PATENT DOCUMENT

Publication Number: US-8570270-B2
Application Number: US-70995710-A
Country: US
Kind Code: B2

Title: Backlight unit color compensation techniques

Abstract:
An edge-lit backlight unit for displays is provided. In one embodiment, the backlight may include a light guide configured to receive light from a light source along a first lateral edge. The received light propagates towards a second opposite lateral edge. The backlight unit may include an arrangement of light-extracting elements configured to extract a portion of the propagating light and to allow the remaining portion to reach the second edge. In one embodiment, a specular reflector disposed at the second edge causes the light reaching the second edge to retro-propagate back towards the first edge. In certain embodiments, the retro-propagating light may be between approximately 5 to 35 percent of the total light received by the light guide. The retro-propagating light may be extracted by multiple light-extracting elements and mixed with the extracted propagating light to provide improved color uniformity along an axis of the display.

Claims:
What is claimed is: 
     
       1. A display backlight unit comprising:
 a light guide configured to receive light from a light source, wherein the light comprises a first portion comprising more of a first color and less of a second color and a second portion comprising less of the first color and more of the second color, wherein the light guide is configured to cause the light to propagate from a first lateral edge towards a second opposite lateral edge of the light guide, and wherein the light guide is configured to cause the first portion to reach the second lateral edge and to retro-propagate from the second lateral edge back towards the first lateral edge; and 
 a plurality of light-extracting elements configured to extract the second portion substantially prior to the second lateral edge and to extract the retro-propagating first portion of the light; 
 wherein the plurality of light-extracting elements is configured to extract the first and second portions of the light to mix the first and second portions to provide substantially uniform color along an axis of the light guide. 
 
     
     
       2. The display backlight unit of  claim 1 , comprising the light source, wherein the light source is arranged along the first lateral edge. 
     
     
       3. The display backlight unit of  claim 1 , wherein the second lateral edge comprises a specular reflector configured to cause retro-propagation of the first portion of the propagating light received at the second lateral edge. 
     
     
       4. The display backlight unit of  claim 1 , wherein the second lateral edge comprises a reflector having a combination of diffuse and specular components configured to cause retro-propagation of the first portion of the propagating light received at the second lateral edge. 
     
     
       5. The display backlight unit of  claim 1 , wherein the light guide is configured to allow between approximately 5 to 35 percent of the light received by the light guide from the light source to reach the second lateral edge and to retro-propagate from the second lateral edge back towards the first lateral edge. 
     
     
       6. The display backlight unit of  claim 1 , wherein the plurality of light-extracting elements includes light-extracting elements formed on a rear surface of the light guide. 
     
     
       7. The display backlight unit of  claim 1 , wherein the plurality of light-extracting elements comprises printed dots, micro-lenses, micro-prisms, circular light-extracting elements, non-circular light-extracting elements, elliptical light-extracting elements, square light-extracting elements, rectangular light-extracting elements, groove-shaped light-extracting elements, or some combination thereof. 
     
     
       8. The display backlight unit of  claim 1 , comprising the light source, wherein the light source comprises a plurality of light emitting diodes (LEDs), at least one of the plurality of LEDs exhibits chromaticity variations across a distribution of viewing angles from optical axes of the respective LEDs, and the plurality of LEDs comprise phosphor-coated LEDs. 
     
     
       9. The display backlight unit of  claim 8 , wherein the plurality of LEDs comprise a blue-light emitting die with a yellow phosphor coating. 
     
     
       10. The display backlight unit of  claim 9 , the chromaticity variations comprise bluer light emitted substantially along the optical axes, and more yellow light emitted away from the optical axes. 
     
     
       11. The display backlight unit of  claim 1 , wherein the first color comprises blue. 
     
     
       12. A method, comprising:
 receiving light from a light source at a first lateral edge of a light guide of a backlight unit, wherein the received light exhibits a non-uniform color characteristic across a distribution of viewing angles from an optical axis of the light source;
 propagating the received light through a propagation medium and towards a second lateral edge of the light guide, wherein the second lateral edge is opposite the first lateral edge, and wherein a first portion of the propagating light reaches the second lateral edge; 
 
 using a reflective element disposed at the second lateral edge to cause substantially the entire first portion of the propagating light reaching the second lateral edge to retro-propagate towards the first lateral edge;
 using a plurality of light extracting features formed on a rear surface of the light guide to extract a second portion of the propagating light and to extract the retro-propagating first portion of the light; and 
 mixing the extracted second portion of the propagating light and the extracted retro-propagating first portion of the light to improve color uniformity in light emitted from a front surface of the light guide, wherein the retro-propagating first portion of the light comprises more of the light emitted along the optical axis and less of the light emitted away from the optical axis, and the second portion of the light comprises less of the light emitted along the optical axis and more of the light emitted away from the optical axis. 
 
 
     
     
       13. The method of  claim 12 , wherein the propagation medium comprises polymethyl-methacrylate or air, or some combination thereof. 
     
     
       14. The method of  claim 12 , wherein the light source comprises at least one light-emitting diode (LED), and wherein the non-uniform color characteristic is attributable to chromaticity variations of the at least one LED across the distribution of viewing angles. 
     
     
       15. The method of  claim 12 , wherein the light emitted from the backlight unit has a Δu′v′ of less than approximately 0.005 based upon the CIE 1976 color space. 
     
     
       16. A display device comprising:
 a liquid crystal display panel; and 
 an edge-lit backlight unit comprising: 
 a light source; 
 a light guide comprising a first lateral edge, a second lateral edge opposite the first lateral edge, a front surface, and a rear surface, at least one of the front surface and the rear surface having a plurality of light-extracting elements, wherein the light guide is configured to receive light emitted from the light source along the first lateral edge and to propagate the received light towards the second lateral edge, and wherein the plurality of light-extracting elements is configured to allow a first portion of the propagating light to reach the second lateral edge and to extract a second portion of the propagating light, wherein the first portion comprises more of a first color and less of a second color, and the second portion comprising less of the first color and more of the second color; and 
 a specular reflector disposed at the second lateral edge and configured to cause substantially the entire first portion of the propagating light to retro-propagate back towards the first lateral edge; 
 wherein the light guide is configured such that the retro-propagating first portion of the light is extracted by the plurality of light-extracting elements, wherein the extracted retro-propagating first portion of the light and the extracted second portion of the propagating light is mixed and emitted from the light guide, and wherein the mixed light emitted from the light guide is substantially uniform in color. 
 
     
     
       17. The display device of  claim 16 , wherein the light source comprises one or more light-emitting diodes (LEDs). 
     
     
       18. The display device of  claim 17 , wherein at least one of the one or more LEDs comprises a phosphor-coated LED that exhibits chromaticity variations across a distribution of viewing angles, and wherein the mixing of the extracted second portion of the propagating light and the extracted retro-propagating first portion of the light reduces color shift caused by the chromaticity variations of the phosphor-coated LED. 
     
     
       19. The display device of  claim 18 , wherein the phosphor-coated LED is configured to emit generally white light and comprises one of an LED having a blue-light emitting die with a yellow phosphor coating or an LED having a blue-light emitting die with a red and green phosphor coating. 
     
     
       20. The display device of  claim 18 , wherein the phosphor-coated LED is configured to emit a bluer light along an optical axis and a less blue light away from the optical axis. 
     
     
       21. The display device of  claim 17 , wherein the light source is configured such that the one or more LEDs are facing the first lateral edge. 
     
     
       22. The display device of  claim 21 , comprising a first reflector disposed on a first side of the one or more LEDs and a second reflector disposed on a second side of the one or more LEDs, wherein light that is not projected directly into the light guide at the first lateral edge by the one or more LEDs is directed into the light guide by the first or second reflectors. 
     
     
       23. A method of reducing color shift in an edge-lit backlight unit, the method comprising:
 receiving light emitted from a light source at a first lateral edge of a light guide; 
 propagating the light towards a second lateral edge opposite the first lateral edge; 
 extracting a second portion of the light during propagation towards the second lateral edge using one or more light-extracting elements; 
 receiving, at the second lateral edge, a first portion of the light not extracted by the one or more light-extracting elements during propagation; 
 retro-propagating the first portion of the light received at the second lateral edge towards the first lateral edge; 
 extracting the retro-propagating the first portion of the light using the one or more light-extracting elements; and emitting the extracted second portion of the propagating light and the extracted retro-propagating first portion of the light from a front surface of the light guide, wherein the first portion comprises more of a first color and less of a second color, and the second portion comprising less of the first color and more of the second color. 
 
     
     
       24. The method of  claim 23 , wherein propagation and retro-propagation occur via total internal reflection within the light guide. 
     
     
       25. The method of  claim 23 , wherein retro-propagating the first portion of the light received at the second lateral edge comprises using a specular reflector disposed at the second lateral edge. 
     
     
       26. The method of  claim 23 , wherein the first portion of the light that is retro-propagated back towards the first lateral edge comprises between approximately 5 to 35 percent of the light received by the light guide from the light source. 
     
     
       27. An electronic device, comprising:
 one or more input structures; 
 a storage structure encoding one or more executable routines; 
 a processor capable of receiving inputs from the one or more input structures and of executing the one or more executable routines when loaded in a memory; and 
 a display device configured to display an output of the processor, wherein the display device comprises:
 a liquid crystal display panel; and 
 a backlight unit comprising:
 a light source configured to provide direct lighting to the backlight unit by emitting light that exhibits a non-uniform color characteristic; and 
 a light guide configured to receive the light from the light source, wherein the light comprises a first portion comprising more of a first color and less of a second color, and a second portion comprising less of the first color and more of the second color, wherein the light guide is configured to cause the light to propagate from a first lateral edge towards a second opposite lateral edge of the light guide, to cause the first portion to reach the second lateral edge and to retro-propagate from the second lateral edge back towards the first lateral edge, to extract the second portion of the propagating light substantially prior to the second lateral edge, and to extract the retro-propagating first portion of the light; 
 wherein the extracted second portion of the propagating light and the extracted retro-propagating first portion of the light are mixed to improve color uniformity in light emitted from the light guide towards the liquid crystal display panel. 
 
 
 
     
     
       28. The electronic device of  claim 27 , wherein the second lateral edge comprises a specular reflector configured to cause retro-propagation of the first portion of the propagating light received at the second lateral edge. 
     
     
       29. The electronic device of  claim 27 , wherein the light guide has a thickness of between approximately 1 to 6 millimeters. 
     
     
       30. The electronic device of  claim 27 , wherein the electronic device comprises a desktop computer, a laptop computer, a tablet computer, a digital media player, or a mobile telephone. 
     
     
       31. A display backlight unit comprising:
 a light guide configured to receive light from a light source, wherein the light comprises chromaticity variations across a distribution of viewing angles from an optical axis of the light source, wherein the light guide is configured to cause the light to propagate from a first lateral edge towards a second opposite lateral edge of the light guide, and to cause a first portion of the propagating-light to reach the second lateral edge and to retro-propagate from the second lateral edge back towards the first lateral edge, wherein the retro-propagating first portion comprises more of the light emitted along the optical axis and less of the light emitted away from the optical axis; and 
 a plurality of light-extracting elements configured to extract a second portion of the propagating light substantially prior to reaching the second lateral edge and to extract the retro-propagating first portion of the light, wherein the extracted second portion of the propagating light comprises less of the light emitted along the optical axis and more of the light emitted away from the optical axis, the plurality of light-extracting elements are arranged on a rear surface of the light guide and are configured to reduce the disruption of total internal reflection of the propagating light, wherein a size of the plurality of light extracting elements increases from the first lateral edge to the second lateral edge, and a spacing of the plurality of light extracting elements decreases from the first lateral edge to the second lateral edge; 
 wherein the extracted first and second portions of the light are mixed and emitted from the plurality of light-extracting features as light output to provide substantially uniform color along an axis of the light guide, wherein the light output has a Δu′v′ of less than approximately 0.005 based upon the CIE 1976 color space. 
 
     
     
       32. The display backlight unit of  claim 31 , comprising the light source, wherein the light source comprises a plurality of light emitting diodes arranged along the first lateral edge. 
     
     
       33. The display backlight unit of  claim 31 , wherein the plurality of light-extracting elements are arranged to increase the first portion of the propagating light received at the second lateral edge to between approximately 5 to 35 percent of the light received by the light guide. 
     
     
       34. A display backlight unit comprising:
 a light source comprising one or more light-emitting diodes (LEDs), wherein at least one of the one or more LEDs exhibits chromaticity variations across a distribution of viewing angles from optical axes of the respective LEDs, wherein the chromaticity variations comprise bluer light emitted substantially along the optical axes, and more yellow light emitted away from the optical axes; 
 a light guide comprising a first lateral edge, a second opposite lateral edge, and a forward surface disposed between the first lateral edge and the second lateral edge, wherein the light guide is configured to receive light from the light source, to cause the light to propagate from the first lateral edge towards the second opposite lateral edge of the light guide, and to cause a first portion of the light to reach the second lateral edge and to retro-propagate from the second lateral edge back towards the first lateral edge, wherein the retro-propagating first portion comprises more of the bluer light and less of the more yellow light; and 
 a plurality of light-extracting elements configured to extract a second portion of the light substantially prior to reaching the second lateral edge, to extract the retro-propagating first portion of the light, wherein the second portion of the light comprises more of the more yellow light and less of the bluer light, and to mix the first and second portions of the light as mixed light, wherein the plurality of light-extracting elements is configured to emit the mixed light through the forward surface of the light guide to provide a substantially uniform color light output from the forward surface. 
 
     
     
       35. The display backlight unit of  claim 34 , wherein the light output has a Δu′v′ of less than approximately 0.005 based upon the CIE 1976 color space.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 61/253,060, entitled “Backlight Unit Color Compensation Techniques”, filed Oct. 19, 2009, which is herein incorporated by reference. 
    
    
     BACKGROUND 
     The present disclosure relates generally to display devices and, more particularly, to techniques for improving color uniformity in display devices. 
     This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present techniques, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
     In recent years, light-emitting diodes (LEDs) have begun to replace fluorescent lighting, such as cold cathode fluorescent lamps (CCFLs), as a light source for backlight units of liquid crystal displays (LCDs) used in a wide variety of electronic devices, including such consumer electronics as televisions, computers, and handheld devices (e.g., cellular telephones, audio and video players, portable gaming systems, and so forth). This is due at least in part to a number of advantages that LEDs exhibit over CCFLs, including improved efficiency and higher light output, lower power consumption, reduced heat output, and longer operational life. Additionally, LEDs are generally more environmentally friendly relative to CCFLs (e.g., CCFLs may contain mercury, whereas LEDs do not). 
     While LEDs retain several advantages over CCFLs, due to the manner in which certain LEDs are fabricated, particularly phosphor-coated LEDs, chromaticity variations may be present in the light emitted from an LED over a range of angles (e.g., a “viewing angle”) relative to an optical axis. In certain backlight units, such as edge-lit backlight units, these variations in chromaticity may negatively affect the color uniformity of the light emitted from a light guide of the backlight unit, such as by causing a color shift along an axis of the backlight unit. As such, it may be beneficial to provide a technique for reducing color shift caused by chromaticity variations of LED lighting, thereby improving color uniformity in LCD displays that utilize LEDs as a light source. 
     SUMMARY 
     A summary of certain embodiments disclosed herein is set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of these certain embodiments and that these aspects are not intended to limit the scope of this disclosure. Indeed, this disclosure may encompass a variety of aspects that may not be set forth below. 
     The present disclosure generally relates to an edge-lit backlight unit for a display device, such as an LCD display. In one embodiment, the edge-lit backlight unit includes a light guide configured to receive light from a light source along a first lateral edge and to propagate the received light towards a second opposite lateral edge. The backlight unit may include a light-extracting layer having multiple light-extracting elements configured to extract a portion of the propagating light and to allow the remaining portion to reach the second lateral edge. For instance, the light-extracting layer may generally be understood to be an arrangement of the light-extracting elements and may be formed on a rear surface of the light guide. The light-extracting surface area provided by the light-extracting elements on the rear surface may be less relative to the arrangement of light-extracting elements on rear surfaces of light guides in conventional backlight units. In one embodiment, a specular reflector disposed at the second lateral edge may cause retro-propagation of the light reaching the second lateral edge back towards the first lateral edge. The retro-propagating light may be extracted by the multiple light-extracting elements and mixed with the extracted propagating light to provide improved color uniformity along an axis of the display. In certain embodiments, the retro-propagating light may be between approximately 5 to 35 percent of the total light received by the light guide. 
     Various refinements of the features noted above may exist in relation to various aspects of the present disclosure. Further features may also be incorporated in these various aspects as well. These refinements and additional features may exist individually or in any combination. For instance, various features discussed below in relation to one or more of the illustrated embodiments may be incorporated into any of the above-described aspects of the present disclosure alone or in any combination. Again, the brief summary presented above is intended only to familiarize the reader with certain aspects and contexts of embodiments of the present disclosure without limitation to the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects of this disclosure may be better understood upon reading the following detailed description and upon reference to the drawings in which: 
         FIG. 1  is a block diagram of exemplary components of an electronic device that may incorporate certain aspects of the present disclosure; 
         FIG. 2  is a perspective view of a computer, in accordance with aspects of the present disclosure; 
         FIG. 3  is a front-view of a handheld electronic device, in accordance with aspects of the present disclosure; 
         FIG. 4  is a simplified exploded perspective view of a liquid crystal display (LCD) including an edge-lit backlight unit, in accordance with aspects of the present disclosure; 
         FIG. 5  is a cross-sectional view showing the operation of a light-emitting diode (LED); 
         FIG. 6  is a graph depicting chromaticity variation with respect to angles at which light is emitted from the LED of  FIG. 5 ; 
         FIG. 7  is a diagram showing the CIE 1976 color space; 
         FIG. 8  is a simplified cross-sectional view of an edge-lit backlight unit and depicts how the propagation of light emitted from the LED of  FIG. 5  may result in color shift along an axis of the edge-lit backlight unit; 
         FIG. 9  depicts a rear surface of the edge-lit backlight unit shown in  FIG. 8 ; 
         FIG. 10  is a simplified cross-sectional view of an edge-lit backlight unit configured to provide for retro-propagation of light emitted from the LED of  FIG. 5  to reduce color shift along an axis of the edge-lit backlight unit, in accordance with aspects of the present disclosure; 
         FIGS. 11 and 12  depict embodiments of rear surfaces of the edge-lit backlight unit shown in  FIG. 10 , in accordance with aspects of the present disclosure; and 
         FIG. 13  is a flowchart depicting a method for reducing color shift in light emitted from a backlight unit of a display device, in accordance with aspects of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     One or more specific embodiments will be described below. These described embodiments are provided only by way of example, and do not limit the scope of the present disclosure. Additionally, in an effort to provide a concise description of these exemplary embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     When introducing elements of various embodiments described below, the articles “a,” “an,” and “the” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Moreover, while the term “exemplary” may be used herein in connection to certain examples of aspects or embodiments of the presently disclosed subject matter, it will be appreciated that these examples are illustrative in nature and that the term “exemplary” is not used herein to denote any preference or requirement with respect to a disclosed aspect or embodiment. Additionally, it should be understood that references to “one embodiment,” “an embodiment,” “some embodiments,” and the like are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the disclosed features. 
     As will be discussed below, the present disclosure is generally directed to edge-lit backlight units for display devices. Particularly, the present application discloses edge-lit backlight units that provide for reduced color shift and, therefore, improved color uniformity compared to certain traditional backlight units. The presently disclosed backlight units may compensate for color shift caused by chromaticity variations in light emitted by certain types of light-emitting diodes (LEDs), particularly at angles farther from an optical axis. As discussed in greater detail below, in some embodiments, edge-lit backlight units in accordance with the present techniques include a light guide configured to provide for propagation of light received from a light source from a first lateral edge to a second opposite lateral edge. A portion of the received light is allowed to reach the second lateral edge and is retro-propagated back towards the first lateral edge. Multiple light-extracting elements are provided to extract and mix the propagating and retro-propagating light, such that the light emitted from the light guide exhibits improved color uniformity. 
     Keeping the above points in mind, a general description of electronic devices that may employ such an edge-lit backlight unit is provided below. As may be appreciated, such an electronic device may include various internal and/or external components which contribute to the function of the device. For instance,  FIG. 1  is a block diagram illustrating components that may be present in one such electronic device  10 , and which may allow the device  10  to function in accordance with the techniques discussed herein. Those of ordinary skill in the art will appreciate that the various functional blocks shown in  FIG. 1  may include hardware elements (including circuitry), software elements (including computer code stored on a computer-readable medium, such as a hard drive or system memory), or a combination of both hardware and software elements. It should further be noted that  FIG. 1  is merely one example of a particular implementation and is merely intended to illustrate the types of components that may be present in the electronic device  10 . For example, in the presently illustrated embodiment, these components may include a display  12 , input/output (I/O) ports  14 , input structures  16 , one or more processors  18 , memory device(s)  20 , non-volatile storage  22 , expansion card(s)  24 , networking device  26 , and power source  28 . 
     With regard to each of these components, it is first noted that the display  12  may be used to display various images generated by the electronic device  10 . In various embodiments, the display  12  may be a liquid crystal display (LCD), such as an LCD utilizing light-emitting diodes (LEDs) as a backlight source. Additionally, in certain embodiments of the electronic device  10 , the display  12  may be provided in conjunction with a touch-sensitive element, such as a touchscreen, that may be used as part of a control interface for the device  10 . As will be discussed below, the display  12  may include an edge-lit backlight unit implementing one or more of the techniques described herein to provide for reduced color shift and thus improved color uniformity across the display  12 . 
     The I/O ports  14  may include various ports configured to connect to a variety of external devices, such as a power source, headset or headphones, or other electronic devices (such as handheld devices and/or computers, printers, projectors, external displays, modems, docking stations, and so forth). The I/O ports  14  may support any interface type, such as a universal serial bus (USB) port, a video port, a serial connection port, an IEEE-1394 port, an Ethernet or modem port, and/or an AC/DC power connection port. In one embodiment, the I/O ports  14  may include a proprietary port from Apple Inc. of Cupertino, Calif., that may function to charge the power source  28  (which may include one or more rechargeable batteries) of the device  10 , or to transfer data between the device  10  and an external source. 
     The input structures  16  may include the various devices, circuitry, and pathways by which user input or feedback is provided to the processor(s)  18 . Such input structures  16  may be configured to control various functions of the electronic device  10 , applications running on the device  10 , and/or any interfaces or devices connected to or used by the device  10 . For example, the input structures  16  may allow a user to navigate a displayed user interface (e.g., a graphical user interface) or application interface. Non-limiting examples of the input structures  16  include buttons, sliders, switches, control pads, keys, knobs, scroll wheels, keyboards, mice, touchpads, and so forth. User interaction with input structures  16 , such as to interact with a user or application interface displayed on display  12 , may generate electrical signals indicative of user input, which may be routed via suitable pathways, such as an input hub or bus, to the processor(s)  18  for further processing. 
     Additionally, in certain embodiments, one or more of the input structures  16  may be provided together with the display  12 , such an in the case of a touchscreen, in which a touch sensitive mechanism is provided in conjunction with the above-mentioned display  12 . In such embodiments, the user may select or interact with displayed interface elements via the touch sensitive mechanism. In this manner, the displayed interface may provide interactive functionality, allowing a user to navigate the displayed interface by touching the display  12 , such as by way of the user&#39;s finger or a stylus. 
     The processor(s)  18  may include one or more microprocessors, such as one or more “general-purpose” microprocessors, one or more special-purpose microprocessors and/or ASICS, or some combination of such processing components, and may provide the processing capability to execute the operating system, programs, user and application interfaces, and any other functions of the electronic device  10 . For example, processor(s)  18  may include one or more reduced instruction set (RISC) or x86 processors, as well as graphics processors (GPU), video processors, audio processors, and the like. As will be appreciated, the processor(s)  18  may be communicatively coupled to one or more data buses or chipsets for transferring data and instructions between various components of the electronic device  10 . 
     Programs or instructions executed by the processor(s)  18  may be stored in any suitable manufacture that includes one or more tangible, computer-readable media at least collectively storing the executed instructions or routines, such as, but not limited to, the memory devices and storage devices described below. Also, these programs (e.g., an operating system) encoded on such a computer program product may also include instructions that may be executed by the processor(s)  18  to enable the device  10  to provide various functionalities, including those described herein. 
     The instructions or data to be processed by the processor(s)  18  may be stored in a computer-readable medium, such as the memory  20 . The memory  20  may include a volatile memory, such as random access memory (RAM), and/or a non-volatile memory, such as read-only memory (ROM), or a combination of RAM and ROM devices. The memory  20  may store a variety of information and may be used for various purposes. For example, the memory  20  may store firmware for the electronic device  10  (such as basic input/output system (BIOS)), an operating system, and various other programs, applications, or routines that may be executed on the electronic device  10 , including user interface functions, processor functions, and so forth. In addition, the memory  20  may be used for buffering or caching during operation of the electronic device  10 . 
     The components of the device  10  may further include other forms of computer-readable media, such as the non-volatile storage  22 , for persistent storage of data and/or instructions. The non-volatile storage  22  may include, for example, flash memory, a hard drive, or any other optical, magnetic, and/or solid-state storage media. By way of example, the non-volatile storage  22  may be used to store firmware, data files, software programs, wireless connection information, and any other suitable data. 
     The embodiment illustrated in  FIG. 1  may also include one or more card or expansion slots. The card slots may be configured to receive one or more expansion cards  24  that may be used to add functionality, such as additional memory, I/O functionality, or networking capability, to the electronic device  10 . Such expansion cards  24  may connect to the device  10  through any type of suitable connector, and may be accessed internally or external to the housing of the electronic device  10 . For example, in one embodiment, the expansion card(s)  24  may include a flash memory card, such as a SecureDigital (SD) card, mini- or microSD, CompactFlash card, Multimedia card (MMC), or the like. Additionally, the expansion cards  24  may include one of the processor(s)  18  of the device  10 , such as a video graphics card having a GPU for facilitating graphical rendering by the device  10 . 
     The components depicted in  FIG. 1  also include the network device  26 , such as a network controller or a network interface card (NIC). For example, the network device  26  may be a wireless NIC providing wireless connectivity over any 802.11 standard or any other suitable wireless networking standard. The network device  26  may allow the electronic device  10  to communicate over a network, such as a personal area network (PAN), a local area network (LAN), a wide area network (WAN), such as an Enhanced Data Rates for GSM Evolution (EDGE) network or a 3G data network (e.g., based on the IMT-2000 standard), or the Internet. By way of the network device  26 , the electronic device  10  may connect to and send or receive data with any device on the network, such as portable electronic devices, personal computers, printers, and so forth. In some embodiments, the network device  26  may be added as one expansion card  24  to provide for the networking capability as described above. 
     As also illustrated in  FIG. 1 , the device  10  may include the power source  28 . In one embodiment, the power source  28  may be one or more batteries, such as a lithium-ion polymer battery or other type of suitable battery. The battery may be user-removable or may be secured within the housing of the electronic device  10 , and may be rechargeable. Additionally, the power source  28  may include AC power, such as provided by an electrical outlet, and the electronic device  10  may be connected to the power source  28  via a power adapter. In embodiments where the power source  28  also includes one or more batteries, such a power adapter may used to recharge the one or more batteries. 
     Having described the exemplary components depicted in  FIG. 1 , it should be appreciated that the electronic device  10  may take the form of a computer system or some other type of electronic device. Such computers may include computers that are generally portable (such as laptop, notebook, tablet, and handheld computers), as well as computers that are generally used in one place (such as conventional desktop computers, workstations and/or servers). In certain embodiments, the electronic device  10  in the form of a computer may be a model of a MacBook®, MacBook® Pro, MacBook Air®, iMac®, Mac® Mini, or Mac Pro®, available from Apple Inc. By way of example, the electronic device  10  in the form of laptop computer  30  is illustrated in  FIG. 2  in accordance with one embodiment. The depicted computer  30  includes a housing or enclosure  33 , the display  12  (e.g., LCD  34  or some other suitable display), I/O ports  14 , and input structures  16 . 
     In one embodiment, the input structures  16  (such as a keyboard and/or touchpad mouse) may be used to interact with the computer  30 , such as to start, control, or operate a graphical user interface (GUI) or applications running on the computer  30 . For example, a keyboard and/or touchpad may allow a user to navigate a user interface or application interface displayed on the display  12  (e.g., LCD  34 ). 
     As depicted, the electronic device  10  in the form of the computer  30  may also include various I/O ports  14  to allow connection of additional devices. For example, the I/O ports  14  may include a USB port, a DVI port, a DisplayPort port, or some other port suitable for connecting to another electronic device, a projector, a supplemental display, and so forth. In addition, computer  30  may include network connectivity, memory, and storage capabilities, as described with respect to  FIG. 1 . As a result, computer  30  may store and execute a GUI and other applications. 
     As mentioned above, the computer  30 , in other embodiments, may also be a desktop computer or workstation/server computer that is generally designed to be less portable than the illustrated laptop computer  30  of  FIG. 2 . In such embodiments, the display  12  may be a standalone display that interfaces with the computer  30  using one of the I/O ports  14 , such as via a DisplayPort, DVI, or analog (D-sub) interface. For instance, in certain embodiments, such a standalone display  12  may be a model of an Apple Cinema Display®, available from Apple Inc. As will be discussed below, the display  12  may include an edge-lit backlight unit implementing one or more of the techniques described herein to provide for reduced color shift and, therefore, improved color uniformity across the display  12 . 
     While the electronic device  10  is generally depicted in the context of a computer in  FIG. 2 , it should be appreciated that the electronic device  10  may also take the form of other types of devices. In some embodiments, various electronic devices  10  may include mobile telephones, media players for playing music and/or video, personal data organizers, handheld game platforms, cameras, and/or combinations of such devices. For instance, as generally depicted in  FIG. 3 , the device  10  may be provided in the form of a handheld electronic device  32  that includes various functionalities (such as the ability to take pictures, make telephone calls, access the Internet, communicate via email, record audio and/or video, listen to music, play games, connect to wireless networks, and so forth). By way of further example, the handheld device  32  may be a model of an iPod® or iPhone® available from Apple Inc. In other embodiments, however, other types of handheld devices (such as other media players for playing music and/or video, personal data organizers, handheld game platforms, and/or combinations of such devices) may also be suitably provided as the electronic device  10 . 
     The handheld device  32  of the presently illustrated embodiment includes an enclosure or body  33  that protects the interior components from physical damage and shields them from electromagnetic interference. The enclosure  33  may be formed from any suitable material such as plastic, metal or a composite material and may allow certain frequencies of electromagnetic radiation to pass through to wireless communication circuitry within the handheld device  32  to facilitate wireless communication. 
     The handheld electronic device  32  may also include various input and output (I/O) ports  14  that allow connection of the handheld device  32  to external devices. For example, one I/O port  14  may be a port that allows the transmission and reception of data or commands between the handheld electronic device  32  and another electronic device, such as a computer. Such an I/O port  14  may be a proprietary port from Apple Inc. or may be an open standard I/O port (e.g., USB, FireWire, etc.). 
     In the depicted embodiment, the enclosure  33  includes the user input structures  16  through which a user may interface with the device  32 . Each user input structure  16  may be configured to help control one or more device functions when actuated. For example, in a mobile telephone implementation, one or more input structures  16  may be configured to invoke a “home” screen or menu to be displayed, to toggle between a sleep and a wake mode, to silence a ringer for a cell phone application, to increase or decrease a volume output, and so forth. 
     In the depicted embodiment, the handheld device  32  includes the display  12 , which may be in the form of an LCD  34 . The LCD  34  may display various images generated by the handheld device  32 . For example, the LCD  34  may display various system indicators  36  that provide feedback to a user with regard to one or more states of the handheld device  32 , such as power status, signal strength, call status, external device connections, and so forth. 
     The LCD  34  may also be configured to display a graphical user interface (GUI)  38  that allows a user to interact with the handheld device  32 . As can be appreciated, the GUI  38  may include various layers, windows, screens, templates, or other graphical elements that may be displayed in all, or a portion, of the LCD  34 . Generally, the GUI  38  may include graphical elements that represent applications and functions of the electronic device, such as icons  40 , as well as other images representing buttons, sliders, menu bars, and the like. The icons  40  may correspond to various applications of the electronic device that may open upon selection of a respective icon  40 . Furthermore, selection of a particular icon  40  may lead to a hierarchical navigation process, such that selection of the particular icon  40  leads to a screen that includes one or more additional icons or other GUI elements. In certain embodiments, the icons  40  may be selected via a touchscreen provided with the display  12 , or may be selected by one or more of the user input structures  16 , such as a wheel or button. Of course, the LCD display  34  may also be used to display other data, images, or visual outputs, such as digital photographs or video data. 
     Continuing now to  FIG. 4 , one example of an LCD display  34  is depicted in accordance with one embodiment. As shown, the LCD display  34  may generally include an LCD panel  46  and a backlight unit  48 , which may be assembled within a frame or enclosure  50 . The LCD panel  46  may include numerous pixels configured to selectively modulate the amount and color of light passing from the backlight unit  48  through the LCD panel  46 . For example, the LCD panel  46  may include a liquid crystal layer, one or more thin film transistor layers configured to control orientation of liquid crystals of the liquid crystal layer via an electric field, and polarizing films, which cooperate to enable the LCD panel  46  to control the amount of light emitted by each pixel. The LCD panel  46  may be a twisted nematic (TN) panel, an in-plane switching (IPS) panel, a fringe-field switching (FFS) panel, variants of the foregoing types of panels, or any other suitable panel. 
     In the illustrated embodiment, the backlight unit  48  is depicted as being an edge-lit backlight unit, and includes a light guide  52 , such as a light guiding plate, one or more optical films  54 , such as one or more brightness enhancement films, and a light source  56  having a number of lighting elements  58 , such as LEDs. As shown, the light source  56  is positioned to provide light to a lateral edge  60  of the light guide  52 . In certain embodiments, the light guide  52  may be formed using polymethyl-methacrylate (“PMMA”), an acrylic glass, and the backlight unit  48  may thus be referred to as a “PMMA” backlight. It is noted, however, that the light guide  52  may be formed of other suitable materials. Further, in some embodiments, the light guide  52  may be an air guide (e.g., rather than using a material like PMMA, the propagation medium within the light guide may be air). 
     Light from the light source  56  generally propagates through the light guide  52  towards an opposite lateral edge  62  via total internal reflection at forward  64  and rear  66  surfaces of the light guide  52 , and may ultimately be emitted therefrom towards the optical film(s)  54  and LCD panel  46 . For example, the light guide  52  may include certain optical features on either or both of the forward  64  and rear  66  surfaces that disrupt total internal reflection. The optical features, which may be referred to as “light-extracting elements,” may be specular, diffuse, or a combination of both specular and diffuse elements. The light-extracting elements generally function to reflect and/or refract the propagating light. For instance, in one embodiment, the light guide  52  may include a light-extracting layer (not shown in  FIG. 4 ) formed along the rear surface  66  that includes the light-extracting elements, such as printed dots, micro-lenses, and/or micro-prisms, or any other type of optical feature suitable for disrupting total internal reflection. The light-extracting elements may have specular or diffuse properties, or a combination of such properties, and may reflect and/or refract light towards a back reflector (not shown in  FIG. 4 ). The back reflector, which may be a diffuse reflector, may be disposed adjacent the light-extracting elements on the rear surface  66  to direct at least some of the light impacting the light-extracting elements towards the front surface  64  of the light guide  52 , where it may emitted therefrom in the forward direction  68 . 
     Before continuing, it should be understood that the use of the positional terms, such as forward, rear, top, and bottom, etc., are intended to refer to the orientation of the light guide  52  shown in  FIG. 4 . For instance, as mentioned above, the surfaces  64  and  66  may be referred to as the forward and rear surfaces, respectively. Additionally, the lateral edges  60  and  62  may be referred to as the top/upper and bottom/lower edges, respectively, and the edges  70  and  72  may be referred to as side edges. While these terms are used within the present disclosure for clarity, it should be understood that these positions may change depending on the actual orientation of the LCD display  34 . 
     As discussed above, due to the nature in which certain LEDs are fabricated, chromaticity variations may exist in the light emitted therefrom. Accordingly,  FIGS. 5-9  are provided herein to more clearly illustrate how such chromaticity variations may result in an undesirable color shift being present along an axis of a backlight unit. 
     Referring to  FIG. 5 , a simplified cross-sectional view of one of the LEDs  58  of the light source  56  shown in  FIG. 4  is depicted. In accordance with aspects of the present disclosure, the LED  58  may be a phosphor-coated LED that is configured to emit generally white light. For instance, the LED  58  may include a substrate  78  upon which a semiconductor material  76  (a “die”) is formed and coated with a phosphor layer  80 . By way of example, an LED configured to emit generally white light may include a blue-light emitting die, such as indium gallium nitride (InGaN) or gallium nitride (GaN), coated with a yellow phosphor layer, such as cerium-doped yttrium aluminum garnet (Ce 3+ :YAG). While certain embodiments described in the present disclosure will refer to the LED  58  as including a blue-light emitting die and yellow phosphor coating, it should be understood that the LED  58  may include other types of phosphor-coated LEDs that emit generally white light, such as LEDs having a blue-die with a red and green phosphor coating, and may also include other phosphor-coated LEDs configured to emit light having a color other than white. 
     In operation, the LED  58  becomes forward biased when switched on, thereby allowing electrons to recombine with holes to release energy in the form of light. The light is emitted from the LED  58  (e.g., through the lens  82 ), and may be received by the light guide  52 . Due to the different distances in which blue light emitted from the die  76  travels through the phosphor layer  80  of the LED  58 , the light ultimately emitted from the LED  58  may exhibit chromaticity variations along an angular distribution (e.g., along the positive angular direction  83  and negative angular direction  85 ) relative to an optical axis  84 . That is, color variations may be apparent depending on the angle (e.g., a “viewing angle”) at which a light ray is emitted from the LED  58  relative to the optical axis  84 . 
     For instance, as shown in  FIG. 5 , the light ray  86  emitted closest to or directly along the optical axis  84  travels a relatively short distance d 1  through the yellow phosphor layer  80 . Similarly, a light ray  88 , which is emitted at a first angle (e.g., approximately 45 degrees relative to the optical axis  84 ), travels a farther distance d 2  through the yellow phosphor layer  80 , and a light ray  90 , which is emitted at a greater second angle (e.g., approximately 85 degrees relative to the optical axis  84 ), travels an even further distance d 3  through the yellow phosphor layer  80 . 
     Thus, while the overall light emitted from the LED  58  is generally white in color, the emitted light may be more blue in color near the optical axis  84  due to the relatively short distance (d 1 ) at which it travels through the yellow phosphor layer  80 , while light emitted further from the optical axis  84  becomes increasingly yellow in color due to the increasingly greater distances (d 2 , d 3 ) at which it travels through the yellow phosphor layer  80 . Accordingly, while the terms “blue” and “yellow” are used to describe the light emitted from the LED  58 , it should be understood that these terms are not intended to imply that the emitted light is actually blue or yellow. Rather, the terms “blue” and “yellow” are meant to imply that the generally white light, when emitted closer to the optical axis  84  has a bluer tint relative to light emitted further away from the optical axis  84 , which may have a more yellowish tint. 
     The above-described chromaticity variations in the blue-die and yellow phosphor-coated LED  58  of  FIG. 5  may be better understood with respect to  FIG. 6 , which depicts a graph  94  showing the LED chromaticity variation over a range of viewing angles, and  FIG. 7 , which provides a diagram  97  of the CIE 1976 color space. As shown in the diagram  97 , a white point  98  in the CIE 1976 diagram is approximately located at the coordinates 0.23 on the u′ axis and 0.48 on the v′ axis. As discussed above, the light emitted from the LED  58  exhibits increasing yellow characteristics as the viewing angle increases away from the optical axis  84 , which this corresponds to a shift in the positive direction along the v′ axis of the CIE 1976 color space diagram  97 . Accordingly, the graph  94  shows that the Δv′ (trace line  96 ) of the emitted light from the LED  58  increases as the viewing angle increases in both positive  83  and negative  85  angular directions with respect to the optical axis (e.g., 0 degrees), while the Δu′ (trace line  95 ) remains generally constant. Again, it should be understood that the graph  94  is intended to illustrate the behavior of a blue-die and yellow phosphor-coated LED, and that the chromaticity variations shown in graph  94  may be different for other types of LEDs. 
     With the above points in mind,  FIGS. 8 and 9  illustrate how the chromaticity variations of the LED  58  may result in a color shift along an axis with regard to light emitted from the forward surface  64  of a light guide  52  in an edge-lit backlight unit  48 . Particularly,  FIG. 8  depicts a simplified cross-sectional showing the propagation of the light rays  86 ,  88 , and  90  emitted from the LED  58  (of the light source  56 ) through a light guide  52 . In the present figure, the light guide  52  has a height depicted by reference numeral  112  and a thickness depicted by reference numeral  113 .  FIG. 9  depicts the rear surface  66  having an arrangement of light-extracting elements  102  configured to disrupt total internal reflection of light rays within the light guide  52 . In conjunction with a back reflector  100  (e.g., a diffuse reflector), the disruption of total internal reflection by the light-extracting elements  102  may cause light to be emitted from the forward surface  64  of the light guide  52  of  FIG. 8 . It should also be understood that in some embodiments, the light-extracting elements  102  may be formed on the forward surface  64  instead of or in addition to being formed on the rear surface  66 . 
     As shown in  FIG. 8 , the light rays  86 ,  88 , and  90  are initially emitted from the LED  58  into an air gap  108  between the LED  58  and the lower edge  60  of the light guide  52 . Though not specifically depicted in  FIG. 8 , it should be understood that due to the different refractive indexes of the air gap  108  and the material forming the light guide  52  (e.g., PMMA), the light rays  86 ,  88 , and  90  may turn slightly toward the normal of the lower edge  60  as the light rays  86 ,  88 , and  90  cross the air gap-light guide interface. As will be appreciated, the amount by which the light rays  86 ,  88 , and  90  turn depends on the angle of incidence at which each respective light ray impacts the lower edge  60 . 
     In the illustrated embodiment, the rear surface  66  having an upper edge  104  and lower edge  106  may include an arrangement of light-extracting elements  102 , as shown more clearly in  FIG. 9 . Thus, as the light rays  86 ,  88 , and  90  propagate from the lower edge  60  towards the upper edge  62  of the light guide  52  via total internal reflection, the light-extracting elements  102  may serve to disrupt total internal reflection to allow for some portion of the light rays  86 ,  88 , and  90  to escape from the light guide  52  and be emitted from the forward surface  64 . For instance, light impacting the light-extracting elements  102  may be refracted out of the light guide  52  to the back reflector  100 . The back reflector  100  then reflects the light back into the light guide  52  and towards the forward surface  64  of the light guide  52 , as shown in  FIG. 8 . 
     As will be appreciated, in some edge-lit backlight units, the light-extracting elements  102  on the rear surface  66  may be arranged to disrupt total internal reflection such that a substantial majority (e.g., greater than 95%) of the light entering the lower edge  60  escapes from the light guide  52  (e.g., through forward surface  64 ) before reaching the upper edge  62 , and such that a substantial minority (e.g., less than 5%) of the light entering the lower edge  60  reaches the upper edge  62 . For the portion of light that does reach the upper edge  62 , a light guide  52  may include a diffuse reflector  126  on the upper edge  62 , such that very little of the light that reaches the upper edge  62  is reflected back a substantial distance towards the lower edge  60  (e.g., a diffuse reflector generally causes reflected light to diffuse and spread evenly). In some light guides, diffuse reflectors may also be provided along the side edges  70  and  72  ( FIG. 4 ) of the light guide  52 . 
     By way of example, a rear surface  66 , as shown in  FIG. 9 , may be configured such that the concentration of the light-extracting elements  102  prevents more than a substantial minority (e.g., less than 5%) from reaching the upper edge  62 . Further, in the depicted rear surface  66 , it should be noted that the spacing between the light-extracting elements  102  decreases along the y-axis from the bottom edge  106  to the top edge  104 , such that the total surface area provided by the light-extracting elements is generally more concentrated near the top edge  104 . This may function to provide a substantially uniform brightness by extracting a greater percentage of the remaining light near the top portion of the light guide  52 . 
     Referring back to  FIG. 8 , the light ray  86 , which generally constitutes “bluer” light relative to the light rays  88  and  90 , propagates towards the upper edge  62  in the direction  110  and impacts the light-extracting element  102   a , located at a generally higher position of the light guide  52  (relative to the height  112 ). It should be understood that the propagation of the light rays  86 ,  88 , and  90  shown in  FIG. 8  is intended to be a simplified depiction of the operation of the light guide  52 . In practice, the light ray  86  may actually undergo total internal reflection against the forward  64  and rear surfaces  66  of the light guide  52  several times prior to impacting the light-extracting element  102   a . Similarly, the light ray  88 , which may be more yellow relative to the light ray  86 , impacts the light-extracting element  102   b , located at a generally middle position along the height  112 , and the light ray  90 , which may be even more yellow relative to both the light rays  86  and  88 , impacts the light-extracting element  102   c , located at a generally lower position along the height  112  of the light guide  52 . 
     As will be appreciated, each of the light-extracting elements  102  may (in conjunction with the back reflector  100 ) be configured to extract the light that strikes it and cause the light to escape, and thus be emitted from the surface  64  in the forward direction  68 . Further, due to the chromaticity variations in the light rays ( 86 ,  88 ,  90 ) emitted from the LED  58 , as discussed above, the blue light (e.g.,  86 ) has a greater probability of impacting light-extracting elements (e.g.,  102   a ) located at a higher position on the rear surface  66 , while the yellow light (e.g.,  90 ) has a greater probability of impacting light-extracting elements (e.g.,  102   c ) located at a lower position on the rear surface  66 . Further, the light (e.g.,  88 ) intermediate the blue and yellow light (e.g., blue-yellow light) has a higher probability of impacting light-extracting elements (e.g.,  102   b ) located intermediate the higher (e.g.,  102   a ) and lower (e.g.,  102   c ) light-extracting elements, as shown in  FIG. 8 . Based on these characteristics, the light  116  emitted from the top of the light guide  52  may include a higher concentration of the blue light (e.g.,  86 ), while the light  118  emitted from the middle of the light guide  52  may include a higher concentration of the blue-yellow light (e.g.,  88 ), and the light  120  emitted from the bottom of the light guide  52  may include a higher concentration of the yellow light (e.g.  90 ). Thus, from the perspective of a viewer  124  viewing the emitted light  116 ,  118 , and  120 , a color shift along the vertical axis (e.g., along optical axis  84 ) may be present in the form of a yellow-to-blue trend from the bottom to the top of the LCD display  34 , thus negatively impacting color uniformity across the display. 
     Before continuing, it should it be understood that a yellow-to-blue trend from the bottom to the top of the light guide  52  is based upon the illustrated configuration, in which the LED light source  56  is positioned along the lower lateral edge  60 . As will be appreciated, if the LED light source  56  were to be positioned along the upper lateral edge  62 , the yellow-to-blue trend would appear from the top to the bottom of the light guide  52  and, similarly, if the LED light source  56  were to be positioned along a side edge, such as edge  70 , then yellow-to-blue trend would appear across a horizontal axis of the light guide  52 , i.e., from edge  70  to edge  72 . 
     Having described the operation of one LED-based backlight unit and the drawbacks thereof regarding color non-uniformity,  FIGS. 10-12  are provided to illustrate various aspects of the presently disclosed techniques that may be applied to a backlight unit  48  to reduce color shift caused by chromaticity variations in light emitted from LEDs  58  and, thus, improve overall color uniformity of light emitted from the backlight unit  48 . 
     To provide an overview, an edge-lit backlight unit  48 , in accordance with certain embodiments of the present technique, may include a light guide  52  having a specular reflector disposed at a lateral edge (e.g., edge  62 ) opposite the lateral edge (e.g., edge  60 ) that receives light from the light source  56 . The reflector may be provided in conjunction with an arrangement of light-extracting elements (e.g.,  102  on rear surface  66 ) that allows for a portion of the light propagating within the light guide  52  to reach the opposite lateral edge (e.g., edge  62 ), that portion being greater relative to the substantial minority of light (e.g., less than 5%) that reaches the opposite lateral edge in the light guide of  FIGS. 8 and 9 . In certain embodiments, the reflector along the edge  62  may be a specular reflector, or may be a reflector having a combination of specular and diffuse reflective components. The light that reaches the specular reflector at the opposite lateral edge is directed back towards the first lateral edge via retro-propagation. As discussed below, this retro-propagation technique improves the uniformity in which the blue and yellow light is extracted by the light-extracting elements, thus providing a light output from the forward surface of the light guide  52  that appears more uniform relative to the output of the light guide discussed above in  FIGS. 8 and 9 . 
     Referring to  FIG. 10 , a simplified cross-sectional view of a light guide  52  for an edge-lit backlight unit  48  is illustrated, in accordance with one embodiment. The light guide  52  of  FIG. 10  is shown in more detail (e.g., larger) to better illustrate the retro-propagation technique, but it should be understood that the presently depicted light guide may have a height  112  that is generally identical to the light guide of  FIG. 8 . In certain embodiments, it should be noted that the thickness  141  of the light guide  52  in  FIG. 10 , which may also be formed from PMMA or any other suitable material, or may be an air guide, may be less relative to certain conventional light guides. By way of example, the light guide  52  of  FIG. 10  may have a thickness  141  of between approximately 1 to 10 millimeters or, in some embodiments, between approximately 1 to 6 millimeters. 
     As shown in  FIG. 10 , the LED  58  emits light into an air gap  108 , which is then received by a lower edge  60  of the light guide  52 . Though not specifically illustrated in  FIG. 10 , as discussed above, the light rays may turn slightly due to the different refractive indexes of the air gap  108  and the material forming the light guide  52 . Further, due to the decreased thickness  141  of the light guide  52  (e.g., relative to certain conventional light guides), the depicted embodiment also provides mirrors  142  disposed adjacent to the LED  58  and parallel to the forward surface  64  and rear surface  66  which serve to direct light that would have otherwise not crossed the air gap-light guide interface (e.g., at lower edge  60 ). 
     Referring briefly to  FIGS. 11 and 12 , embodiments of a rear surface  66  that may be used in conjunction with the light guide  52  of  FIG. 10  are illustrated. As mentioned above, the rear surface  66  may include a configuration of light-extracting elements  102  which allow for a greater portion of the light propagating through the light guide  52  (in the propagation direction  110 ) to reach the upper edge  62 . For instance, in the embodiment shown in  FIG. 11 , the light-extracting elements  102  may be arranged such that they are less concentrated relative to the arrangement of the light-extracting elements  102  in the alternative rear surface  66  of  FIG. 9 . For instance, while the spacing between the light-extracting elements  102  in  FIG. 11  generally decreases in the y-direction from the lower edge  106  to the upper edge  104 , the overall spacing between the light-extracting elements  102  of  FIG. 11  is generally greater than the spacing of the light-extracting elements  102  shown in  FIG. 9  in both the x- and y-directions. Thus, the overall surface area of the rear surface  66  that is covered by light-extracting elements  102  is less relative to the surface area of that depicted in  FIG. 9 . 
     As can be appreciated, due to this lowered concentration, and thus surface area, of light-extracting elements  102 , the rear surface  66  of  FIG. 11  provides for less disruption of total internal reflection, thereby allowing a greater portion of the propagating light to reach the upper edge  62  of the light guide  52 .  FIG. 12  depicts an alternate embodiment of the rear surface  66  in which the spacing of the light-extracting elements  102  is also greater in the x- and y-directions relative to the spacing of the light-extracting elements  102  in the rear surface  66  of  FIG. 9 . Additionally, the rear surface  66  of  FIG. 12  is also configured such that the size of the light-extracting elements  102  generally increases and such that the spacing between the light-extracting elements  102  generally decreases in the y-direction from the lower edge  106  to the upper edge  104 . 
     It should be understood that by concentrating the surface area provided by the light-extracting elements  102  closer to the top edge  104  of the rear surface  66 , brightness uniformity may be better maintained. Further, while the light-extracting elements  102  in  FIGS. 11 and 12  are shown as being generally circular in shape (e.g., circular micro-lenses and/or printed dots), any suitable type of light-extracting elements for disrupting total internal reflection may be provided, such as micro-prisms, grooves, elliptical-shaped elements, square or rectangular-shaped elements, and so forth. As will be appreciated, these light-extracting elements  102  may be configured to refract and/or reflect light towards a back reflector, shown here by reference numeral  140 . Light reflected by the back reflector  140  may be directed towards the surface  64 , and may be emitted therefrom. 
     Referring back to  FIG. 10 , the light-extracting elements  102  may be configured to reduce the disruption of total internal reflection in the propagation direction  110 , and allows for a greater portion of the light propagating from the lower edge  60  to reach the upper edge  62 . For example, as shown in  FIG. 10 , a blue light ray  86   a  emitted from the LED  58  may propagate towards the upper edge  62  and impact a specular reflector  144  disposed at the upper edge  62 . By way of example, the specular reflector  144  may be provided as a silver reflector or an enhanced spectral reflector (ESR) film, such as a model of Vikuiti® ESR film, available from 3M Company. As discussed above, in some embodiments, the reflector  144  could also include a combination of specular and diffuse components to provide for retro-propagation. 
     Upon impacting the specular reflector  144 , the blue light ray  86   a  may be directed back towards the lower edge  60  via retro-propagation (e.g., in direction  148 ) and total internal reflection, as indicated by reference number  86   a ′, ultimately impacting the light-extracting element  102   e , which is located at a generally lower position along the height  112  of the light guide  52 . As can be appreciated, because the light rays emitted closer to the optical axis  84  undergo total internal reflection less frequently than light rays emitted further from the optical axis  84 , the light rays that ultimately reach the upper edge  52  may include a greater proportion of blue light rays. Accordingly, the majority of the retro-propagated light may include blue-tinted light, such as light ray  86   a′.    
     Meanwhile, a yellow light ray  90   a  emitted from the LED  58  impacts the mirror  142  and is directed into the light guide  52 , propagating via total internal reflection until also impacting the light-extracting element  102   e . Thus, the light-extracting element  102   e  extracts light from the propagating yellow ray  90   a  and the retro-propagating blue ray  86   a ′, thereby causing a mixture of blue and yellow light to be emitted from the lower position of the light guide  52 , as indicated by reference number  152 . Again, it should be understood that the use of the term “blue” and “yellow” are meant to imply that the generally white light emitted from the LED  58  may have either a bluish or yellowish tint (e.g., depending on the viewing angle at which the light is emitted from the LED  58 ), and not that the light is actually a “true” blue or yellow colored light. 
     The present technique also provides for the mixture and extraction of yellow light with blue light at generally higher positions of the light guide  52  to provide enhanced color uniformity in the light output. For example, as shown in  FIG. 10 , the blue light ray  86   b  and yellow light ray  90   b  are also emitted from the LED  58 . The blue light ray  86   b  undergoes total internal reflection (not shown in this simplified illustration) until it impacts the light-extracting element  102   d , located generally at a higher position along the height  112  of the light guide  52 . Meanwhile, since the total light-extracting surface area of the rear surface  66  is reduced relative to that of  FIGS. 8 and 9 , the probability of disrupting total internal reflection of a yellow light ray  90   b  in the lower region of the light guide  52  is reduced, thus allowing the yellow light ray  90   b  to travel further in the propagation direction  110 . In the depicted embodiment, the yellow light ray  90   b  also impacts the light-extracting element  102   d . As such, the light-extracting element  102   d  extracts light from the propagating yellow ray  90   b  and the propagating blue ray  86   b , thereby also causing a mixture of blue and yellow light to be emitted from a higher position of the light guide  52 , as indicated by reference number  150 . 
     Though not shown in  FIG. 10 , it should be understood that intermediate “blue-yellow” light rays (e.g.,  88 ) are also present and may undergo total internal reflection within the light guide  52  and mix with yellow and/or blue light in a similar manner. Further, in some instances, some of the yellow light rays (e.g.,  90   a ,  90   b ) may also reach the upper edge  62  and retro-propagate towards the lower edge  60 , impacting with light-extracting elements  102  as it is retro-propagated in the direction  148  and mixing with propagating and/or retro-propagating blue (e.g.,  86 ). 
     While  FIG. 10  only depicts the mixture of emitted blue and yellow light at a high-positioned location (e.g.,  102   d ) and a low-positioned location (e.g.,  102   e ) along the height  112  of the light guide  52  in order to illustrate certain aspects of the present technique, it should be appreciated that the present technique of using a specular reflector  144  and light-extracting elements  102  configured to provide for significant retro-propagation may result in improved color uniformity in the vertical direction (e.g., along axis  84 ) across the entire height  112  of the light guide  52 . Thus, compared to the light output of the light guide shown in  FIG. 8 , a light guide  52  using the presently disclosed retro-propagation technique reduces color shift caused by chromaticity variations of the LED  58 , thereby providing for an overall improved color uniformity in the light output from the LCD display  34 . 
     The mixture of the blue and yellow light, as described herein, may be accomplished via additive mixing. For instance, referring to the extraction and mixture of light impacting the light-extracting element  102   e  of  FIG. 10 , the light extracted from the propagating yellow light  90   a  may be expressed by the variables x 1 , y 1 , and Y 1 , and the light extracted from the retro-propagating blue light  86   a ′ may be expressed by the variables x 2 , y 2 , and Y 2 , wherein the variables x 1 , x 2 , y 1 , and y 2  represent CIE 1931 color space chromaticity coordinates of their respective light beams, and wherein the variables Y 1  and Y 2  represent the luminance of their respective light beams. Thus, the (x, y) CIE 1931 chromaticity coordinates of the resulting mixed light may be expressed as: 
                   x   =           m   1     ⁢     x   1       +       m   2     ⁢     x   2             m   1     +     m   2                 (     Equation   ⁢           ⁢   1     )                 y   =           m   1     ⁢     y   1       +       m   2     ⁢     y   2             m   1     +     m   2           ,           (     Equation   ⁢           ⁢   2     )               
wherein the variables m 1  and m 2  may be computed as follows:
 
                     m   1     =       Y   1       y   1               (     Equation   ⁢           ⁢   3     )                 m   2     =       Y   2       y   2               (     Equation   ⁢           ⁢   4     )               
Additionally, the total luminance of the mixed light may be expressed as follows
 
 Y   total   =Y   1   +Y   2   (Equation 5)
 
     The CIE 1931 chromaticity coordinates (x, y) of the mixed light output may be converted to coordinates within the CIE 1976 color space shown in  FIG. 7  (which is generally more uniform relative to the CIE 1931 color space), by applying the following transfer functions to the CIE 1931 x and y coordinates to obtain corresponding CIE 1976 u′ and v′ coordinates: 
     
       
         
           
             
               
                 
                   
                     u 
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                           12 
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                         3 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     6 
                   
                   ) 
                 
               
             
             
               
                 
                   
                     v 
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                       9 
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                       y 
                     
                     
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                             2 
                           
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                           x 
                         
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                           y 
                         
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                     Equation 
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     Obtaining an acceptable color uniformity in the mixed propagating and retro-propagating light may be accomplished, in certain embodiments, by achieving a Δu′v′ of less than about 0.005 (referring to the graph  94  of  FIG. 6  and the diagram  97  of  FIG. 7 ). As can be appreciated, light output having a Δu′v′ of less than about 0.005 may appear as being substantially uniform in color. In such embodiments, this may be accomplished by configuring the light guide  52 , such that the portion of the propagating light that reaches the upper edge  62  and retro-propagates back towards the lower edge  60  is between approximately 5% to 35% of the total luminosity, i.e., total light from the light source  56  that enters the light guide  52 . For example, the percentage of the retro-propagated light may be approximately 5%, 10%, 15%, 20%, 25%, 30%, or 35% of the total luminosity. 
     One technique for measuring the luminosity extracted from the propagating light (e.g., in direction  110 ) is hereinafter described. The technique may include replacing the specular reflector  144  with a diffuse reflector or black absorption material. The backlight unit luminosity measured with the absorption material will be equivalent to the light extracted from the propagating light (e.g., in direction  110 ), denoted by the variable L prop . The specular reflector  144  may then be replaced, and the total luminosity of the backlight unit may be measured to obtain a total luminosity, L total . Accordingly, the retro-propagating light, L retro , may be calculated as follows:
 
 L   retro   =L   total   −L   prop   (Equation 8)
 
     The percentage of retro-propagated light (L retro ) relative to total luminosity (L total ) may then be determined as follows: 
                   c   =       L   retro       L   total               (     Equation   ⁢           ⁢   9     )               
As discussed above, an acceptable color uniformity level may be achieved when c is between approximately 5% to 35%. As will be appreciated, c may be adjusted by either varying reflective properties of the specular reflector  144  (e.g., using specular reflectors having different reflection properties, or by adjusting a combination of specular and diffuse reflector components at the lateral edge  62 ) or by configuring the arrangement of the light-extracting elements  102  on the rear surface  66 , or both.
 
     With the foregoing points in mind,  FIG. 13  depicts a method  160  for improving color uniformity of an edge-lit backlight unit of a display device, in accordance with aspects of the present disclosure described herein. Beginning at block  162 , light is received at a first lateral edge (e.g., edge  60 ) of a light guide panel  52  of the backlight unit  48 . As discussed, the light may be provided by one or more LEDs arranged along the first lateral edge. Further, although the embodiments above depict the LEDs (e.g.,  58 ) as being arranged along a lower lateral edge  60  of the light guide  52 , it should be understood that the LEDs may be arranged along any one of the lateral edges of the light guide, such as the edges  62 ,  70 , or  72 . 
     Next, at block  164 , light enters the light guide  52  at the first edge (e.g., edge  60 ) and propagates via total internal reflection towards a second edge (e.g., edge  62 ) opposite the first edge. From block  164 , the method  160  then branches to both of blocks  166  and  168 . As shown at block  166 , light-extracting elements (e.g.,  102 ) may extract some of the propagating light by disrupting total internal reflection. At block  168 , propagating light that reaches the second lateral edge (e.g., upper edge  62 ) is reflected, such as by using a specular reflector  144 , and is thus retro-propagated back towards the first edge via total internal reflection. As indicated at block  170 , one or more light-extracting elements  102  may extract the retro-propagated light. For instance, retro-propagating light may impact the light-extracting elements that extracted the propagating light (block  166 ). As discussed above the retro-propagating light may include light emitted close to an optical axis (e.g.,  84 ) of an LED (e.g., blue light) or farther from the optical axis of the LED (e.g., yellow light), and retro-propagating light may potentially be extracted by any of the light-extracting elements  102  of the rear surface  66 . The extracted propagating and retro-propagating light is mixed, as shown at block  172 , and may be subsequently emitted from the light guide  52 , to provide a more uniform light output. 
     The specific embodiments described above have been shown by way of example, and it should be understood that these embodiments may be susceptible to various modifications and alternative forms. It should be further understood that the claims are not intended to be limited to the particular forms disclosed, but rather to cover all modifications, equivalents, and alternatives falling within the spirit and scope of this disclosure.

Metadata:
Filing Date: 20100222
Publication Date: 20131029
Grant Date: 20131029
Priority Date: 20091019
Inventors: YOU CHENHUA
WANG SHENGMIN
Assignee: APPLE INC
CPC Classifications: [{"code": "G02B6/003", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/0061", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/0031", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/003", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/0043", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B6/0031", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/0061", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/0055", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/0043", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B6/0055", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 43878893