Abstract:
Various embodiments of corner-coupled backlights are described, where one or more white light LEDs are optically coupled to a truncated corner edge of a solid rectangular light guide backlight. The one or more LEDs are mounted in a small reflective cavity, whose output opening is coupled to the truncated corner of the light guide. The reflective cavity provides a more uniform light distribution at a wide variety of angles to the face of the truncated corner to better distribute light throughout the entire light guide volume. To enable a thinner light guide, the LED die is positioned in the reflective cavity so that the major light emitting surface of the LED is parallel to the top surface of the light guide. The reflective cavity reflects the upward LED light toward the edge of the light guide.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a continuation application based on U.S. application Ser. No. 11/467,499, filed on Aug. 25, 2006, entitled Backlight Using High Powered Corner LED. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to backlights for liquid crystal displays and, in particular, to illuminating the backlight using a light emitting diode (LED). 
       BACKGROUND 
       [0003]    There are many type of backlights used for liquid crystal displays (LCDs). Generally, for full color backlights, the light used to illuminate the backlight has red, green, and blue components. Fluorescent lamps are most commonly used as the light source. With the development of high power LEDs, such LEDs have been replacing the fluorescent lamps in some applications. A combination of red, green, and blue LEDs may be used, or “white light” LEDs may be used. A white light LED uses a blue or UV LED coated with a wavelength-converting phosphor so that the resulting light appears white. 
         [0004]    A typical backlight for a small or medium size LCD uses a solid, transparent light guide formed of a polymer. The light source, either a fluorescent bulb or LEDs, is optically coupled to one side edge of the rectangular light guide. The light guide may be in the shape of a wedge or have facets or other types of reflectors that uniformly leak light out of the face of the light guide onto the liquid crystal layers. The red, green, and blue pixel locations of the liquid crystal layer are controlled by electrical signals to effectively act as light shutters for the RGB pixels to create a color image on the LCD screen. 
         [0005]    In backlights where only one or a small number of LEDs are used, it is known to form the light guide to have a flattened corner edge, such as a 45 degree angle corner, and mount the LED package in contact with the face of the truncated corner. This is described in U.S. Pat. No. 7,001,058. A suitable LED package for mounting at the corner may be that described in U.S. Pat. No. 6,953,952, which shows an LED package with a window having an area less than 3 mm 2 . Both patents are incorporated herein by reference. By coupling the LED to the flattened corner, rather than to a relatively long side of the light guide, the light more fully spreads throughout the light guide volume to provide a more uniform illumination of the liquid crystal layers. However, with such corner-coupled LEDs, there is still significant optical coupling inefficiency and nonuniformity, and such backlights have only been suitable for small backlights due to the small coupling area. The nonuniformity is partially a result of a point source of light being applied to a surface of the light guide, rather than a wide homogeneous light source, such as a fluorescent bulb, applied to the surface of the light guide. Further, there are variations in the light emitted by the LED, such as due to color variations across the LED due to uneven deposition of phosphor, or variations in the emission profile of the LED die itself, or varying properties in the LED package. Additionally, if high power LEDs are used, it is difficult to remove heat from the LED without incurring additional area requirements. 
       SUMMARY 
       [0006]    Various embodiments of corner-coupled backlights are described, where one or more LEDs are optically coupled at or proximate to a corner of a rectangular light guide backlight. 
         [0007]    In one embodiment, a high-power, white light LED is mounted in a small reflective cavity, whose open end is then optically coupled to the face of a truncated (e.g., flattened) corner of the light guide. The reflective cavity provides a more uniform light distribution at a wide variety of angles to the face of the truncated corner to better distribute light throughout the entire light guide volume. 
         [0008]    Because the LED is housed in the reflective cavity and thus separated from the light guide surface, there is a reduced angle between the light guide coupling surface and the LED. Thus, the maximum angle of light from the LED directly impinging on the light guide surface without being reflected by the reflective cavity is relatively small (e.g., 30-60 degrees relative to a line normal to the LED surface). The light emitted by the LED at wider angles is first reflected by the reflective cavity prior to being incident upon the surface of the light guide. The reflective cavity thus acts to spread out the LED light before impinging on the light guide surface so that the light fills the entire volume of the light guide. Further, variations in the light emitted by the LED itself are smoothed out by the reflective cavity and the separation of the LED from the light guide surface. This creates a more uniform light guide emission into the liquid crystal layers. 
         [0009]    Various types of reflective cavities are described, along with various shapes of the truncated corner portion of the light guide. 
         [0010]    One or more LEDs may be mounted in the same cavity. In one embodiment, to create white light, a wavelength-converting phosphor is located over a blue or UV LED. 
         [0011]    In another embodiment, red, green, and blue LEDs are located in the same reflective cavity. Other combinations of LED types are described. 
         [0012]    LEDs may be coupled to any number of the four corners of the light guide for additional brightness or uniformity. 
         [0013]    In other embodiments, a side-emitting LED is mounted in a small cavity (e.g., a hole) near a corner of the light guide, and a reflector is mounted on the corner of the light guide. 
         [0014]    Other embodiments are also described. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a top down view of an LCD having a backlight, where the backlight comprises a substantially rectangular light guide with one or more LEDs housed in a reflective cavity optically coupled to a corner of the light guide. 
           [0016]      FIG. 2  is a side view of the LCD of  FIG. 1  showing a wedge shaped rectangular light guide. 
           [0017]      FIG. 3  is a side cross-sectional view of an LED encapsulated by a lens housed in a reflective cavity, optically coupled to a corner of a light guide, such as the light guide of  FIG. 2 . 
           [0018]      FIG. 4  is a cross-sectional view of an LED, with no lens, housed in a reflective cavity, optically coupled to a corner of a light guide. 
           [0019]      FIG. 5  is a top down cross-sectional view of a flattened corner portion of a backlight showing the LED, the reflective cavity, and light rays emitted by the LED. 
           [0020]      FIG. 6  is a top down cross-sectional view of a concave corner portion of a backlight showing the LED and the reflective cavity. 
           [0021]      FIG. 7  is a top down cross-sectional view of a scalloped corner portion of a backlight showing the LED and the reflective cavity. 
           [0022]      FIG. 8  is a top down cross-sectional view of a concave corner portion of a backlight showing the LED, a tapered sidewall reflective cavity, and light rays emitted by the LED. 
           [0023]      FIG. 9  is a top down cross-sectional view of a concave corner portion of a backlight showing the LED, a tapered sidewall diffusing-reflective cavity, and light rays emitted by the LED. 
           [0024]      FIG. 10  is a top down cross-sectional view of a flattened corner portion of a backlight showing the LED, a reflective cavity, and light rays with angles emitted by the LED. 
           [0025]      FIG. 11  is a graph showing the incident ray angles and resulting refracted ray angles for the structure of  FIG. 10  for materials with various indexes of refraction. 
           [0026]      FIG. 12  is a top down cross-sectional view of a flattened corner portion of a backlight showing a single white light LED, a reflective cavity, and a diffuser at the cavity exit. 
           [0027]      FIG. 13  is a top down cross-sectional view of a flattened corner portion of a backlight depicting at least two white light LEDs, a reflective cavity, and a diffuser at the cavity exit. 
           [0028]      FIG. 14  is a CIE color diagram showing how the YAG phosphor color component from a white light LED can be combined with the blue light of the LED to achieve a color that can substantially match an accepted standard color point for a backlight. 
           [0029]      FIG. 15  is a top down cross-sectional view of a flattened corner portion of a backlight depicting a white light or phosphor-converted green LED along with a red LED in the reflective cavity. A feedback sensor is used for color point control. 
           [0030]      FIG. 16  is a top down cross-sectional view of a flattened corner portion of a backlight depicting red, green, and blue LEDs in the reflective cavity. 
           [0031]      FIGS. 17 ,  18 , and  19  are side cross-sectional views of a flattened corner portion of a backlight showing an LED within a reflective cavity, where the LED is mounted on a surface parallel to a top or bottom surface of the light guide to achieve a thinner backlight. 
           [0032]      FIGS. 20 ,  21 , and  22  are top down cross-sectional views of a flattened corner of a backlight where a phosphor plate is located near an exit of the reflective cavity, and the LED(s) is a blue or UV LED. 
           [0033]      FIG. 23  is a side cross-sectional view of a backlight using a side-emitting LED mounted within a cavity formed near a corner of the light guide. 
           [0034]      FIGS. 24 ,  25 , and  26  are top down views of a light guide having a side-emitting LED(s) mounted in a cavity near a corner of the light guide and a reflector mounted around the corner of the light guide. 
           [0035]      FIG. 27  is a top down view of a light guide having a side-emitting LED mounted in a cavity near a corner of the light guide, where the corner has a shaped edge to better reflect the light back into the light guide and to achieve a desired intensity profile in the light guide to achieve good uniformity. 
           [0036]      FIGS. 28 ,  29 , and  30  are side cross-sectional views of a backlight showing LEDs within reflective cavities mounted in different configurations and showing various ways to integrate the reflective cavity with a reflector on the back surface of the light guide and techniques for sinking heat from the LED. 
           [0037]      FIGS. 31 ,  32 , and  33  are side cross-sectional views of a backlight showing different types of LEDs mounted in a cavity in the light guide near a corner and integrating a heat sink with a reflector on the back surface of the light guide. 
           [0038]      FIGS. 34 ,  35 , and  36  are similar to  FIG. 32  but show various shapes of top reflectors above the LED to provide more uniform light in the light guide. 
       
    
    
       [0039]    Elements labeled with the same numerals in the various figures may be the same or equivalent. 
       DETAILED DESCRIPTION 
       [0040]      FIG. 1  is a top down view of a color LCD  10 , shown in greater detail in  FIG. 2 . One or more LEDs are housed in a reflective cavity  12 . The cavity  12  is affixed to a flattened (or truncated) corner  14  of a rectangular light guide  16 . 
         [0041]      FIG. 2  is a side view of the LCD  10  of  FIG. 1  with various layers separated for ease of explanation. Light emitted from the reflective cavity  12  enters a corner of the light guide  16 . The light guide  16  is typically a transparent polymer (e.g., PMMA), or may be another transparent material, such as glass. A reflective film  18 , such as a thin aluminum film or sheet, combined with total internal reflection by the light guide, reflects all light upward and out of a top surface of the light guide  16 . The exiting light is diffused by a diffuser  20 . An enhancement film  22  may be used to brighten the light within a narrow viewing angle. The resulting light is then incident upon the back surface of liquid crystal layers  24 , which are very well known and need not be described in detail. One type of liquid crystal layers basically consist of a bottom polarizer, a transparent ground layer, a liquid crystal layer, a transparent thin film transistor (TFT) array, an RGB filter corresponding to the RGB pixel locations, and a second polarizer. Signals applied to the TFT array at selected pixel locations change the polarization of light and cause light at that pixel location to be output from the LCD as a red, green, or blue pixel. By proper selection of the TFT signals, color images are produced by the LCD. Other types of liquid crystal layers for use with a backlight are known, and the invention may be used with any display using a backlight. 
         [0042]      FIG. 3  is a cross-section of one embodiment of the LED and reflective cavity  12 . The LED die  26  emits blue light and is typically formed of an AlInGaN based material. A UV LED may also be used. Over the LED  26  is formed a phosphor layer  28 . A phosphor plate may be used instead of a phosphor layer formed over the LED. The phosphor is typically a YAG phosphor that emits a yellow-green light when excited by blue light. Some of the blue light leaks through the phosphor layer  28 , and the combination of the blue light and YAG emission produces white light. In another embodiment, a green phosphor (SSON) in combination with a red (eCas) or red-orange (BSSN) phosphor can be used to achieve a higher color gamut. 
         [0043]    The LED die  26  is mounted on a submount  30 . Metal pads on the LED die  26  are bonded to corresponding pads on the submount  30 . The LED die  26  may be a flip-chip, or the bonds may be by wires. The submount  30  has terminals that electrically connect to a power supply. The submount  30  may be bonded to a printed circuit board (PCB) by direct bonding or wire bonding, where the PCB has an electrical connector for connection to the power supply. In addition to the submount  30  acting as an electrical interface between the power supply and the LED die, the submount  30  also acts as a heat sink to remove heat from the LED die  26 . The top surface of submount  30  may be reflective to reflect the LED light towards the light guide  16 . 
         [0044]    Typical sizes for LED dies are 0.3 mm to 1 mm. The LED die  26  is preferably a high power LED so that only one LED is needed for the backlight. In one embodiment, the LED emits between 10-200 lumens and can handle drive currents ranging between 100 mA and 1.5 Amps. It is anticipated that the typical LEDs used in most embodiments will have a maximum power rating of at least 0.5 W. In the future, a single LED may replace a single fluorescent bulb typically used for backlights in laptop computers and other medium size LCDs. Currently, 2-4 LEDs are required to replace a single fluorescent lamp, depending on the size of the backlight. 
         [0045]    The LED die  26  is encapsulated by a lens  32 . The lens  32  may have a diameter of 2-6 mm. For very thin light guides, even the 2 mm lens is too big and may be deleted.  FIG. 4  illustrates an embodiment of a very thin (e.g., 2 mm) light guide  16  where the lens is not used. By not using a lens, there is a lower efficiency of light exiting the LED die  26  but a higher in-coupling efficiency into the light guide  16 . In all of the embodiments, either a lensed or unlensed LED may be used. 
         [0046]    The inner walls of the reflective cavity  12  may be highly reflective enhanced aluminum or silver coated mirrors or other reflective material so that virtually all light entering the cavity  12  exits into the light guide  16 . Suitable aluminum mirrors are available from Alanod Corporation and reach reflectivities of 92-97%. The cavity  12  may be clamped on or affixed onto the light guide  16  with an adhesive. The submount  30  may be snapped into place on the cavity  12  or affixed with an adhesive or by other means. 
         [0047]    For optimum uniformity of light output by the light guide, the opening of the reflective cavity  12  should approximately match the face of the flattened portion of the corner (or other shape of the corner) so there are no light voids at the edges of the corner in the light guide. Also by providing a large opening of the cavity  12 , the light becomes incident upon the light guide surface with fewer reflections by the cavity  12 . For purposes of this disclosure, approximately matching the opening of the cavity to the face of the truncated corner is a matching within 75-100%, meaning that at least 75% of the corner face is covered by the cavity  12 . The light output opening of the reflective cavity will typically be at least 4 mm 2 , corresponding to a relatively small corner coupling area. 
         [0048]      FIG. 5  illustrates two light rays from the LED die  26 , where one of the rays is reflected off a wall of the cavity  12 , and the other ray just clears the far edge of the cavity. The ray that does not get reflected by the cavity  12  has a maximum angle α relative to a line normal to the surface of the LED die. Any light that reflects off the light guide is recycled by the cavity  12 . 
         [0049]    By locating the LED die  26  further away from the light guide  16 , the angle α becomes smaller. This angle α is in accordance with the equation tan α=w/2d, where w is the total width of the flattened corner and d is the distance from the LED die  26  to the middle of the flattened corner. A good tradeoff between uniformity, size of cavity, and in-coupling efficiency is to cause the angle α to be between about 30-60 degrees. This results in the distance d to be in the range of about w/3.5 to w/2; however, a distance d of w/5 (corresponds to α=68 degrees) or greater may be suitable. The larger the corner face, the larger the reflective cavity  12  will be. The LED die will typically be within a distance of 1-15 mm from the in-coupling surface of the light guide. A distance of 1 mm would correspond to a corner edge width of 3.5 mm for a 60 degree beam illumination, and 15 mm would correspond to a corner edge width of 17.5 mm for a 30 degree beam illumination. 
         [0050]    Thus, the maximum angle α of light from the LED directly impinging on the light guide surface without first being reflected is smaller compared to the prior art, where the LED is essentially adjacent the corner face. The light emitted by the LED at angles wider than a is first reflected by the reflective cavity  12  prior to being incident upon the surface of the light guide  16 . The reflective cavity  12  thus acts to spread out the LED light over the entire opening of the cavity  12 , similar to the effect of a fluorescent bulb. This creates a more uniform distribution of light in the light guide volume, as opposed to a point source of light being coupled into the light guide. 
         [0051]    Any number of corners ( 1 - 4 ) of the light guide  16  may be flattened and have LEDs housed in reflective cavities mounted thereon as shown in  FIG. 5 . This increases brightness and uniformity. A flat corner face is typically at a 45 degree angle with both adjacent sides of the rectangular light guide. 
         [0052]      FIG. 6  illustrates an embodiment where the truncated corner is concave. This improves the in-coupling efficiency since the incident light is at a smaller angle relative to a line normal to the surface of the light guide. Accordingly, the corner surface will be less than 45 degrees relative to the nearest adjacent side of the light guide. 
         [0053]      FIG. 7  illustrates an embodiment where the truncated corner is scalloped to further improve the in-coupling efficiency. 
         [0054]      FIG. 8  illustrates an embodiment where the walls of the reflective cavity  12  are tapered. Benefits of this embodiment are that the reflective cavity  12  is smaller than a rectangular cavity, and a higher in-coupling efficiency is obtained due to the smaller angle of incident light upon the surface of the light guide  16 . 
         [0055]      FIG. 9  illustrates an embodiment where the walls of the reflective cavity  12  are tapered and the walls are diffusively reflective, as illustrated by the scattering of the light ray  34 . Alanod Corporation manufactures aluminum mirrors with different scattering properties that are suitable. The cavity walls may even be coated with a suitable white paint. Benefits of this embodiment are that the diffused light creates a more uniform illumination over the entire surface of the corner face to increase the uniformity of light emitted by the light guide into the liquid crystal layers. 
         [0056]      FIG. 10  is the same as  FIG. 5  but identifies the indexes of refraction (n 1  and n 2 ) of the reflective cavity medium (e.g., air or epoxy) and the light guide material (e.g., glass or plastic), as well as the entrance angle α 1  and exit angle α 2  of a light ray at the cavity/light guide interface point A. 
         [0057]      FIG. 11  is a graph of the angles α 1  and α 2  for different indexes of refraction of the cavity medium and light guide material. For optimum uniformity of light emitted by the light guide into the liquid crystal layers, there should be an even light distribution across the flattened corner face of the light guide through α 2 =+/−45 degrees. The 45 degree target line is illustrated in  FIG. 11 . In the case of an unencapsulated cavity  12  (the cavity is filled with air, n=1), the light guide edge has to be illuminated with large angles to come close to the 45 degree target. In these cases, it is beneficial to use a slight curvature on the light guide edge, as shown in  FIG. 6 , to make sure that the light reaches all the corners of the light guide, especially if it is taken into account that there will be less light in these large angles due to more losses in the reflective cavity  12 . 
         [0058]    The complexity of achieving good uniformity out of the light guide can be greatly reduced if good uniformity is achieved over the in-coupling edge. In  FIG. 12 , by using a diffuser  36  between the LED die  26  and the coupling edge, a much better uniformity can be accomplished. It is in particular beneficial to place the diffuser  36  close to the in-coupling edge, as the LED die  26  in combination with the reflective cavity  12  will be able to illuminate the diffuser  36  more uniformly. The diffuser  36  may be a roughened translucent plate or other type of diffuser. 
         [0059]      FIG. 13  is similar to  FIG. 12  but there are two or more white LED dies  26  for added brightness. Using multiple LEDs also creates an average light to mitigate variations in the individual LED&#39;s output from a target brightness and color point. This effectively improves the yield of suitable LED dies  26 . The LED dies  26  may be mounted on the same submount  30  or on different submounts. 
         [0060]      FIG. 14  is a CIE color diagram, including the so called black body curve showing the color points of a black body when emitting light at very high temperatures. Two points are indicated in particular, the 6500K color point, and the 9500K color point. For backlighting, these are two relevant color temperatures, as the 6500K color point is the white point specified by international display standards, and the 9500K color point is a very popular display color point. Also indicated in this curve is the color point of a typical phosphor YAG:Ce used in a white LED. The color points of white LEDs lie on a line between this YAG:Ce color point, and the color points of the blue LEDs being used. Three lines are shown for three different color points of the blue LEDs (460 nm, 465 nm, and 470 nm). In actual LED manufacturing, blue wavelength varies, and in order to reach a high yield, blue LEDs of different color points have to be used. In addition, not only the blue wavelength varies, but also the Ce concentration in the phosphor, as well as the thickness of the phosphor, varies so that in practice there can be a large spread in the white points of white LEDs, as indicated by the ellipse in this graph. By determining the white point of the LEDs in advance, and using two or more white LEDs, the LEDs can be chosen such that the combined white point of the two or more LEDs has much less spread than if a single white LED were used for the backlight. 
         [0061]    As shown in  FIG. 15 , to get good color gamut, it is beneficial to use a separate red LED  38  and a separate white LED or phosphor-converted green LED  40 . The phosphor-converted green LED uses a blue LED die and a phosphor that emits green light when energized by blue light and also leaks a portion of the blue light. It is beneficial to use a phosphor-converted green LED, with sufficient blue leakage, such that all the primary color components (red, green and blue) are available to the LCD. Depending on the color balance and the efficiency of the LEDs, also two phosphor-converted green LEDs can be used with one red LED, or two red LEDs and one phosphor-converted green LED can be used, or a combination of phosphor-converted red and a phosphor-converted green (one or more of each) may be used. The main benefit of using a phosphor-converted light instead of direct red or green LEDs is the color stability. The direct red (AlInGaP) and direct green (InGaN) color points depend on drive current and temperature, whereas most of the phosphors have only limited dependence on temperature and no dependence on drive current. 
         [0062]    For color point control, an optical feedback sensor  42  can be integrated into the reflective cavity  12 , for example near the bottom of the side mirrors. The feedback sensor  42  may detect the relative intensities of the red, green, and blue color components and control currents to each of the LEDs to achieve a target color point. As an alternative to placing the optical feedback sensor  42  in the LED cavity, the sensor  42  can also be mounted on one of the edges of the light guide  16 , opposite the light source. 
         [0063]    In  FIG. 16 , one or more separate blue, green, and red LEDs  44 ,  45 , and  46  are used (either phosphor-converted or direct). Very high color gamuts can be achieved in this way and, by independent control of each of the LEDs, the white point can be changed in real time. Further, the backlight can be illuminated in a rapid red, green and blue sequence, as for example required for field-sequential color operation, where the LCD is used without any color filters, and the color images are made by displaying the red, green and blue image content in rapid sequence, synchronized with the red, green and blue illumination modes. 
         [0064]    As in all embodiments, additional LEDs may be used and additional corners may be coupled to the reflective cavities. Such would be the case for large LCD displays. 
         [0065]    To reduce the total thickness of the LCD, the LED module can be placed in the same plane as the light guide plate, as shown in  FIGS. 17-19 . Each of the LEDs in  FIGS. 17-19  is a white LED comprising a blue LED die  26  and a phosphor layer  28 . Red, green, and blue LEDs may be used instead, as described above, to create white light. For a lens  32  with a diameter of 5 mm, the height of the lens  32  is only 2.5 mm, and the thickness of the light guide  16  can be reduced by 2.5 mm compared to the light guides in previous embodiments. 
         [0066]      FIG. 17  illustrates the use of a rectangular reflective cavity  50 . In  FIG. 18 , the reflective cavity  52  is made approximately elliptical to improve coupling efficiency to the light guide  16  by reducing the angle of the incident light on the edge of the light guide  16 . One focal point of the ellipse would be located at the position of the LED die and the other at the center of the in-coupling edge. The cavity may also be parabolic. 
         [0067]      FIG. 19  illustrates an embodiment similar to  FIG. 18  but no lens is used on the LED die  26 . As such, the LED module is smaller, and a smaller reflective cavity  54  is used. The reflective cavities in  FIGS. 17-19  are coupled to truncated corners of the light guide  16  for maximum light uniformity in the light guide. 
         [0068]    As illustrated in  FIG. 20 , instead of putting the phosphor layer on the blue LED die  26 , the phosphor can also be made in the form of a diffusing phosphor plate  56 , or screen printed on a transparent carrier or diffuser plate (alternatively identified as  56 ), which is separate from the LED die and placed in proximity to the edge of the light guide  16 . Thus plate  56  acts both as a diffuser and color conversion means. This has the benefit that very good uniformity is obtained. 
         [0069]      FIG. 21  illustrates the use of two blue LED dice  26  and  58  in the reflective cavity  12  of  FIG. 20 . By using two (or more) blue LED dice, a higher luminance is achieved for larger displays, and the light from two LEDs is averaged to achieve a desired color point. This increases the yield of the blue LEDs, since a blue LED with a short wavelength and a blue LED with a long wavelength can be used together to achieve a desired color point. 
         [0070]    As in all embodiments, the LEDs can be various types using different phosphors and different direct LED colors to achieve the desired white point. For example, the phosphor may be YAG, or a combination of red and green phosphor, or a YAG and red phosphor. Instead of two blue LEDs being used, a blue and a red LED may be used, where a YAG or green phosphor plate near the cavity&#39;s exit is excited by the blue LED and allows the red light to pass through with little absorption. UV LEDs may also be used with suitable phosphors providing the red, green, and blue components. 
         [0071]      FIG. 22  illustrates the embodiment of  FIG. 20  or  21 , but no lens is used and the cavity  12  is filled with an encapsulant  60  (e.g., silicone). This has the benefit that the LED die and optional wirebonds are protected, and a higher extraction efficiency is achieved out of the blue die by providing interfacing materials with similar indexes of refraction. 
         [0072]      FIG. 23  illustrates an embodiment where a side-emitting LED  62  is inserted into a cavity (such as a round hole) formed near a corner of the light guide  16 . The cavity is preferably within 15 mm from the corner of the light guide. The LED  62  uses a standard white LED (blue die plus phosphor layer) with a side-emitting lens  64  that reflects and refracts light such that light is emitted at a low angle 360 degrees around the LED. Multiple LEDs supplying red, green, and blue color components may also be used instead of one white LED. High power side emitting LEDs are available from Philips Lumileds Lighting, LLC, under the Luxeon trade name. Due to the omnidirectional nature of the light emission, all corners of the light guide are supplied with direct light from the LED so the light guide supplies uniform brightness to the liquid crystal layers. To prevent any light from the top of the lens  64  creating a bright spot, a diffusing reflector  66  is placed above the LED  62  on the light guide  16 . A specular reflector  68  is provided around the corner of the light guide  16  near the LED, since the light emitted by the LED is at a small angle relative to the near corner edges and would be transmitted out through the sides of the light guide rather than being internally reflected had the reflector  68  not been employed. The LED submount  30  may be affixed to the light guide by an epoxy or any other adhesive, or may be snapped into place using appropriate fasteners. 
         [0073]      FIG. 24  is a top down view of the backlight of  FIG. 23 . 
         [0074]      FIG. 25  illustrates the use of two or more side-emitting LEDs in the configuration of  FIG. 23 . The LEDs may be the same type to average out the light, or may be different types as described above. 
         [0075]      FIG. 26  illustrates the backlight of  FIG. 25  but where the corner of the light guide  16  is flattened to better distribute the light throughout the light guide. The reflection by reflector  68  reflects light directly toward the far end of the light guide. Other shapes of the corner may also be employed to evenly distribute the light. 
         [0076]      FIG. 27  illustrates the use of a side-emitting LED embedded in a light guide, where the corner is scalloped shaped. The shape of the corner may be made such that almost all light incident on the corner area is at a large angle (greater than the critical angle) so is internally reflected, which obviates the need for an external reflector around the corner. The shape of the corner may also be optimized to get an optimal distribution of light throughout the light guide for achieving good uniformity and efficiency, wherein an external reflector film may be applied over the shaped edge. 
         [0077]      FIGS. 28-33  illustrate how a wall of the highly reflective cavity  12  can also be continued and expanded to serve as a bottom reflector for the light guide  16  to simplify construction and to cool the LED. 
         [0078]      FIG. 28  illustrates a white LED in the reflective cavity  12  similar to that shown in  FIG. 4 . The bottom one of the reflective walls is extended out and expanded to cover the entire bottom surface of the light guide  16 , as reflector  70 , opposite to the side of the liquid crystal layers. Cooling of high powered LEDs is always a difficult problem in a small enclosure. Not only is the LED cooled by heat being removed by the metal reflective cavity  12  contacting the submount  30 , but the LED is further cooled by the bottom metal wall extending over the bottom of the light guide  16 . An additional metal heat sink  72  may be thermally coupled to the back of the submount  30  and contact the metal reflector  70  to further cool the LED. The heat sink  72  may be a metal bracket connected to the reflector  70  using a thermally conductive tape or paste. 
         [0079]    It is preferable that the thermal resistance of the submount  30  material is 25K/W per LED or less. 
         [0080]      FIG. 29  illustrates an embodiment similar to  FIG. 17  but where a wall of the reflective cavity  12  is extended and expanded to become the bottom reflector  70  of the light guide  16 . As in  FIG. 28 , a heat sink  72  additionally thermally couples the submount  30  to the large reflector  70 . 
         [0081]      FIG. 30  illustrates an embodiment where the LED submount  30  is mounted over the bottom wall of the reflective cavity  12 , and the bottom wall of the reflective cavity  12  extends and expands to become the bottom reflector  70  for the light guide  16 . This results in a particularly thin backlight with very good thermal conductivity between the LED die and the reflector  70 . 
         [0082]    In the embodiments of  FIGS. 29 and 30 , and the similar embodiments of  FIGS. 17-19 , the LED may be a side-emitting LED. 
         [0083]      FIG. 31  illustrates a side-emitting LED, having an LED die  62  with a side-emitting lens  64 , similar to  FIG. 23 . The LED submount  30  is directly mounted on an extension of the metal reflector  70  for removing heat from the LED die  62 . A corner reflector may be employed for added efficiency as in  FIGS. 23 and 24 . 
         [0084]      FIG. 32  is similar to  FIG. 31  but the white LED does not use a side emitting lens. This is not as efficient as using a side-emitting lens, since much of the light emitted by the LED must be first reflected by the top reflector  66  and the submount  30  before entering the light guide  16 . The heat sink for the submount  30  extends and expands to become the bottom reflector  70  of the light guide  16 . 
         [0085]      FIG. 33  is similar to  FIG. 32  except that a lens  32  encapsulates and protects the LED die  26 . 
         [0086]    In the embodiments of  FIGS. 28-33 , the bottom surface of the light guide may have a thin reflective film, and the wall of the cavity need not extend over the entire back surface of the light guide. The heat sinking wall of the cavity may be made thicker for added heat sinking. 
         [0087]      FIGS. 34-36  are similar to  FIG. 32  except that the top reflector over the LED is shaped to cause the reflected light to be coupled into the light guide with the minimum number of reflections and to reduce reflections back to the LED. This improves the efficiency of the backlight. 
         [0088]    In  FIG. 34 , the top reflector  76  is cone shaped. 
         [0089]    In  FIG. 35 , the top reflector  78  is cusp shaped so that the reflected light is at a very low angle with respect to the inner walls of the cavity. This greatly reduces reflections off the inner walls of the cavity. 
         [0090]    In  FIG. 36 , the top reflector  80  has microstructures to reflect light away from the LED. 
         [0091]    Any combination of the embodiments is possible. All of the backlights may be used in the LCD embodiment of  FIG. 2 . The LCD may be color or monochromatic. For very large LCDs, additional LEDs may be mounted in conventional ways along the sides of the light guide, and the corner LEDs are essentially used to improve the uniformity across the entire light guide surface. 
         [0092]    Having described the invention in detail, those skilled in the art will appreciate that, given the present disclosure, modifications may be made to the invention without departing from the spirit of the inventive concept described herein. Therefore, it is not intended that the scope of the invention be limited to the specific embodiments illustrated and described.