FIG. 3  is a schematic cross-sectional view of another exemplary embodiment of a backlighting system according to the present disclosure, illustrating the use of a spatially graded diffuser; 
       FIG. 4  is a schematic cross-sectional view of another exemplary embodiment of a backlighting system according to the present disclosure, illustrating the use of a variable thickness lightguide; 
       FIG. 5  is a schematic cross-sectional view of another exemplary embodiment of a backlighting system according to the present disclosure; 
       FIG. 6  is a schematic cross-sectional view of an exemplary configuration for testing some of the concepts of the present disclosure; 
       FIGS. 7A-C  show data illustrating properties of light extracted from the top lightguide, where the top lightguide is a bare wedge ( FIG. 7A ), a wedge laminated with DBEF-M ( FIG. 7B ), or a wedge laminated with DRPF ( FIG. 7C ); 
       FIGS. 8A-F  show data representing output polarization of DRPF-laminated lightguide; 
       FIGS. 9A-F  show data representing output polarization of DBEF-M-laminated lightguide; 
       FIG. 10A  shows a gain profile of a lightguide system with a loose sheet of DRPF; 
       FIG. 10B  shows a gain profile of a lightguide system with a laminated sheet of DRPF; 
       FIG. 10C  shows a gain profile of a lightguide system with a loose sheet of DBEF-M; and 
       FIG. 10D  shows a gain profile of a lightguide system with a laminated sheet of DBEF-M. 
   

   DETAILED DESCRIPTION 
     FIG. 1  illustrates the structure and components of a traditional direct-lit backlight  10 , such as those presently used in LCD televisions. The traditional backlight  10  includes an array of light bulbs  15 , typically CCFLs, and a shaped reflector  17  located behind the array of light bulbs  15  for directing more light toward a viewer. A thick diffuse plate  18  is usually placed over the array of light bulbs  15  to diffuse light from the individual bulbs, e.g., CCFLs, in order to hide them from the viewer. A typical diffuser plate  18  has a large amount of absorption associated with it, as well as a large amount of back scattering, the effects of which grow exponentially if light-recycling enhancement films (described below) are added to the backlight. To further aid in hiding individual light bulbs from the viewer, diffuser plates have been patterned, which resulted in additional losses of light. 
   The traditional backlight  10  further includes a thin diffuser sheet  16  and a layer of enhancement film  14  having prismatic surface structures, such as Vikuiti™ Brightness Enhancement Film BEF, available from 3M Company. The enhancement film  14  refracts light within a certain angle toward the viewer. Light outside that angle is “recycled,” i.e., reflected back into the backlight  10 , where it travels within the system until reaching the proper angle for exiting the system. In addition, the traditional backlight  10  includes a layer of reflective polarizer  12  placed over the enhancement film  14 . The reflective polarizer  12  is usually a multilayer reflective polarizer, such as Vikuiti™ Dual Brightness Enhancement Film (DBEF), also available from 3M Company. The reflective polarizer  12  transmits light with a predetermined polarization, while reflecting light with a different polarization into the backlight  10  where the polarization state is altered and the light is fed back to the reflective polarizer  12 . This process is also referred to as “recycling.” 
     FIG. 2  shows a schematic cross-sectional view of a backlighting system  100  constructed according to an exemplary embodiment of the present disclosure. The backlighting system  100  includes a first lightguide  130  and a second lightguide  140 . In the exemplary embodiment shown in  FIG. 2 , pairs of light sources  165   a  and  165   b  are placed at the edges  132   a  and  132   b  of the first lightguide  130 , so that at least a portion of the light emanating from the sources  165   a ,  165   b  is coupled into the interior of the lightguide  130  and propagates along its length by reflecting from the surfaces  130   a  and  130   b , e.g., by total internal reflection. Lamp cavity reflectors  156   a ,  156   b  may be provided as illustrated in  FIG. 2 , for increasing coupling efficiency from the sources  165   a ,  165   b  into the interior of the lightguide  130 . As will be understood by those of ordinary skill in the art, shape and structure of the reflectors  156   a ,  156   b  may vary. 
   Referring further to  FIG. 2 , in the exemplary backlighting system  100 , pairs of light sources  185   a  and  185   b  are placed at the edges  142   a  and  142   b  of the second lightguide  140 , so that at least a portion of the light emanating from the sources  185   a ,  185   b  is coupled into the interior of the lightguide  140  and propagates along its length by reflecting from the surfaces  140   a  and  140   b , e.g., by total internal reflection. Lamp cavity reflectors  176   a ,  176   b , similar to the lamp cavity reflectors  156   a  and  156   b , may be provided as illustrated in  FIG. 2  for increasing coupling efficiency from the sources  185   a ,  185   b  into the interior of the lightguide  140 . As will be understood by those of ordinary skill in the art, shape and structure of the reflectors  176   a ,  176   b  may also vary. 
   Although the exemplary backlighting system  100  illustrated in  FIG. 2  shows pairs of light sources  165   a ,  165   b  placed at the edges  132   a ,  132   b  of the lightguide  130  and pairs of light sources  185   a ,  185   b  placed at the edges  142   a ,  142   b  of the lightguide  140 , the present disclosure also contemplates using one, three or more light sources at an edge of the lightguide  130  and one, three or more light sources at one or more edges of the lightguide  140 . In addition, although for ease of illustration light sources  165   a  and  165   b  are shown to be aligned with the light sources  185   a  and  185   b , the present disclosure contemplates placing light sources at any one or more edges of each lightguide, as desired for a specific application. For example,  FIGS. 2A and 2B , illustrate two exemplary ways of arranging light sources  135   a ′,  135   a ″,  145   a ′,  145   a ″ and  135   b ′,  135   b ″,  145   b ′,  145   b ″ at the edges of generally rectangular lightguides  130   a ,  140   a  and  130   b ,  140   b  respectively. In  FIG. 2A , light sources  135   a ′and  135   a ″ are disposed at the opposing edges  125   a ′ and  125   a ″ of the lightguide  130   a  and light sources  145   a ′ and  145   a ″ are disposed at the opposing edges  155   a ′ and  155   a ″ of the lightguide  140   a . In this exemplary embodiment, each of the opposing edges  125   a ′ and  125   a ″ is not aligned with any of the opposing edges  155   a ′ and  155   a ′. In  FIG. 2B , light sources  135   b ′ and  135   b ″ are disposed at the adjacent edges  125   b ′ and  125   b ″ of the lightguide  130   b  and light sources  145   b ′ and  145   b ′ are disposed at the adjacent edges  155   b ′ and  155   b ″ of the lightguide  140   b . In this exemplary embodiment, each of the adjacent edges  125   b ′ and  125   b ″ is not aligned with any of the adjacent edges  155   b ′ and  155   b″.    
   Light sources suitable for use with embodiments of the present disclosure include any source that emits light, such as a fluorescent lamp (e.g., CCFL), a hot cathode fluorescent lamp (HCFL), an incandescent lamp, an electroluminescent light source, a phosphorescent light source, an external electrode fluorescent lamp, a light emitting diode (LED), including organic LEDs (OLEDs), an array of LEDs, any other suitable light source(s), or any appropriate number or combination thereof. 
   The number and type of lightguides may also vary. For example, three or more lightguides may be used in accordance with the present disclosure and any one or more of the constituent lightguides may be hollow. Increasing the number of lightguides in backlighting systems according to exemplary embodiments of the present disclosure would lead to corresponding increases in weights and thicknesses of displays. However, most manufacturers of large panel LCDs typically consider display thickness and weight to be secondary concerns. Lifetime, brightness, spatial uniformity, ease of assembly, and reduction in warp of enhancement films are usually considered to be more important. 
   The number and type of light sources arranged at an edge of a lightguide, e.g.,  130  or  140 , as well as the number, dimensions and type of lightguides will depend on the specific application and luminance target, as well as practical considerations such as the size of the specific source as compared to the dimensions of the lightguide. For example, assuming that lightguides  130  and  140  have about the same thicknesses as those typically seen in traditional single-lightguide edge-lit displays, up to six bulbs of typical CCFLs may be used per lightguide (e.g., three bulbs at each of the edges  132   a ,  132   b ,  142   a , and  142   b ). Thus, a 29″ direct-lit LCD television backlight having 12 light bulbs can be replaced with a two-lightguide system illustrated in  FIG. 2 , with three bulbs arranged at each lightguide edge (e.g.,  132   a ,  132   b ,  142   a  and  142   b ). A 32″ direct-lit LCD television with 16 light bulbs would require a three-lightguide system to make it completely edge-lit in order to produce comparable luminance. 
   Referring further to  FIG. 2 , the backlighting system  100  may include a first recycling enhancement structure  112  disposed at a surface of the first lightguide  130 . In the context of the present disclosure, “a recycling enhancement structure” may be any structure that is capable of “recycling” light in a manner similar or equivalent to the enhancement films  12  and  14 , described with reference to  FIG. 1 . Preferably, the first recycling enhancement structure  112  is disposed at the surface  130   a  and includes a reflective polarizer, such as a multilayer reflective polarizer Vikuiti™ Dual Brightness Enhancement Film (DBEF), available from 3M Company. Most preferably, the first recycling enhancement structure  112  also includes a diffuser, which may be integrated within the reflective polarizer or be included as a separate component, such as a matte surface or a layer of pressure sensitive adhesive (PSA). One function of the diffuser is the randomization of the polarization and direction of the light reflected by the reflective polarizer back into the backlighting system  100 . Exemplary components suitable for use within the first recycling enhancement structure  112  include Vikuiti™ Diffuse Reflective Polarizer Film (DRPF) and Vikuiti™ Dual Brightness Enhancement Film-Matte (DBEF-M), both available from 3M Company. 
   The first recycling enhancement structure  112  preferably is attached to a surface of the first lightguide  130 , e.g., surface  130   a . The first recycling enhancement structure  112  may be attached to a surface of the first lightguide  130  by lamination, molding the enhancement structure  112  or any of its constituent components into the lightguide or by any suitable bonding technique. If the first recycling enhancement structure  112  includes a matte surface, e.g., as in DBEF-M, the first recycling enhancement structure  112  preferably is attached to the lightguide  130  so that the matte surface faces the surface  130   a . In exemplary embodiments of the present disclosure, in which the first recycling enhancement structure  112  is attached to the first lightguide  130 , light may be extracted from the interior of the first lightguide  130  through its interactions with the first recycling enhancement structure  112 . For example, if DRPF or DBEF-M is included into the structure  112 , light is diffused by either of these films and it is either transmitted to the LCD in the proper polarization state or scattered back into the backlight  100 , where it can be recycled as explained above. Alternatively, DBEF may be attached to a surface of the first lightguide  130  with a layer of PSA. In that case, PSA would also facilitate the extraction of light from the interior of the first lightguide  130 . 
   In appropriate exemplary embodiments of the present disclosure, both DBEF and DRPF may be included into the first recycling enhancement structure  112  and preferably attached, e.g., laminated, to the surface  130   a  of the lightguide  130 . In that case, the polarization axes of both reflective polarizers, i.e., DBEF and DRPF should be aligned. As a result, DRPF will facilitate extraction of light from the lightguide  130 , while DBEF will enhance the contrast. Alternatively, BEF or another suitable prismatic film or structure may be used in combination with a reflective polarizer, e.g., DBEF, as a part of the first recycling enhancement structure  112 . BEF would facilitate light extraction, while DBEF would ensure that light exits the backlight  100  with the appropriate polarization. 
   Additionally or alternatively, the backlighting system  100  illustrated in  FIG. 2  may include a second recycling enhancement structure  114 , which may be disposed between the first lightguide  130  and the second lightguide  140 . Preferably, the second recycling enhancement structure  114  includes prismatic structures, e.g., prismatic structured film, that would aid in redirecting and recycling light to increase on-axis brightness of the backlight  100  by refracting toward the viewer light within a certain angle and reflecting back light outside that angle. One example of such prismatic structured films suitable for use within the second recycling enhancement structure  114  is Vikuiti™ Brightness Enhancement Film (BEF), available from 3M Company. The second recycling enhancement structure  114  also may include prismatic structures oriented so that the prism apexes are facing generally away from the lightguide  130 . 
   In the appropriate exemplary embodiments of the present disclosure, two BEFs or similar prismatic films or structures may be used in the second recycling enhancement structure  114 . In such exemplary embodiments, the directions of the prismatic films&#39; grooves preferably are crossed, and a thin layer of adhesive joins the films so that only small portions of the prismatic structures are immersed into the adhesive. The second recycling enhancement structure  114  preferably is attached, e.g., laminated, molded or bonded using any other suitable technique, to the surface  130   b  of the lightguide  130 . This feature would create added extraction from the first lightguide  130  and reduce warping of the second recycling enhancement structure  114 , which may occur due to temperature variations, handling and other causes. 
   Placing an extractor  143 , preferably a diffuse extractor, at a surface of the lightguide  140  may facilitate light extraction from the second lightguide  140 .  FIG. 2  illustrates the use of such an extractor  143 , which in this exemplary embodiment includes an array of dots disposed on the surface  140   b  of the lightguide  140 . Preferably, the pattern of dots is optimized to compensate for potential spatial non-uniformities of light extraction from the entire backlighting system  100 . For example, the dot pattern may be adjusted so that more light is extracted toward the center of the lightguide  140  by gradually increasing the size of dots toward the center of the lightguide  140 . 
   The backlighting system  100  may further include a diffuser sheet  116  and a reflector sheet  127 . The diffuser sheet primarily serves to increase spatial uniformity of the light exiting the second lightguide  140 , as well as to aid in randomizing polarization of the light reflected back into the backlight  100 . The reflector sheet  127  may further increase efficiency of the backlighting system  100  by reflecting back light that escapes through the side  140   b  of the lightguide  140 , so that the light may be directed toward the viewer and/or recycled. 
     FIG. 3  is a schematic cross-sectional view of a backlighting system  200  constructed according to another exemplary embodiment of the present disclosure. The backlighting system  200  includes a first lightguide  230  and a second lightguide  240 . Pairs of light sources  265   a  and  265   b  are placed at the edges  232   a  and  232   b  of the first lightguide  230 , so that at least a portion of the light emanating from the sources  265   a ,  265   b  is coupled into the interior of the lightguide  230  and propagates along its length by reflecting from the surfaces  230   a  and  230   b , e.g., by total internal reflection. Lamp cavity reflectors  256   a ,  256   b  may be provided for increasing coupling efficiency from the sources  265   a ,  265   b  into the interior of the lightguide  230 . As will be understood by those of ordinary skill in the art, shape and structure of the reflectors  256   a ,  256   b  may vary. 
   Referring further to  FIG. 3 , in the exemplary backlighting system  200 , pairs of light sources  285   a  and  285   b  are placed at the edges  242   a  and  242   b  of the second lightguide  240 , so that at least a portion of the light emanating from the sources  285   a ,  285   b  is coupled into the interior of the lightguide  240  and propagates along its length by reflecting from its surfaces  240   a  and  240   b , e.g., by total internal reflection. Lamp cavity reflectors  276   a ,  276   b , similar to the lamp cavity reflectors  256   a  and  256   b , may be provided for increasing coupling efficiency from the sources  285   a ,  285   b  into the interior of the lightguide  240 . As will be understood by those of ordinary skill in the art, shape and structure of the reflectors  276   a ,  276   b  may also vary. As it has been explained in reference to the exemplary embodiments illustrated in  FIG. 2 , the number, type and configuration of light sources and lightguides may vary as well. 
   Referring further to  FIG. 3 , the backlighting system  200  may include a first recycling enhancement structure  212  disposed at a surface of the first lightguide  230 . Preferably, the first recycling enhancement structure  212  is disposed at the surface  230   a  and includes a reflective polarizer  212   b , such as DBEF. Most preferably, the first recycling enhancement structure further includes a diffuser  212   a , such as a loaded PSA structure, which also may be used to attach the reflective polarizer to a surface, e.g., surface  230   a , of the first lightguide  230 . As illustrated in  FIG. 3 , the diffuser  212   a  may be spatially graded to improve the overall uniformity of the output from the backlighting system  200 . The backlighting system  200  may also include optically clear surface extraction features  245 , such as step-wedge structures disposed on the surface  230   b , which would facilitate extraction of light from the first lightguide  230 . Those of ordinary skill in the art will readily recognize that such surface extraction features  245  may be used as appropriate in other exemplary embodiments of the present disclosure. 
   The remainder of the backlighting system  200  may have a structure similar to that of the embodiments illustrated in  FIG. 2  or a different suitable structure. For example, the backlighting system  200  may include a second recycling enhancement structure  214 , which may be disposed between the first lightguide  230  and the second lightguide  240 . Preferably, the second recycling enhancement structure  214  includes prismatic structures, e.g., prismatic structured film such as BEF, which redirect and recycle light to increase on-axis output brightness of the backlighting system  200  by refracting toward the viewer light within a certain angle and reflecting back light outside that angle. Similar to the backlighting system  100 , the backlighting system  200  may further include a diffuser sheet  216  and a reflector sheet  227 . 
   Light extraction from the second lightguide  240  may be accomplished by placing an extractor  243 , preferably a diffuse extractor, at a surface of the lightguide  240 .  FIG. 3  illustrates the use of such an extractor  243 , which in this exemplary embodiment includes an array of dots disposed on the surface  240   b  of the lightguide  240 . Preferably, the pattern of dots is optimized to compensate for potential spatial non-uniformities of light extraction from the entire backlighting system  200 . For example, the dot pattern may be adjusted so that more light is extracted toward the center of the lightguide  240  by gradually increasing the size of dots toward the center of the lightguide  240 . 
     FIG. 4  illustrates a backlighting system  300  constructed according to another exemplary embodiment of the present disclosure. The backlighting system  300  includes a first lightguide  330  and a second lightguide  340 . As it has been explained in reference to other exemplary embodiments, pairs of light sources  365   a  and  365   b  are placed at the edges  332   a  and  332   b  of the first lightguide  330 , and pairs of light sources  385   a  and  385   b  are placed at the edges  342   a  and  342   b  of the second lightguide  240 . Preferably, lamp cavity reflectors  356   a ,  356   b  and  376   a ,  376   b  are provided for increasing coupling efficiency from the sources  365   a ,  365   b  and  385   a ,  385   b  into the lightguides  330  and  340 . As will be understood by those of ordinary skill in the art, shape and structure of the reflectors may vary. The number, type and configuration of light sources and the number and configuration of lightguides may vary as well. 
   In the exemplary backlighting system  300 , the first lightguide  330  may include two wedge lightguides  336   a  and  336   b  joined at a juncture or seam  336   c , or a single lightguide molded so that the surface  330   a  is generally flat while the surface  330   b  has a cross-section approximating the shape of an inverted V, with the thickness of the lightguide  330  tapering away from the light sources, as illustrated in  FIG. 4 . A first recycling enhancement structure  312  may be disposed at a surface of the first lightguide  330 . Preferably, the first recycling enhancement structure  312  is disposed at the surface  330   a  and includes a reflective polarizer, such as DBEF. Most preferably, the first recycling enhancement structure  312  also includes a diffuser, which may be integrated within the reflective polarizer or be included as a separate component, such as a matte surface or a layer of PSA. Examples of structures suitable for use within the first recycling enhancement structure  312  in exemplary embodiments of the present disclosure include DRPF and DBEF-M. 
   The first recycling enhancement structure  312  preferably is attached to the surface  330   a  of the first lightguide  330 , e.g., by lamination, molding the enhancement structure  312  or any of its constituent components into the lightguide or by any suitable bonding technique. Extraction of light from the first lightguide  330  in such exemplary embodiments may be achieved by total internal reflection failure and interactions with the attached first recycling enhancement structure  312 . 
   The remainder of the backlighting system constructed according to this exemplary embodiment may have a structure similar to the embodiments illustrated in  FIGS. 2 and 3 , or a different suitable structure. For example, the backlighting system  300  may include a second recycling enhancement structure  314  disposed between the first lightguide  330  and the second lightguide  340 . Preferably, the second recycling enhancement structure  314  includes prismatic structures, e.g., prismatic structured film such as BEF, that redirect and recycle light to increase on-axis brightness of the backlight  300  by refracting toward the viewer light within a certain angle and reflecting back light outside that angle. In the appropriate exemplary embodiments, the second recycling enhancement structure  314  may include prismatic structures having prism apexes that face generally away from the light guide  330 . Similar to the backlighting system  100 , the backlighting system  300  may further include a diffuser sheet  316  and a reflector sheet  327 . 
   Light extraction from the second lightguide  340  may be accomplished by placing an extractor  343 , preferably a diffuse extractor, at a surface of the lightguide  340 . The extractor  343  may include an array of dots disposed on the surface  340   b  of the lightguide  340 . Preferably, the pattern of dots is optimized to compensate for potential spatial non-uniformities of light extraction from the entire backlighting system  300 . For example, if two wedge lightguides  336   a  and  336   b  are used to form the lightguide  330 , extraction of light from the second lightguide  340  may be adjusted to hide the juncture or seam  336   c  from the viewer by a flood of light. This may be accomplished by increasing the size of dots in a dot pattern toward the center of the lightguide  340 . 
     FIG. 5  shows a schematic cross-sectional view of a backlighting system  400  according to another exemplary embodiment of the present disclosure. The backlighting system  400  includes a first lightguide  430  and a second lightguide  440 . As it has been explained in reference to other exemplary embodiments, pairs of light sources  465   a  and  465   b  are placed at the edges  432   a  and  432   b  of the first lightguide  430 , and pairs of light sources  485   a  and  485   b  are placed at the edges  442   a  and  442   b  of the second lightguide  440 . Lamp cavity reflectors  456   a ,  456   b  and  476   a ,  476   b  may be provided for increasing coupling efficiency from the sources  465   a ,  465   b  and  485   a ,  485   b  into the lightguides  430  and  440 . As will be understood by those of ordinary skill in the art, shape and structure of the reflectors may vary. The number, type and configuration of light sources and lightguides may vary as well. 
   Referring further to  FIG. 5 , the backlighting system  400  may include a recycling enhancement structure  426  disposed at the surface  430   a  of the first lightguide  430 . Preferably, the recycling enhancement structure  426  includes a reflective polarizer, such as DBEF, and prismatic structures, e.g., a prismatic structured film such as BEF. The prismatic structures may be introduced into the backlight  400  by appropriately molding the first lightguide  430 , laminating a sheet of prismatic film onto the surface  430   a , or by any other suitable technique. The reflective polarizer, e.g., DBEF, also may be attached to the lightguide  430 , e.g., by lamination, molding or another suitable bonding technique, preferably over the prismatic structures. Variations may be introduced into the recycling enhancement structure  426 , and particularly into the prismatic structures, to enhance extraction of light from the first lightguide  430  as well as to increase off-axis brightness. See, e.g., U.S. Pat. No. 6,354,709, the disclosure of which is incorporated by reference herein to the extent not inconsistent with the present disclosure. In the appropriate exemplary embodiments, the recycling enhancement structure may include prismatic structures having prism apexes that face generally forward the first lightguide  430 . 
   The second lightguide  440  may include an extractor  443 , preferably a diffuse extractor, disposed at a surface of the lightguide  440 . As in other embodiments described herein, the extractor  443  may be disposed on the surface  440   b  of the lightguide  440  and may include an array of dots. Preferably, the pattern of dots is optimized to compensate for potential spatial non-uniformities of light extraction from the entire backlighting system  400 . The backlighting system  400  may further include a diffuser sheet  416 , which would aid in hiding the diffuse extractor  443  from the viewer and randomizing polarization of recycled light, and a reflector sheet  427 . 
   Series of experiments were conducted to test various aspects of exemplary embodiments of the present disclosure.  FIG. 6  illustrates a testing configuration  700  for exemplary embodiments of the present disclosure. The testing configuration  700  includes a bottom lightguide  740 , illuminated by two CCFL light source assemblies  780   a ,  780   b , with a reflector  727  disposed below the lightguide  740  and a diffuser sheet  716  disposed over the lightguide  740 . Crossed BEFs  714   a ,  714   b  were also included into the testing configuration  700  and positioned over the diffuser sheet  716 . The top lightguide  730  in this configuration was a wedge lightguide laminated with strips of DBEF-M (diffuse side toward lightguide) and DRPF, designated as  712  and located side-by-side. The light source  760  for illuminating the top lightguide  730  was an incandescent fiber line source. An absorbing polarizer  772  was placed over the top lightguide  730  so that it could be aligned or anti-aligned with the reflective polarizer or completely removed. 
   Conoscopic measurements were then taken using ELDIM EZContrast160. All measurements were made at a constant distance from the fiber source to eliminate effects of down-wedge spatial non-uniformities. Performance improvements were seen despite the fact that the output luminance of the wedge lightguide  730  and fiber source  760  was more than an order of magnitude smaller than that of the bottom lightguide  740  illuminated by the CCFL source assemblies  780   a  and  780   b.    
     FIGS. 7A-7C  show results of the first set of measurements, illustrating properties of light extracted from the top lightguide  730  laminated with the reflective polarizers  712 . The absorbing polarizer  772  was not used in these measurements.  FIG. 7A  represents light extraction form the bare top lightguide  730  (no reflective polarizers),  FIG. 7B  represents light extraction from the top lightguide  730  laminated with DBEF-M, and  FIG. 7C  represents light extraction from the top lightguide  730  laminated with DRPF. Extraction from the bare lightguide  730  occurs by total internal reflection failure. The data represented in  FIGS. 7A-7C  demonstrate effectiveness of light extraction from the top lightguide  730  via interactions with the laminated DBEF-M and DRPF.  FIGS. 8A-E  and  9 A-E show results of measurements comparing polarization content of the extracted light to that of light transmitted through the reflective polarizer  712 .  FIG. 8  shows the data for DRPF laminated onto the top lightguide  730  and  FIG. 9  shows the data for DBEF-M laminated onto the top lightguide  730 . Measurements corresponding to  FIGS. 8A ,  8 D,  9 A and  9 D were made without the absorbing polarizer  772 , measurements corresponding to  FIGS. 8B ,  8 E,  9 B and  9 E were made with the absorbing polarizer  772  aligned with the pass axis of the reflective polarizer  712 , and  FIGS. 8C ,  8 F,  9 C and  9 F were made with the absorbing polarizer  772  anti-aligned with the pass axis of the reflective polarizer  712 . In each configuration, two measurements were made: 1) fiber source turned on and CCFL sources turned off ( FIGS. 8A ,  8 B,  8 C,  9 A,  9 B and  9 C), and 2) fiber source turned off and CCFL sources turned on ( FIGS. 8D ,  8 E,  8 F,  9 D,  9 E and  9 F). Overall, the data shown in  FIGS. 8A-E  and  9 A-E demonstrate that light extracted from the top lightguide  730  is polarized predominantly with the same orientation as the light transmitted through the reflective polarizer  712 . 
     FIGS. 10A-D  show measurements performed to verify that the gain of the reflective polarizer  712  in the system  700  is not diminished because it is laminated to the top lightguide  730 . First, the top lightguide was removed and the reflective polarizer was positioned at  712 ′ in  FIG. 6 . Measurements were then made with a loose sheet of DBEF-M and then with the same lot of DBEF-M laminated to the top lightguide  730  (the fiber source was not turned on). The gain due to both the loose sheet and laminated DBEF-M was then computed by taking a ratio of the measurements with DBEF-M to the measurement with DBEF-M removed. The procedure was then repeated for DRPF.  FIGS. 10A-D  show the results of these gain measurements. As can be seen, the gain for the systems with laminated reflective polarizers was slightly higher than that for the loose sheet versions. This occurred due to the fact that an air interface had been removed and some of the diffusivity of the samples had been wetted-out against the lightguide. 
   Thus, the backlighting systems constructed according to the present disclosure allow achieving high output luminances and address various problems encountered with the presently known backlights for LCDs. For example, the present disclosure mitigates the risks of using variable lifetime light sources, so that burnout or aging of an individual light source would not be catastrophic to the display viewing quality. Thus, if an individual light source ages or burns out in a multiple-lightguide system according to an embodiment of the present disclosure, the effect on spatial brightness and color uniformity will be relatively insignificant due to the enhanced light mixing. 
   The present disclosure eliminates the need for a thick diffuser plate traditionally used in direct-lit backlights to hide individual sources from the viewer, thus providing additional gains in brightness. The present disclosure also eliminates the need for a structured reflector surface traditionally used in single-cavity direct-lit backlights, resulting in cost reduction and increased ease of manufacturing. In addition, light extracted directly from the top lightguide is likely to exit at a wide range of angles, which would enhance off-axis viewability of the display. Moreover, the present disclosure makes possible inclusion of additional features for preventing warp and physical damage to various recycling enhancement structures, which may be used in exemplary embodiments of the present disclosure. 
   Although the backlighting systems of the present disclosure have been described with reference to specific exemplary embodiments, those of ordinary skill in the art will readily appreciate that changes and modifications may be made thereto without departing from the spirit and scope of the present invention. For example, the number, type and configuration of light sources, lightguides, and recycling enhancement structures used in exemplary embodiments of the present disclosure may vary. Any of the lightguides used in exemplary embodiments of the present disclosure may be a hollow lightguide or have another suitable structure. See, e.g., U.S. patent application entitled “Hybrid Lightguide Backlight,” Attorney Case No. 59399US002, filed concurrently herewith and incorporated by reference herein to the extent not inconsistent with the present disclosure. 
   In addition, it will be understood by those of ordinary skill of the art, that the terms “prismatic structures,” “prismatic films” and “prisms” encompass those having structural and other variations, such as those described in U.S. Pat. No. 6,354,709, as well as prismatic structures having rounded peaks. Furthermore, although the present disclosure is particularly advantageous for use in large area, high luminance applications typically associated with LCD televisions, it could also encompass LCD monitors and point of sale devices.