Abstract:
One or more embodiments of the present invention provide apparatuses and systems to form edge-illuminated LED backlight units for flat-panel LCD displays. The backlight unit is able to achieve uniform color and brightness distribution with very small dimensions of depth and bezels. One or more embodiments of the present invention include a light guide coupled to a light guide plate, which, by operating together, provide simple, efficient, few LEDs and low cost backlight units. Effective coupling structures provide high system efficiency.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This non-provisional U.S. Patent Application claims the benefit of U.S. Provisional Patent Application No. 61/081,287 entitled “LED BACKLIGHT HAVING EDGE ILLUMINATOR FOR FLAT PANEL LCD DISPLAYS” filed on Jul. 16, 2008, the disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Statement of the Technical Field 
         [0003]    This invention pertains to innovative LED-based backlight units used for flat panel LCD displays, in particular pertaining to light guide architectures having low cost, high efficiency, few LEDs, and uniform light distribution across a display screen. 
         [0004]    2. Description of the Related Art 
         [0005]    Light emitting diodes (LEDs) are gradually extending their applications in LCD backlight units from small panels to large screen LCD displays. One typical approach, known as direct backlighting, disposes an array of LEDs directly on the back surface of a backlight unit. A drawback of direct backlighting is that hot spots from individual LEDs may be visible on the screen unless either the LED array is dense, or the backlight unit is deep. Preferably the backlight unit is compact, therefore the LED array should be dense in order to eliminate hot spots for uniformly illuminating LCD displays such as flat panel HDTVs. Some large screen LCD TVs use hundreds or thousands of LEDs to form a matrix of point light sources on the backplane of a backlight unit for LCD panel illumination. The use of such a huge number of LEDs results in complex control systems and significantly increases the cost of LED backlight units, making it difficult for LED-based backlight units to compete with conventional low-cost CCFL backlighting systems. 
       SUMMARY OF THE INVENTION 
       [0006]    Compared to direct backlighting, an edge-lit arrangement has advantages such as reducing the depth and making the backlight unit thinner by placing LEDs along the sides and using very thin light guides to uniformly distribute light across the LCD panel. While it is easy to implement LEDs which emit white light in a backlighting system, red, green and blue (RGB) LEDs can provide a much wider color gamut and result in superior color displays. In an edge-lit based backlight unit, light rays from small RGB LED light sources are first coupled into light guides and then the light guides carry light internally. The light guide may also be referred to as a guide or a light guide plate. The guide has input surfaces from which light is coupled into the light guide. 
         [0007]    Simple and thin light guide architectures with fewer LEDs can greatly reduce the cost of LED backlight units and promotes adoption of this technique into the mainstream in flat panel LCD displays. Light from the LED light sources can be coupled into a rectangular light guide from both lateral sides of the guide. A transition region known as the color mixing distance is provided between the LEDs and the illumination region of the guide. When the transverse dimension of the rectangular light guide is small, the associated color mixing distance is very short and color light from the individual RGB LEDs can be mixed very well before the light is extracted out from the guide. It is desirable that the color mixing distance be kept short so that the size of the bezel surrounding the flat panel LCD display can be kept small. 
         [0008]    The backlight illumination is provided by a light guide plate. The light guide plates used in the backlighting units are made from a substantially transparent bulk plastic material. An example of a bulk material used in injection-molded light guides is acrylic (PMMA) for low cost, lightweight, and less light absorption. The light guide plate will further include micro-structures (i.e., micro-lenses) on the top and bottom surfaces to extract light out from the light guide for illuminating the viewing area of the LCD display. Because the micro-lenses can be made very small and dense, uniform light distribution can be achieved on large LCD screens even the backlight units are very slim. 
         [0009]    One or more embodiments of the present invention are able to achieve good brightness and good color uniformity by use of light-reshaping structures that produce a well-mixed and color-balanced distribution of light. The light-reshaping structures allow the design to use fewer LEDs, and fewer electrical components, thereby producing a design that is low-cost, thin and simple. 
         [0010]    One or more embodiments of the present invention provides an edge-illuminated LED backlight apparatus that produces uniform light distribution for LCD displays, including an elongated light-transmissive bulk material having a first end, a second end, a central axis running from the first end to the second end, a first edge surface connecting the first end and the second end, and a second edge surface opposing said first edge surface; further including first and second input windows on the first and second ends, respectively, the first and second input windows configured to allow light from a plurality of light sources to enter the bulk material; further including a plurality of light extractors disposed on the first edge surface of the bulk material, each of said light extractors configured to allow a portion of light to escape from the interior of the bulk material; further including a flat light-transmissive bulk material having a first major surface, a second major surface, and an edge surface transverse to the first major surface and the second major surface, wherein the edge surface of the flat light-transmissive bulk material is optically coupled to the light extractors of the elongated light-transmissive bulk material; and further including a plurality of micro-lenses disposed on at least one major surface of the flat light-transmissive bulk material. 
         [0011]    One or more embodiments of the present invention provides a method for coupling and re-distributing light in a LCD backlight unit, the method including a step of communicating light from a plurality of light sources, through an input window, into an elongated light-transmissive bulk material; further including a step of emitting said light from the elongated bulk material, through a plurality of light extractors disposed on a first edge of the elongated bulk material, into a flat light-transmissive bulk material; and further including a step of emitting said light from the flat light-transmissive bulk material through a plurality of micro-lenses disposed on at least one major surface of the flat light-transmissive bulk material. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Features illustrated in the figures are not drawn to scale unless explicitly stated otherwise, and the relative sizes of certain features may be exaggerated to better illustrate the features. Embodiments will be described with reference to the following figures, in which like numerals represent like items throughout the figures, and in which: 
           [0013]      FIG. 1  shows a front view of an embodiment of the present invention, wherein the light guide is positioned to inject light into the bottom edge of a light guide plate. 
           [0014]      FIG. 2  shows a front view of usage of the present invention, wherein an LCD panel with bezel covers the light guide plate. 
           [0015]      FIG. 3  shows an edge illuminator with dual RGB LED chipsets and a rectangular light guide. 
           [0016]      FIG. 4  shows a perspective view of a rectangular light guide having micro-structures on the top surface for edge lighting of the light guide plate. 
           [0017]      FIG. 5  shows a rectangular light guide with tapered elements for edge lighting. 
           [0018]      FIG. 6  shows a perspective view of a tapered light guide with micro structures on a top surface of the light guide. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    One or more embodiments of the present invention includes an elongated light guide apparatus adapted to accept light from two RGB LED chipsets, one on each end of the light guide, in order to produce highly uniform light that has a highly uniform color distribution and highly uniform intensity distribution. The highly uniform light is produced with high efficiency and at low cost by use of structures as described below. 
         [0020]    One or more embodiments of the present invention includes a light guide plate adapted to accept light from the edge lighting apparatus, in order to produce backlight illumination for an LCD display. 
         [0021]      FIG. 1  shows an exemplary front view of an embodiment of the present invention, wherein the light guide is positioned to inject light into the bottom edge of a light guide plate. 
         [0022]      FIG. 1  shows a front view of a first exemplary LED backlight unit  100 , constructed from a light guide  101  and a light guide plate  121 . Both light guide and light guide plate are relatively flat and thin, so that the overall LED backlight unit  100  can be very slim. As referred herein, the light guide  101  has: a first end  103  which received light from the set  102  of LEDs; a second end  104  which is opposite to the first end  103  and which received light from the set  122  of LEDs; an axis  105  which runs along the length of the light guide  101  from the first end  103  to the second end  104 ; a lateral edge surface  124  that abuts an edge  125  of light guide plate  121 ; a width  106  determined by a dimension of the light guide  101 , transverse to the axis  105  and parallel to the plane of  FIG. 1 ; and a thickness determined by a dimension of the light guide  101 , transverse to the axis  105  and perpendicular to the plane of  FIG. 1 . Set  102  and set  122  of LEDs each includes at least one LED chip of each of red, green, and blue (“RGB”) color. A reflective surface (not shown) may be provided adjacent to the set  102  or set  122  of LEDs, in order to improve recycling of light toward light guide  101 . Light guide  101  and light guide plate  121  are both made from a substantially transparent bulk plastic material such as PMMA. Each set  102  and  122  of RGB LEDs may have three or more surface-mounted RGB chips which can be driven at high current to provide sufficient light output with required white color when RGB light is well mixed. 
         [0023]    Each set  102  and  122  of RGB LEDs is depicted here as one blue LED, one green LED, and one red LED, but persons of skill in the art will recognize that additional quantities of LEDs and/or varying ratios of the different colors of LEDs including white LEDs, may be used to achieve different desired brightness or color balance. The individual LEDs will ordinarily be arranged in an elongated pattern in one direction, such as one row or a small number of rows, in order to help maintain the overall flat and thin shape of light guide  101 . 
         [0024]    Light guide  101  accepts light from both set  102  and set  122  of RGB LEDs. After light enters the light guide  101 , light rays  128  propagate through light guide  101 . As light rays  128  first enter light guide  101 , the color of light rays  128  will have a relatively strong dependency upon a linear position transverse to axis  105 , because the color will be dominated by the color of the LED closest to that portion of the linear position. As light rays  128  continue propagating along light guide  101 , colors will become mixed due to TIR reflections described below, thereby producing substantially white light. A color mixing region may be provided in the portions of light guide  101  that are adjacent to set  102  and set  122  of RGB LEDs, such that light will not be extracted from light guide  101  in the color mixing region. Because the transverse dimension of the light guide is small, the color mixing region is very short. If white LEDs are used, there is no need for the color mixing region. 
         [0025]    There exists a difference in the refractive index of the bulk material of light guide  101  and the medium that it is immersed in (usually air). The difference in refractive index creates a critical angle with respect to the perpendicular of a surface formed at a boundary between the bulk material and the medium. At angles less than the critical angle (i.e., close to perpendicular), light may be able to pass through the surface. At angles greater than the critical angle (i.e., shallow angles with respect to the surface, and away from the perpendicular), light will be totally reflected by the surface. This condition of total reflectance is called total internal reflection (TIR). Light rays  128  are contained within light guide  101  by use of TIR on the top, bottom, left and right interior surfaces of light guide  101 , until the light is extracted by use of microstructures on one side of light guide  101 , as described below in relation to  FIG. 4 . As light propagates through light guide  101 , the color and intensity of the light will become more uniform. 
         [0026]    The light guide plate  121  is optically coupled to the light guide  101  through the lateral edge surface  125  of light guide plate  121 , which abuts the edge  124  of light guide  101 . The light guide plate  121  has a first major surface  127  visible in  FIG. 1 , and a second major surface (not visible in  FIG. 1 ) that is opposite from the first major surface. At least one of the first and second major surfaces has disposed thereon a plurality of micro-lens  126 , forming a micro-lens pattern array  123 . For sake of clarity, only a portion of the micro-lens pattern array  123  is illustrated in  FIG. 1 .  FIG. 1  illustrates micro-lens  126  as having a circular shape, but the shape is not limited in this regard. Other shapes may be used, such as oval, rectangular, hexagonal, etc. Generally, substantially the entire surface area of the first major surface  127  or the second major surface is covered by the micro-lens pattern array  123 . The function of micro-lens pattern array  123  is to extract light from light guide plate  121  in order to provide backlight illumination for an LCD panel overlying one major surface of the light guide plate  121 . The sizes and placement of individual micro-lenses  126  across first major surface  127  or the second major surface will vary by design and/or as-measured manufacturing tolerances, based on a desired profile of light intensity to be extracted by the micro-lens pattern array  123 . 
         [0027]    In order to improve efficiency, one major surface, which is not immediately overlaid by the LCD panel, will have disposed thereon one or more recycling enhancement films, and the opposite major surface, which is immediately overlaid by the LCD panel, will have disposed thereon brightness enhancement films (“BEF”), and/or dual brightness enhancement films (“DBEF”) in order to improve the recycling of light toward the LCD panel. BEF is known as a microreplicated prism film that is used to increase display brightness by managing the exit angle of light. DBEF is known as a multi-layer reflective polarizing film which increases the amount of polarized light available for illuminating the LCD panel by recycling light that would normally be absorbed by a pre-polarizer in the LCD panel. The recycling enhancement film may also be formed from a reflective surface integrated together with a white diffuse reflector. Such recycling enhancement approach can increase the brightness on the LCD panel without additional increase of light output or power of LEDs. A light recycling film as used herein can be referred to a specular reflector or a mirror. The recycling enhancement component abuts the one major surface with no intended air gap. 
         [0028]    The design of the micro-lens pattern array  123  takes into account both the calculated light intensity within light guide plate  121 , and the brightness distribution that will be pleasing to a user and perceived by the user as being uniform across an overlying LCD panel. The perceived brightness of light emitted through the micro-lens pattern array  123  will be substantially balanced over light guide plate  121  by designing individual micro-lens  126  with a size and location that is matched to the intensity distribution within light guide plate  121  at the location of the individual micro-lens  126 . The perceived uniformity of light distribution provided to illuminate a LCD display relates to optimal usage of available light. It is acceptable if the central region of a display is designed to be brighter than the periphery of the display (i.e., edges and corners) by about 10%-20%. From the center to the periphery, actual brightness gradually and symmetrically decreases, following a specified profile, so that the brightness variation across the LCD panel is not noticeable and visible by the viewers. 
         [0029]    Design features of the micro-lens pattern array  123 , of one or more embodiments of the present invention, may be varied to control the extraction of light from within the light guide plate  121 . These design features include the size of a micro-lens  126 ; density of micro-lenses  126 ; and placement of micro-lenses  126  used to form the micro-lens pattern array  123 . For equivalent light intensities within a light guide plate  121 , a larger diameter micro-lens  126  will emit more light than a small diameter micro-lens  126 , due to the difference in scattering or diffusing areas. Similarly, for equivalent light intensities within a light guide plate  121 , a greater density of micro-lens  126  of a predetermined size will emit more light over an area than a smaller density of micro-lens  126  of the same predetermined size. Micro-lens patterns with increased micro-lens density and reduced micro-lens size play a very important role in the depth reduction of backlight units. 
         [0030]    The features of micro-lens pattern array  123  may also vary as a function of position within the light guide plate  121 . For instance, light intensity within the light guide plate  121  initially will be greater near the edges of light guide plate  121 , because of reflections and light injection at surface  125 , than the light intensity in the center of the light guide plate  121 . Therefore, the sizes of micro-lenses are correspondingly adjusted to remove hot spots and to provide uniform perceived brightness distribution. As light propagates within the light guide plate  121 , light mixing arising from internal reflections will cause the light intensity to become more uniform. Light extraction by micro-lens pattern array  123  through the light guide plate  121  also causes the light intensity propagating within the light guide plate  121  to change as a function of position. 
         [0031]    To compensate for the variations of light intensity within the light guide plate  121 , and to provide a perceived uniformity of brightness over a viewing area, the size of micro-lenses  126  may vary, with the larger micro-lenses  126  allowing more light to be released. The size of a micro-lens  126  at a predetermined location, the density of micro-lenses  126 , and their placement within array  123  is designed by simulating the light intensity distribution within the light guide plate  121  at the predetermined location, and determining micro-lens  126  sizes that provide a desired brightness profile resulting in a perceived brightness uniformity. 
         [0032]    Manufacturing tolerances and imperfections will affect the actual brightness distribution profile. Therefore, iterative processes may be used during manufacturing, such that an as-manufactured brightness profile can be measured, and the size of individual micro-lenses  126  are then adjusted (i.e., adapted) to provide a new-iteration as-manufactured brightness profile that is closer to the desired brightness profile. An exemplary process uses a diamond-turning machine, under the control of a CAD software program, to produce optimal micro-lens  126  patterns. 
         [0033]    Micro-lens  126  may be a roughened area, such that the TIR condition is broken, thereby allowing light to be locally extracted out from the light guide. The degree of local roughness in a localized portion of the micro-lens  126  affects the angular distribution of light extracted from the localized portion of the micro-lens  126 . The micro-lens  126  also may be a concavity formed in the surface of light guide plate  121  by a process such as milling, drilling, etching, laser ablation, etc., such that the TIR condition is broken. Micro-lens  126  may also be a convex protrusion from the surface of light guide plate  121  such that the TIR condition is broken. Embodiments of the invention are not limited by the method of making micro-lenses  126 . 
         [0034]      FIG. 2  illustrates an exemplary front view of usage of the present invention, wherein an LCD panel  201  with bezel  202  covers the light guide plate. Bezel  202  can be very small on three sides and functions to provide an aesthetically pleasing cover over a peripheral portion of light guide plate  121  that is not covered by the micro-lens pattern array  123 . The LCD panel  201  is arranged substantially parallel to light guide plate  121 . Light extracted by the micro-lens pattern array  123  passes through the LCD panel  201  to provide images. A bezel  203  or similar covering may also be provided over light guide  101 . 
         [0035]      FIG. 3  illustrates exemplary light rays extracted from an edge illuminator with dual RGB LED chipsets and a rectangular light guide. Light guide  101  as illustrated in  FIG. 3  has an upper surface  301 , and a lower surface  302  that is opposite from upper surface  301 . The upper surface  301  of the light guide  101  optically interfaces with the light guide plate  121  (not shown). Both the upper surface  301  and lower surface  302  are substantially perpendicular to the plane of  FIG. 3 . Light guide  101  also has a top surface (not marked), and a bottom surface (not marked) that is opposite from the top surface. Both the top surface and the bottom surface are substantially parallel to the plane of  FIG. 3 . 
         [0036]      FIG. 3  further illustrates a plurality  303  of light rays extracted from upper side  301  and a plurality  304  of light rays escaping from the lower side  302 . Upper surface  301  includes microstructures to extract light from light guide  101 , as discussed below with respect to  FIG. 4 , therefore plurality  303  of light rays is dominantly large compared to plurality  304  of light rays. 
         [0037]    Light rays escape from lower side  302  because the escaping rays do not meet the TIR condition at lower surface  302 . In order to improve efficiency, a recycling enhancement component, such as a high-reflectivity reflector or a Mylar film, may be added to recycle light rays that escape through the lower surface  302 . A back reflector may also be added adjacent to lower surface  302 , in order to improve the light recycling efficiency. Such recycling enhancement approach can increase the brightness on the LCD panel without the increase of light output or power of LEDs. The recycling enhancement component abuts lower surface  302  with no intended air gap. 
         [0038]    There is almost no leakage through the top and bottom surfaces of light guide  101  (i.e., the surfaces parallel to the plane of  FIG. 3 ), because fewer light rays within light guide  101  are able to strike the top and bottom surfaces at an angle that does not meet the TIR condition, compared to upper surface  301  and lower surface  302 . The recycling enhancement component and/or reflector may also be added to the top and bottom surfaces, but the improvement to efficiency will not be as great because not as many light rays escape from the top and bottom surfaces. 
         [0039]      FIG. 4  is a perspective view that further illustrates the processing of light in the light guide  101  and light guide plate  121 . Light rays  401  and  402  are generated by LEDs  102  and  122 , respectively (not shown), and are directed toward light guide  101 . Glass plates  403  and  404  with anti-reflective (“AR”) coatings can be attached to the ends of light guide  101  to improve the light coupling of the sets  102  and  122  of LEDs to light guide  101  with reduced reflection loss. Light is mixed within light guide  101  in order to produce substantially white light of a determinable intensity within the light guide  101 . Disposed on the top surface of light guide  101  is an array of microstructure  405 . Microstructure  405  functions as a plurality of light extractors to allow a portion of light to escape from the interior of the light guide  101 . A microstructure as known in the art may be a roughened area, such that the TIR condition is broken, thereby allowing light to be locally extracted out from the light guide. The degree of local roughness in a localized portion of the microstructure  405  affects the angular distribution of light extracted from the localized portion of the microstructure  405 . The microstructure  405  also may be a concavity formed in the surface of light guide  101  by a process such as milling, drilling, etching, laser ablation, etc., such that the TIR condition is broken. Microstructure  405  may also be a convex protrusion from the surface of light guide  101  such that the TIR condition is broken. Embodiments of the invention are not limited by the method of making microstructure  405 . 
         [0040]    The embodiment of  FIG. 4  illustrates microstructure  405  as a plurality of roughened strips situated along the top side of light guide  101 , each strip substantially transverse to axis  105 . The roughened strips with proper width are spaced to uniformly extract light along the length of light guide  101 . The width of each strip, for a predetermined position along light guide  101 , is determined by the local light intensity within the light guide  101  and a desired light intensity to couple into light guide plate  121  at the predetermined position. Generally, the strips near the center of light guide  101  are wider than the strips near ends  103 ,  104  of light guide  101 , the ends  103 ,  104  being closer to sets  401 ,  402  of LEDs. Wider strips extract more of the available light from light guide  101  than is extracted by narrower strips. 
         [0041]    The light  406  extracted from microstructures  405  is substantially white in color, and is substantially uniform in intensity along the length of microstructures  405 . Light  406  is coupled into light guide plate  121 . For sake of clarity,  FIG. 4  is drawn with an air gap between light guide  101  and light guide plate  121 , but it should be known that in practice light guide  101  and light guide plate  121  will be assembled with a negligible air gap between them. 
         [0042]    Light  406  enters light guide plate  121  through lower edge surface  407 . The interface from light guide  101  to light guide plate  121  may be a light-spreading adaptation to spread light rays over a wider range of angles into light guide plate  121  in the direction parallel to the plane of  FIG. 4 , and thereby achieve more uniform intensity and color of light within light guide plate  121 . In one embodiment, the light-spreading adaptation may include lenticular structures on lower edge surface  407 . Lenticular structures are known in the art as an array of micro-ridges. Micro-lens pattern array  123  (not shown in  FIG. 4 ) is positioned on the front major surface  408  of light guide plate  121 . Micro-lens pattern  123  and microstructure  405  together are designed to provide a desired distribution of extracted light from the light guide plate  121 . 
         [0043]      FIG. 5  illustrates another embodiment of the plurality of light extractors, wherein a plurality of tapered elements  501  direct light from the top surface of light guide  101 , into light guide plate  121 . The tapered elements  501  have tapered surfaces in the plane of  FIG. 5 , with a narrow end adjacent to light guide  101 , and a wider end adjacent to light guide plate  121 . The thickness of the tapered elements  501  is substantially the same as that of the light guide  101 . Tapered elements  501  can be injection-molded together with the light guide  101 . Light to be extracted enters from the bottom of tapered elements  501 , and the amount of extracted light depends on the bottom opening of the corresponding tapered element. Light propagating through tapered elements  501  is TIR-reflected by the tapered surfaces towards the light guide plate  121 . The half cone-angle of tapered elements is in the range between approximately 35° to 42°. Micro-structures may be introduced on the tapered element  501  at the junction with the light guide plate  121  in order to improve the uniformity of light injected into light guide plate  121 . 
         [0044]    In another aspect of the invention,  FIG. 6  illustrates light guide  601  having a lower surface formed from two or more sections  602 ,  604 . The two or more sections  602 ,  604  include at least one section that is situated at a non-parallel angle with respect to axis  605 . The two or more sections  602 ,  604  together form a lower surface in which width  606  varies along the length of light guide  601 . As illustrated in the embodiment of  FIG. 6 , sections  602  and  604  are both angled with respect to axis  605  and join to form one or more protrusions  603  into the interior of light guide  601 . The protrusion  603  may extend into light guide  601  by about half of the width  606  at the end of light guide  601 , but the invention is not limited in this respect and protrusion  603  may extend more than or less than half of the width  606  at the end of light guide  601 . Although  FIG. 6  is illustrated with angles  607  and  608  being substantially equal, embodiments of the invention are not limited in this respect. Angles  607  and  608  may be unequal if the sections  602 ,  604  are of unequal lengths. In other embodiments (not illustrated), lower surface of light guide  601  may comprise a concave curve or a toroidal shape. 
         [0045]    Section  602  functions to direct a portion of light from set  401  of LEDs toward the upper surface of light guide  601 , and section  604  functions to direct a portion of light from set  403  of LEDs toward the upper surface of light guide  601 . By directing the light from sets  401  and  403  in this way, the efficiency of light extraction by microstructure  405  is improved. The angles  607  and  608  with respect to axis  605  are not so great as to cause a loss of TIR condition at sections  602  and  604  for most light rays propagating through light guide  601 . 
         [0046]    In a further embodiment, a thickness  609  of the upper edge surface can be less than the thickness of lower edge surface  407 , thereby providing a wedge shape to light guide plate  121  in a vertical direction. Preferably, the wedge shape is oriented such that front major surface  408  is substantially vertical, and a rear major surface  610  is slanted away from vertical. Providing a wedge shape for the light guide plate  121  has a first advantage of reducing the bulk of light guide plate  121  and the associated cost and weight. A second advantage is that as light propagating from the lower edge surface  407  is reflected from the rear major surface  610 , the reflected light will be directed toward the front major surface  408 . The wedge shape is not so great as to cause a loss of TIR condition at the rear major surface  610  for most light rays propagating through light guide plate  121 . The thickness of lower edge surface  407  is constrained by the thickness of light guide  101 , such that the best coupling of light from light guide  101  into light guide plate  121  is attained when the thickness of lower edge surface  407  is approximately the same as the thickness of the light guide  101 . 
         [0047]    Simulations have been carried out to calculate the intensity distribution of the red, green, and blue components of light produced to illuminate an LCD panel. The simulations provide design parameters for uniform white light intensity distribution over the entire viewing surface of an LCD panel. 
         [0048]    Although the invention has been illustrated and described with respect to one or more implementations, equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. 
         [0049]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Furthermore, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description and/or the claims, such terms are intended to be inclusive in a manner similar to the term “comprising.” 
         [0050]    Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
         [0051]    Embodiments of the present invention have been described herein for proper color mixing to generate white light. However, it will be understood by persons of ordinary skill in the art that the individual red, green, and blue LEDs may be turned on and off sequentially in order to display a color image. A color image is perceived by a user when the individual red, green, and blue LEDs are switched on and off in synchronism with the LCD panel displaying an image tailored for the color(s) that are switched on. This is a special and useful feature if the LCD panel can be switched very fast.