Patent Publication Number: US-6657607-B1

Title: Liquid crystal flat panel display with enhanced backlight brightness and specially selected light sources

Description:
This is a continuation of application(s) Ser. No. 09/087,280 filed on May 29, 1998, now U.S. Pat. No. 6,243,068. 
    
    
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to the field of display devices. More specifically, the present invention relates to the field of flat panel display devices utilizing liquid crystal display (LCD) technology. 
     (2) Prior Art 
     Flat panel displays or liquid crystal displays (LCDs) are popular display devices for conveying information generated by a computer system. The decreased weight and size of a flat panel display greatly increases its versatility over a cathode ray tube (CRT) display. High quality flat panel displays are typically back-lit. That is, a source of illumination is placed behind the LCD layers to facilitate visualization of the resultant image. Flat panel LCD units are used today in many applications including the computer industry where flat panel LCD units are an excellent display choice for lap-top computers and other portable electronic devices. However, because the technology of flat panel LCD units is improving, they are being used more and more in other mainstream applications, such as desktop computers, high-end graphics computers, and as television and other multi-media monitors. 
     In the field of flat panel LCD unit devices, much like conventional cathode ray tube (CRT) displays, a white pixel is composed of a red, a green and a blue color point or “spot.” When each color point of the pixel is illuminated simultaneously, white can be perceived by the viewer at the pixel&#39;s screen position. To produce different colors at the pixel, the intensities (e.g., brightness) to which the red, green and blue points are driven are altered in well known fashions. The separate red, green and blue data that corresponds to the color intensities of a particular pixel is called the pixel&#39;s color data. Color data is often called gray scale data. The degree to which different colors can be achieved by a pixel is referred to as gray scale resolution. Gray scale resolution is directly related to the amount of different intensities to which each red, green and blue point can be driven. 
     The method of altering the relative color intensities of the color points across a display screen is called white balance adjustment (also referred to as color balance adjustment, color temperature adjustment, white adjustment, or color balancing). In other words, the appearance of “white” is a combination of red, green and blue intensities in various contributions of each color. “Color temperature” attempts to correlate the temperature of an object with the apparent color of that object. It is the temperature of the light source that illuminates the object. Ideally, that source is a perfect black body emitter, e.g., a thermally radiating object that absorbs all incident radiation and re-radiates that energy with complete efficiency. A theoretical model of such a black body was derived by Max Planck and is the standard to which any the source is compared. 
     But real life radiators are not so efficient but still tend to follow Planks equation in a relative sense and are known as “gray body” emitters. A tungsten filament is a very good approximation to a gray body and the user of a tungsten filament as a substitute for a black body reference is wide spread. Therefore, the term “color temperature” refers to the emission spectra of a tungsten filament t a given temperature as expressed in degrees Kelvin. In a display, the “color temperature” of white correlates to the relative percentage contributions of its red, green and blue intensity components. Relatively high degree K color temperatures represent “white” having a larger blue contribution (e.g., a “cooler” look). Relatively small degrees K color temperatures represent “white” having a larger red contribution (e.g., a “warmer” look). Generally, the color temperature of a display screen is adjusted from blue to red while avoiding any yellow-ish or green-ish variations within the CIE chromaticity diagram. 
     The white balance adjustment for a display is important because many users want the ability to alter the display&#39;s color temperature for a variety of different reasons. For instance, the color temperature might be varied based on a viewer&#39;s personal taste. In other situations, color temperature adjustment may be needed to perform color matching (e.g., from screen-to-screen or from screen-to-paper or screen-to-film). In some situations, color temperature adjustment can correct for the effects of aging in some displays. Therefore, it is important for a flat panel LCD unit to provide the user with a color balancing adjustment option. 
     One method for correcting or altering the color balance within an LCD unit screen is to alter, on-the-fly, the color data used to render an image on the screen. For instance, instead of sending a particular color point a color value of X, the color value of X is first passed through a function that has a gain and an offset. The output of the function, Y, is then sent to the color point. The function is specifically selected for a particular color temperature result. The values of the above function can be altered as the color temperature needs to be increased or decreased in value. Although offering dynamic color balance adjustment, this prior art mechanism for altering the color balance is disadvantageous because it requires relatively complex circuitry for altering a very large volume of color data. The circuitry adds to the overall cost of production and can increase image generation latency. Secondly, this prior art mechanism may degrade the quality of the image by reducing, e.g., narrowing, the gray-scale range and therefore the gray-scale resolution of the flat panel display. Therefore, it is desirable to provide a color balance adjustment mechanism for a flat panel display screen that does not alter the image data nor compromise the gray-scale resolution of the image. 
     Another method of correcting for color balance within a flat panel display screen is used in active matrix flat panel display screens (AMLCD). This method pertains to altering the physical color filters used to generate the red, green and blue color points. By altering the color the filters, the color temperature of the AMLCD screen can be adjusted. However, this adjustment is not dynamic because the color filters need to be physically (e.g., manually) replaced each time adjustment is required. Therefore, it would be advantageous to provide a color balancing mechanism for a flat panel display screen that can respond, dynamically, to required changes in the color temperature of the display. 
     Within CRT devices, color balancing is performed by independently altering the voltages of the primary electron guns (e.g., red, green and blue guns) depending on the color temperature desired. However, like the prior art mechanism that alters the color data on-the-fly, this prior art color balancing technique reduces the gray-scale&#39;s dynamic range and therefore the gray-scale resolution of the display. Also, this technique for color balancing is not relevant for flat panel LCD units because they do not have primary electron guns. 
     Accordingly, the present invention offers a mechanism and method for providing color balancing within a display that does not require a large amount of complex circuitry and does not reduce the gray-scale resolution of the display. Further, the present invention offers a mechanism and method that dynamically alters the color balance of a display and is particularly well suited for application with flat panel LCD units. These and other advantages of the present invention not specifically described above will become clear within discussions of the present invention herein. 
     SUMMARY OF THE INVENTION 
     Multiple light source systems are described herein for color balancing within a liquid crystal flat panel display unit. The present invention includes a method and system for altering the brightness of two or more light sources, having differing color temperatures, thereby providing color balancing of a liquid crystal display (LCD) unit within a given color temperature range. The embodiments operate for both edge and backlighting systems. In one embodiment, two planar light pipes are positioned, a first over a second, with an air gap between. The light pipes distribute light uniformly and independently of each other. The first light pipe is optically coupled along one edge to receive light from a first light source having an overall color temperature above the predetermined range (e.g., the “blue” light) and the second light pipe is optically coupled along one edge to receive light from a second light source having an overall color temperature below the predetermined range (e.g., the “red” light). 
     In the above embodiment, the color temperatures of the first and second light sources are selected such that the overall color temperature of the LCD can vary within the predetermined range by altering the driving voltages of the first and second light sources. In effect, the LCD color temperature is altered by selectively dimming the brightness of one or the other of the light sources so that the overall contribution matches the desired LCD color temperature. In the selection of the light sources, a constraint is maintained that at any color temperature the brightness of the LCD is not reduced below a given threshold minimum (e.g., 70 percent of the maximum brightness). In the selection of the light sources, a second constraint is maintained that within the predetermined color temperature range, the color temperature is held close to the black body curve of the CIE chromaticity diagram. In a third constraint, the light sources are selected so that their maximum brightness point is set to be near the middle of the predetermined color temperature range. 
     In furtherance of one embodiment of the present invention, the light sources selection process may be implemented in computer readable instructions executable by a computer system and stored in computer readable memory such that a large number of number of light sources candidates may be simulated to obtain their luminance, chromaticity, and color temperature data. Candidates that satisfy the above constraints are selected. In one embodiment, the selection process includes the step of analyzing high-resolution spectral files of R, G, and B phosphors, varying a percentage composition of the R, G, B phosphors to generate multiple sets of light source candidates, matching up the light source candidates to generate a pool of candidate pairs, calculating a color temperature-luminance relationship for each candidate pair, and rejecting the candidate pair unless the color temperature-luminance relationship satisfies the above predefined selection constraints. 
     In another embodiment of the present invention, the selection process specifically includes the steps of calculating a chromaticity relationship for each candidate pair and rejecting the candidate pair if the chromaticity relationship deviates significantly from the black body curve. In yet another embodiment of the present invention, the selection process further includes the steps of examining the color temperature-luminance relationship to determine whether a peak brightness point occurs at the middle of the given color temperature range. 
     One embodiment of the present invention includes a color balancing system within a flat panel display for providing color balancing within a color temperature range, the color balancing system having: a first planar light pipe disposed to provide backlight to a liquid crystal display (LCD) layer; a first light source optically coupled to provide light to the first planar light pipe, the first light source having a color temperature that is below the minimum color temperature of the color temperature range; a second planar light pipe disposed parallel to the first planar light pipe such that an air gap exists between the first and the second planar light pipes, the second planar light pipe also for providing backlight to the LCD layer; a second light source optically coupled to provide light to the second planar light pipe, the second light source having a color temperature that is above the maximum color temperature of the color temperature range; a pre-polarizing film disposed between the light pipes and a rear polarizer of an LCD; and a rear reflector positioned on the other side of the light pipes. The system also has a circuit coupled to the first and the second light sources for setting a color temperature of the flat panel display by selectively and independently varying the brightness of the first light source and the brightness of the second light source. The circuit decreases the brightness of the first light source to increase the color temperature of the flat panel display and decreases the brightness of the second light source to decrease the color temperature of the flat panel display. 
     Significantly, in the present embodiment, brightness in the LCD is enhanced by polarization recycling. According to the present embodiment, the pre-polarizing film first pre-polarizes light emitted from the light pipes to a predetermined orientation that matches the polarization orientation of the rear polarizer of an LCD. Light that is not polarized in the predetermined orientation is reflected by the pre-polarizing film to the reflector where it is rephased to the predetermined orientation. Consequently, brightness of the LCD screen is significantly improved by the recycling. In one embodiment, the pre-polarizing film comprises a layer of DBEF brightness enhancement film, and the rear reflector is made of a PTFF material. In another embodiment, the rear reflector is covered with a film comprising barium sulfate. Brightness may also be significantly enhanced by the addition of a crossed BEF layer between the rear polarizer of the LCD and the light pipes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A illustrates a perspective front view of a display device in accordance with one embodiment of the present invention having a removable backlighting assembly partially inserted. 
     FIG. 1B illustrates a perspective back view of a display device in accordance with one embodiment of the present invention having a removable backlighting assembly removed. 
     FIG. 1C illustrates a front view of a desk top display device in accordance with an embodiment of the present invention having a fixed in place backlighting assembly or module. 
     FIG. 1D illustrates a back perspective view of the desk top display device of FIG.  1 C. 
     FIG. 2A is a cross sectional view of a dual light source and dual light pipe embodiment of an LCD flat panel display in accordance with the present invention. 
     FIG. 2B is a cross sectional view of another implementation of the dual light source and dual light pipe embodiment of FIG.  2 A. 
     FIG. 3A illustrates an extraction pattern disposed on the bottom side of a light pipe in accordance with embodiments of the present invention that use a single edge-disposed light source per light pipe. 
     FIG. 3B illustrates a portion of the LCD panel embodiment of FIG. 2A with oriented extraction patterns in accordance with the present invention. 
     FIG. 3C illustrates a portion of the LCD panel embodiment of FIG. 2B with oriented extraction patterns in accordance with the present invention. 
     FIG. 3D illustrates variation of the embodiment of FIG. 3C having two variable intensity light sources and a single light pipe for both. 
     FIG. 4 is a schematic diagram of the inverter circuitry used to independently control the brightness of light sources within an LCD flat panel display within the embodiments of the present invention. 
     FIG. 5 illustrates the CIE chromaticity diagram including the black body curve from blue to red. 
     FIG. 6 is a graph illustrating the color temperatures achieved by one implementation of the dual light source and dual light pipe embodiment of the present invention for a given color temperature range. 
     FIG. 7A, FIG.  7 B and FIG. 7C are spectrum graphs of the energy distributions over a range of wavelengths representing the color temperature distributions of three exemplary blue light sources selected in accordance with the present invention. 
     FIG.  8 A and FIG. 8B are spectrum graphs of the energy distributions over a range of wavelengths representing the color temperature distributions of two exemplary red light sources selected in accordances with the present invention. 
     FIG. 9A is a graph of color temperature and luminance for one implementation of the dual light source and dual light pipe embodiment of the present invention having a blue source at 11,670 K and a red source at 3,623 K. 
     FIG. 9B is a graph of color temperature versus luminance for one implementation of the dual light source and dual light pipe embodiment of the present invention for a blue source at 15,599 K and a red source at 3,221 K with 2.6 mm cold cathode fluorescent tubes (CCFL). 
     FIG. 9C is a graph of color temperature versus luminance for one implementation of the dual light source and dual light pipe embodiment of the present invention for a blue source at 15,599 K and a red source at 3221 K with 2.4 mm CCFL. 
     FIG. 9D is a graph of color temperature versus luminance for one implementation of the dual light source and dual light pipe embodiment of the present invention for a blue source at 15,005 K and a red source at 3,561 K with 2.6 mm CCFL. 
     FIG. 10A is a cross sectional diagram of an embodiment of the present invention having dual light pipes and four light sources, two blue sources and two red sources. 
     FIG. 10B is a cross sectional diagram of an embodiment of the present invention having a single light pipe and four light sources, two blue sources and two red sources. 
     FIG. 11 is a cross sectional diagram of an embodiment of the present invention having dual light pipes and three light sources, two blue sources associated with one light pipe and one red light source that is adjusted for color balancing. 
     FIG. 12 is a cross sectional diagram of an embodiment of the present invention having two wedge-shaped cross-nested light pipes and two light sources. 
     FIG. 13A illustrates a cross sectional diagram of another embodiment of the present invention in which backlight is recycled to increase luminance of the LCD. 
     FIG. 13B illustrates a cross section of another embodiment of a rear reflector where the reflection material is applied to a plastic substrate or carrier. 
     FIG. 13C illustrates a cross section of a PTFF film reflector according to one embodiment of the present invention. 
     FIG. 14 illustrates an exemplary computer system in which the process of selecting approprate light sources according to one embodiment of the present invention. 
     FIG. 15 is a flow diagram illustrating the process of selecting appropriate light source candidates according to one selection criterion in furtherance of one embodiment of the present invention. 
     FIG. 16 is a flow diagram illustrating the process of selecting appropriate light source candidates according to another selection criterion in furtherance of one embodiment of the present invention. 
     FIG. 17 is a flow diagram illustrating a process of selecting appropriate light source candidates according to yet another selection criterion in furtherance of one embodiment of the present invention. 
     FIG. 18A illustrates a backlighting embodiment of the present invention having an array of CCF light sources. 
     FIG. 18B illustrates a backlighting embodiment of the present invention having an array of CCF light sources and a scallop-shaped rear reflector. 
     FIG. 18C illustrates a backlighting embodiment of the present invention having an array of CCF light sources and a scallop-shaped rear reflector where each scallop has a light source pair. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description of the present invention, a color balancing system for a flat panel LCD unit applying variable brightness to multiple light sources of varying color temperature, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be obvious to one skilled in the art that the present invention may be practiced without these specific details or with certain alternative equivalent circuits and methods to those described herein. In other instances, well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the present invention. 
     Exemplary Display Systems 
     FIG.  1 A and FIG. 1B illustrate front and back perspective views of a display system  5  in which embodiments of the present invention can be implemented. Although exemplary display system  5  utilizes a removable backlighting unit  14  that makes use of edge lighting technology, the color temperature balancing embodiments of the present invention are equally applicable to display systems that use fixed-in-place edge lit backlighting units and/or direct backlighting technology (FIG.  1 C and FIG.  1 D). The color temperature balancing embodiments of the present invention are equally applicable to direct backlighting applications as well as edge lighting applications. As described more fully in U.S. Pat. No. 5,696,529, issued Dec. 9, 1997, by Evanicky et al., and U.S. Pat. No. 5.593,221, issued Jan. 14, 1997, by Evanicky et al, both of which are assigned to the assignee of the present invention and incorporated herein by reference, the high resolution color flat panel display  5  has a backlighting door assembly (“backlighting assembly”) 14  for direct viewing. This backlighting assembly  14  can be removed to expose the transparent active LCD unit screen. Once removed, the transparent active LCD unit screen can be positioned on top of an overhead projector in order to project the displayed image in an enlarged fashion onto a receiving screen. 
     With reference to FIG. 1A, a perspective view of the display subsystem  5  is illustrated with the display side facing toward the viewer. This is the direct viewing configuration. The display system  5  comprises three major assemblies. The base assembly  12  which is coupled to a display assembly  10  via a hinge in order to allow the display assembly  10  to adjust to different angles for direct monitoring or allows the display assembly to lay flat for overhead projection configurations and for storage and transportation. The base assembly  12  supports the display  10  for direct viewing configurations and also contains several electronic circuit systems for providing the display unit with power, audio information, and video information (see FIG.  4 ). Herein, the door assembly  14  is also called a backlighting assembly or a liquid crystal flat panel layer. 
     The display assembly  10  contains two stereo speakers  8   a  and  8   b  as well as an active matrix LCD color screen  20 . Although many different resolutions can be utilized within the scope of the present invention, an implementation utilizes an LCD screen  20  having 1280 pixels by 1024 pixels by RGB color and utilizes amorphous silicon thin film transistors (TFT). The LCD screen  20  is composed of color TFT-LCD panel, driver ICs, control circuitry, and power supply circuitry all contained in a rigid bezel. LCD screen  20  is capable of displaying 2 18  true colors without frame rate modulation in text or graphics mode. Various flat panel LCD screens and screen technologies can be used within the scope of the present invention with proper configuration. 
     As shown in FIG. 1A, the display assembly  10  is back-lit via a separate assembly or removable backlighting assembly  14 . In this view, the door is partially removed from the display assembly  10 . The backlighting assembly  14  is removed so that the display  20  can become transparent for overhead projection configurations. While inserted, the backlighting assembly  14  provides backlighting for the LCD screen  20  for direct viewing configurations. Although a number of lamps can be utilized, one embodiment utilizes cold cathode fluorescent (CCF) tubes which are located within the display assembly to illuminate along the top and bottom edges of one or more light pipes located within the backlighting assembly  14  (as will be discussed further below) when the backlighting assembly  14  is inserted within the display assembly  10 . Hot cathode tubes (HCF) can also be used. Also shown is a snap fit clip  34  which is used to secure the backlighting assembly  14  to the display assembly  10 . 
     FIG. 1B illustrates the back side of the display subsystem  5  with the backlighting assembly  14  completely removed to expose inner components of the display assembly  10 . In this view, with the backlighting assembly  14  removed, the back side of the LCD screen  20  is exposed. Located on the base assembly  12  are inputs for AC power  44  and an audio/video input connector  48 . Power supplied to the subsystem, backlight brightness and audio volume are controlled by the computer system&#39;s software through the audio/video input connector  48 . In an alternative embodiment, in addition to computer control these features can be manually adjusted. For instance, also located on the display subsystem  5  can be (optionally) a power on switch  2   a , a color temperature adjustment knob  2   b  and a volume adjustment knob  2   c  for the stereo speakers  8   a  and  8   b . The audio/video input connector  48  is coupled to the digital audio/video output of a computer system. 
     Located within the display assembly  10  are two lamp assemblies or housings. One lamp housing  40  is shown. Each lamp housing can contain one or more CCF lamps  52 , depending on the particular embodiment of white balancing utilized (described further below). The CCF lamps can optionally be mounted within their respective lamp housing using two rubber shock mounts, as shown,  50   a  and  50   b  to secure lamps  52 . An identical configuration is employed for the top lamp housing (obscured). A reflective film  42  is applied to the inner portions of the lamp housings and this tape extends outside, beyond the positions of the lamps  52 , for providing an optical coupling with components of the backlighting assembly  14  when inserted. The same is true for the upper lamp housing. 
     Also shown in FIG. 1B are two receiving holes  32  located on the right and left sides of the display assembly  10 . These receiving holes  32  fasten to corresponding latches ( 34  not shown) located on the backlighting assembly  14 . There is also a recess associated with these latch holes  32  for removal of the backlighting assembly  14 . Also located within this region of the display assembly  10  is a magnetic reed switch  22  that is responsive to the presence of a magnet  140  (not shown) that is located along the mating edge of the backlighting assembly  14 . Using this switch  22 , the display subsystem  5  determines whether or not the backlighting assembly  14  is inserted or removed from the display assembly  10  and responds accordingly. It is appreciated that the reed switch  22  and sensor, in lieu of being magnetically operated, can also be implemented using and optical sensor (or switch, such as using a LED or fiber-optic device) or a mechanical sensor (or switch, such as a toggle or spring switch). 
     There are also two notches  95   a  and  95   b  located on the top of the display assembly  10 . These notches  95   a  and  95   b  are for mating with corresponding latches located on an overhead projector of the present invention for securing the display subsystem properly over an illuminating screen of the projector. When used in a projector configuration, the display subsystem  5  is extended so that the base assembly  12  and the display assembly  10  are flat and the facing side of the display subsystem, as shown in FIG. 1B, is placed facing down on top of the illuminating screen of the projector. In this way, light is projected through the back side of the LCD screen  20 . 
     As discussed above, the color temperature balancing embodiments of the present invention are equally applicable to display systems that use fixed-in-place edge lit backlighting units and/or direct backlighting technology. FIG. 1C illustrates a front view of a desk top display unit (“monitor”)  36  having installed therein an LCD flat panel display assembly  38  having the color balancing system of the present invention. In- this embodiment, the LCD flat panel display assembly  38  is fixed-in-place and edge lit with light sources along the top horizontal edge  38   a  and bottom horizontal edge  38   b.    
     FIG. 1D illustrates a back perspective view of the desk top display unit  36 . This desk top display unit  36  includes a mounting bracket  39  for mounting on a wall, a mechanical arm or for mounting with a base. 
     Color Balancing Systems of the Present Invention 
     FIG. 2A illustrates a liquid crystal flat panel display (herein “flat panel display”)  110   a  in accordance with one embodiment of the present invention. This flat panel display  110   a  can be used within a flat panel display device having a fixed-in-place backlighting unit or can be used within a flat panel display system  5  (FIG. 1A) using a removable backlighting assembly  14 . The flat panel display  110   a , in accordance with the present invention, provides white balance adjustment by independently varying the brightness of a pair of light sources (e.g., CCF tubes)  132  and  136 . For a predetermined range of color temperatures, having a minimum temperature (e.g., 5,000 K) and a maximum temperature (e.g., 6,500 K), a first light source  132  is provided that has a wavelength spectrum with an overall color temperature less than the minimum temperature of the predetermined range; herein, a light source  132  with this characteristic is called the “red” light source for convenience. Also, a second light source  136  is provided that has a wavelength spectrum with an overall color temperature that is greater than the maximum temperature of the predetermined range; herein, a light source  136  with this characteristic is called the “blue” light source for convenience. 
     As shown in FIG. 2A, the red light source  132  is optically coupled to provide light to a first planar light pipe  130 . The red light source  132  is positioned along an edge of the light pipe  130 . In FIG. 2A, only cross sections of this planar light pipe  130  and the light source  132  are shown. Likewise, the blue light source  136  is optically coupled to provide light to a second planar light pipe  134 . The blue light source  136  is positioned along an edge of the light pipe  134 . Only cross sections of this planar light pipe  134  and the light source  136  are shown. In the embodiment  110   a  of FIG. 2A, the light sources  132  and  136  are long thin tubes which are positioned on opposite sides of the planar light pipes  134  and  130 . The light sources  132  and  136  are positioned to be substantially parallel with each other. The power supply for each light source  132  and  136  receives a separate voltage signal for independently controlling its brightness with respect to the other light source. It is appreciated that the positions of the red tube  132  and the blue tube  136  can be switched without departing from the scope of the invention. 
     In order to maintain independence of the light distribution between the first  130  and second  134  light pipes, an air gap  133  is maintained between the two light pipes. This air gap  133  prevents light from light source  132  being extracted within light pipe  134  and prevents light from light source  136  from being extracted within light pipe  130 . The air gap  133  is particularly important for embodiment  110   a  because each light pipe is illuminated from one edge, and the extraction dot pattern (e.g., a pattern of bumps disposed on the surface of the light pipes) corresponding to that light pipe is specifically tailored for light originating from that edge. On the edges and surrounding the light pipes are reflection tapes  131  and  135 . 
     Within embodiment  110   a , a rear reflector layer  138  is positioned on one side of the light pipes. On the other side of the light pipes a diffuser layer  128  (mylar) is followed by one or more brightness enhancement layers (BEFs)  126 , followed by a double BEF (DBEF) layer  124  which is followed by a cover layer  122  for protection. The DBEF layer  124  redirects light not of the proper polarization to the rear reflector layer  138  for recycling. The LCD panel includes a first polarizer layer  120  followed by a back glass layer  121  followed by the selectively energized transistor layer  119  and an LCD layer  118 , followed by red/green/blue color filter layers  114 , a front glass layer  115  followed by a second polarizer layer  116 . A glass or acrylic protection layer  112  is then used. These layers are described in more detail further below. 
     The white balance or color temperature of the embodiment  110   a  is maintained and adjusted using the two independently controlled light sources  132  and  136 . The white balance is adjusted by altering the brightness of the light sources  132  and  136  independently. The phosphor mix (e.g., contribution of red, green and blue phosphor) of the two light sources  132  and  136  is selected so that the white balance can be adjusted by varying the brightness of the light sources. The light pipes  130  and  134  are acrylic and each contain an extraction system that uniformly and independently distributes the light from each light source across the viewing area of the display. 
     In one implementation, the light pipes  130  and  134  are mounted to address a removable backlight assembly (e.g., assembly  14 ). In another implementation, the light sources are located behind a diffusing system  128  to directly backlight the display rather than “edge” light the light pipe. In one embodiment, the light sources  132  and  136  are cold cathode fluorescent (CCF) tubes and, in another embodiment, hot cathode fluorescent (HCF) tubes are used. Constraints are placed on the amount of brightness variation tolerated during white adjustment such that the overall brightness of the display never decreases below a percentage of the maximum brightness output by the light sources  132  and  136 . In one implementation, this percentage is selected at 70 percent. 
     FIG. 2B illustrates another embodiment  110   b  that is analogous to embodiment  110   a  except for the differences pointed out below. In the embodiment  110   b  of FIG. 2B, the light sources  132  and  136  are long thin tubes which are positioned on the same side of the planar light pipes  134  and  130 . The light sources  132  and  136  are positioned to be substantially parallel with each other. Because the light sources  132  and  136  are positioned on the same side of the planar light pipes  134  and  130 , the position of the reflection element  131  is shifted. It is appreciated that the positions of the red tube  132  and the blue tube  136  can be switched without departing from the scope of the invention. 
     Extraction Pattern Orientation 
     FIG. 3A illustrates a top view of an exemplary extraction pattern  144   a  that can be applied to the bottom of light pipe  130  within embodiment  110   a . The extraction pattern  144   a  is designed to uniformly illuminate the LCD layer  118 , at any brightness, taking into consideration that the red tube  132  is positioned along one edge of the light pipe  130 . To accomplish this uniform distribution of light, extraction dots increase in size in a proportion to their distance from the light source  132  as shown in direction  146 . Extraction dots  150   a  are smaller since they are relatively close to the light source  132 . Extraction dots  150   b  are slightly larger since they are relatively farther from to the light source  132  than dots  150   a . Extraction dots  150   c  are the largest because they are the farthest from light source  132 . It is appreciated that extraction pattern  144   a  also includes larger sized dots  150   d  at the comers near the light source  132  because the tube  132  is not as bright at the ends as in the middle sections of the tube. 
     FIG. 3B illustrates a configuration  160  of light pipes and light sources (of embodiment  110   a  of FIG. 2A) taking into consideration the orientation of the light extraction patterns. Within embodiments  110   a  and  110   b , each light extraction pattern is designed to operate with its own light pipe (e.g., pipe  130 ) independently of the other light pipe (e.g., pipe  134 ). In other words, extraction pattern  144   a  is designed to uniformly distribute light to the LCD layer  118 , independently of light pipe  132 , as the brightness of light source  132  varies. Extraction pattern  144   b  is designed to uniformly distribute light to the LCD layer  118 , independently of light pipe  130 , as the brightness of light source  136  varies. Light extraction pattern  144   a  is shown in FIG. 3B in cross section as a thin line applied to the underside of light pipe  130 . As shown, the dot sizes increase within pattern  144   a  from left to right because the light source  132  is positioned on the left edge of the light pipe  130 . However, the light extraction pattern  144   b  applied to the underside of light pipe  134  is the flipped image of pattern  144   a  with the dot sizes increasing from right to left because light source  136  is positioned along the right edge of the light pipe  134 . 
     Considering the provision of the air gap  133  and that each light extraction pattern  144   a  and  114   b  is tailored for its own light pipe, the light pipes  130  and  134  effectively operate separately and independently to uniformly distribute light over the LCD layer. One function of the light extraction patterns  144   a  and  144   b  is to uniformly distribute light over their associated light pipes even if one lamp is dimmed (or brightened) unilaterally. It is appreciated that the brightness of light source  136  is increased slightly to compensate for the fact that extraction pattern  144   a  resides between the light pipe  134  and any LCD layer and thereby slightly obstructs the light emitted from light pipe  134 . An alternative approach adjusts the sizes of the dots of the relative dot extraction patterns to compensate for the obstruction. 
     FIG. 3C illustrates a configuration  165  of light pipes and light sources (of embodiment  110   b  of FIG. 2B) taking into consideration the orientation of the light extraction patterns. Within embodiments  110   a  and  110   b , each light extraction pattern is designed to operate with their light pipe (e.g., pipe  130 ) independently of the other light pipe (e.g., pipe  134 ). Light extraction pattern  144   b  is shown in FIG. 3C in cross section as a thin line applied to the underside of light pipe  130 . As shown, the dot sizes increase within pattern  144   b  from right to left because the light source  132  is positioned on the right edge of the light pipe  130 . The same light extraction pattern,  144   b , is also applied to the underside of light pipe  134 . As discussed above, extraction pattern  144   b  is the mirror image of pattern  144   a  with the dot sizes increasing from right to left because light source  136  is positioned along the right edge of the light pipe  134 . 
     FIG. 3D illustrates a variation of the embodiment  165  of FIG.  3 C. Alternatively, as shown in FIG. 3D, this embodiment  167  uses both controls for the first  132  and second  136  light sources together to change the display brightness without altering the white balance setting where both the first and second light sources are positioned on the same side of a single light pipe layer  130 ′ and optically coupled to it. A rear reflector  138  is also used. This embodiment  167  can also be used for color temperature balancing. 
     Dual Inverters for Independently Driving Light Sources 
     FIG. 4 is a logical block diagram of electronics  170  of the display subsystem  5 . Although some electrical components are shown (in dashed lines) to be associated with the base assembly  12  or the display assembly  10 , it is appreciated that their locations are exemplary. Apart from the LCD screen  20 , the actual location of the circuits could be in either the display assembly  10  or the base assembly  12 . It is appreciated that circuit  170  includes the provision of separate inverter circuits  175   a  and  175   b  for separately and independently controlling light sources  132  and  136 . A color temperature adjustment knob  2   b  (FIG. 1B) is coupled to circuit  187  which controls the voltages supplied by the inverters  175   a  and  175   b  to the lamps  132  and  136  over bus  180  to separately control their brightness. Another implementation adjusts the brightness through software control by means of a digital potentiometer. Inverter  175   a  controls light source  132  and inverter  175   b  controls light source  136 . 
     In one implementation, within the base assembly, as shown in FIG. 4, are a power supply unit  184  for coupling with an alternating current source  44 . This power supply  184  supplies power via line  182  to an audio board  178  and a video board  176 . The audio board  178  is coupled to the video board  176  via bus  186 . Audio and video information are sent to the display subsystem via input interconnect  48 . It is appreciated that a variety of audio/video information transfer formats and standards can be used within the scope of the present invention, including an IBM compatible standard, a UNIX standard, or Apple Computer standard. 
     Video board  176  is coupled to a bus  185  for communicating and controlling elements of the display assembly  20 . It is appreciated that portions of bus  185  are composed of flex circuits so that base assembly  12  and display assembly  10  can move freely about their common hinge. Among other signals, this bus  185  carries power, control signals and audio and video data signals. The video board  176  is coupled to supply audio signals over bus  185  to stereo speakers  8   a  and  8   b . Video board  176  also supplies a control signal and power over line  185  to a circuit  187  which in turn independently controls two AC to DC inverters  175   a  and  175   b . Each inverter contains a transformer for supplying a high voltage signal, over bus  180 , to the light sources  132  and  136  and also contains a switch circuit for turning the tubes off. Light sources  132  and  136  are separately coupled to power supply lines  180   a  and  180   b , respectively, which are within bus  180 . Return bus  189  contains a separate return lines from source  132  to inverter  175   a  and from source  136  to inverter  175   b . Bus  185  is also coupled to reed switch  22  which carries a digital signal indicating when the backlighting assembly  14  is inserted into the display assembly  10  or not present. 
     Bus  185  of FIG. 4 is coupled to supply video information to column driver circuits  171 . The column driver circuits  171  control information flow to the columns of each of the rows of transistors of the LCD screen  20  to generate an image in the well-known fashion. (There are also separate row driver circuits that are not illustrated but operate in the well-known fashion.) 
     Blackbody Chromaticity Curve 
     FIG. 5 illustrates a CIE chromaticity diagram illustrating chromaticity coordinates along the horizontal and vertical. Within the diagram  190 , the green portion  194  is toward the top with yellow  192  between green  194  and red  198 . Blue  196  is toward the left. A black body curve  200  represents the chromaticity displayed by a tungsten filament heated to various degrees kelvin. For instance, from point D to point A along curve  200 , the curve represents the color emitted from the tungsten filament from 6,500 degrees K to 2856 degrees K. As shown, the blackbody curve  200  traverses from blue  196  to the red  198  without straying much into the yellow  192  or green  194  regions. 
     The light sources  132  and  136  selected in accordance with the present invention are those that illuminate with a color temperature that is near the blackbody curve  200  when their brightness is adjusted within a predetermined color temperature range (e.g., 5,000 to 6,500 K). That is, the color balancing system of the present invention allows adjustments to the color temperature of the flat panel display screen that remain close to the blackbody curve. 
     In addition to following the CIE black body curve (“locus”), light sources  132  and  136  selected in accordance with the present invention follow the daylight color temperature locus when color temperature of the display is adjusted. One advantage of following the daylight color temperature locus during white balancing is that the resulting color temperature tends to be brighter (e.g. having a greater lumen value, Y), and tends to model daylight more accurately. Furthermore, the resulting color temperature tends to be more “green,” giving the display a more natural appeal. 
     FIG. 6 illustrates one exemplary case where the “blue” light source  136  is a CCFL tube having a color temperature of 11,670 K and the “red” light source  132  is a CCFL tube having a color temperature of 3,623 K within a flat panel display having a color balancing system  160  as shown in FIG.  3 B. FIG. 6 illustrates that by independently varying the brightness of the light sources  132  and  136 , the resultant color temperature of the flat panel display can be altered in accordance with line  212 . Line  200  is the same blackbody curve as shown in FIG.  5 . In this case, curve  212  is substantially similar to curve  200  within the predetermined color temperature range of 5,000 K to 6,500 K. In one embodiment, the brightness is varied by holding one light source (e.g.,  132 ) at maximum brightness and dimming the other light source (e.g.,  136 ) until a minimum brightness threshold is met. This steers the color temperature from a mid range value (e.g., 5500 K) into progressively warmer (e.g. smaller) values. To increase the color temperature from the mid range value, light source  136  is held constant and light source  132  is dimmed down until the minimum threshold brightness is reached. 
     Example Red and Blue Light Sources 
     Within the present invention, the light sources  132  and  136  are selected such that their color temperature allows the white balancing within a predetermined range (e.g., 5,000 to 6,500) that (1) follows the blackbody curve  200 , (2) where the overall brightness of the display does not drop below a predetermined threshold over the color temperature range and (3) having a peak brightness (both light sources on) near the middle of the color temperature range. There are many combinations of blue and red tubes that meet the above constraints. Processes described further below illustrate the manner in which light sources  132  and  136  can be selected that meet the above constraints. The following description illustrates some exemplary blue light sources and exemplary red light sources that can be paired (in blue-red combinations) to meet the above constraints. 
     FIG. 7A illustrates a blue light source  136  having percentages of red, green and blue phosphors such that the CCFL tube exhibits the emission spectrum  220  within the 375-775 nm wavelength range. The overall color temperature for the light source of FIG. 7A is very high at 15,600 K. FIG. 7B illustrates a blue light source  136  having percentages of red, green and blue phosphors such that the CCFL tube exhibits the spectrum  224  within the 375-775 nm wavelength range. The overall color temperature for the light source of FIG. 7B is high at 15,000 K. FIG. 7C illustrates a blue light source  136  having percentages of red, green and blue phosphors such that the CCFL tube exhibits the spectrum  226  within the 375-775 nm wavelength range. The overall color temperature for the light source of FIG. 7C is 10,600 K. 
     FIG. 8A illustrates a red light source  132  having percentages of red, green and blue phosphors such that the CCFL tube exhibits the spectrum  230  within the 375-775 nm wavelength range. The overall color temperature for the light source of FIG. 8A is low at 3,560 K. FIG. 8B illustrates a red light source  132  having percentages of red, green and blue phosphors such that the CCFL tube exhibits the spectrum  232  within the 375-775 nm wavelength range. The overall color temperature for the light source of FIG. 8B is low at 3,220 K. 
     Exemplary combinations of the above red and blue light sources that can be used within embodiment  160  of FIG. 3B are shown in Table I below. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 Pair 
                 Red Tube 
                 Blue Tube 
                 Ctemp @ Max Lumin 
               
               
                   
                   
               
             
            
               
                   
                 1 
                 3,560 
                 15,600 
                 5,893 
               
               
                   
                 2 
                 3,220 
                 15,600 
                 5,635 
               
               
                   
                 3 
                 3,560 
                 10,600 
                 5,468 
               
               
                   
                 4 
                 3,560 
                 15,000 
                 5,635 
               
               
                   
                 5 
                 3,220 
                 15,000 
                 5,367 
               
               
                   
                   
               
               
                   
                 Where Ctemp @ Max Lumin is the color temperature of the pair at maximum luminance.  
               
            
           
         
       
     
     FIG. 9A illustrates a color temperature and luminance diagram  240  for various brightness configurations of a color balancing embodiment  160  of FIG. 3B within the color temperature range of 5,000 to 6,500 K. In this example, a blue tube  136  having a color temperature of 11,670 K is used with a red tube  132  having a color temperature of 3,623 K and corresponds to the same configuration described with respect to FIG.  6 . Mid point  246   c  of FIG. 9A represents maximum luminance when both tubes are at their full brightness and a color temperature near 5,500 K is reached. This is roughly in the middle of the color temperature range 5,000 to 6,500 K. 
     The following describes color temperature variations from the mid point  246   c  achieved by dimming one or the other tube. Region  246   a  represents the white balance adjustment where the red tube  132  is left fully on and the blue tube  136  is dimmed down in a range from 5 to 25 percent (of the original full) luminance. Within region  246   a , curve  242  represents the luminance ratio and this value decreases (from 1.0 to 0.8) as the blue tube  136  is dimmed down. Also within region  246   a , the color temperature as shown by curve  248  decreases as the blue tube  136  is dimmed down. Region  246   b  represents the white balance adjustment where the blue tube  136  is left fully on and the red tube  132  is dimmed down from 5 to 25 percent of the original full luminance. Within region  246   b , curve  242  represents the luminance ratio and this value decreases (from 1.0 to 0.8) as the red tube  132  is dimmed down. Also within region  246   b , the color temperature as shown by curve  248  increases as the red tube  132  is dimmed down. 
     FIG. 9B illustrates a color temperature and luminance diagram  260  for various brightness configurations of a color balancing embodiment  160  of FIG. 3B within the color temperature range of 3,400 to 8,250 K using CCFL tubes of 2.6 mm in size. In this example, a blue tube  136  having a color temperature of 15,599 K (FIG. 7A) is used with a red tube  132  having a color temperature of 3,221 K (FIG.  8 B). Curve  262  represents the luminance in Cd/sq m over the given range of color temperatures and curve  264  represents the luminance ratio (from 0 to 1.0). Peak luminance point  266  represents the maximum brightness condition (4,600 K) when both lamps  136  and  132  are fully on. That portion of the curves to the right of point  266  represents the condition when tube  136  is fully on and tube  132  is dimmed down to increase the color temperature. That portion of the curves to the left of point  266  represents the condition when tube  132  is fully on and tube  136  is dimmed down to decrease the color temperature. 
     FIG. 9C illustrates a color temperature and luminance diagram  270  for various brightness configurations of a color balancing embodiment  160  of FIG. 3B within the color temperature range of 3,400 to 8,250 K using CCFL tubes of 2.4 mm in size. In this example, a blue tube  136  having a color temperature of 15,599 K (FIG. 7A) is used with a red tube  132  having a color temperature of 3,221 K (FIG.  8 B). Curve  272  represents the luminance in Cd/sq m over the given range of color temperatures. Peak luminance point  276  represents the maximum brightness condition (5,000 K) when both lamps  136  and  132  are fully on. That portion of the curves to the right of point  276  represents the condition when tube  136  is fully on and tube  132  is dimmed down to increase the color temperature. That portion of the curves to the left of point  276  represents the condition when tube  132  is fully on and tube  136  is dimmed down to decrease the color temperature. 
     FIG. 9D illustrates a color temperature and luminance diagram  280  for various brightness configurations of a color balancing embodiment  160  of FIG. 3B within the color temperature range of 3,400 to 8,250 K using CCFL tubes of 2.6 mm in size. In this example, a blue tube  136  having a color temperature of 15,599 K (FIG. 7A) is used with a red tube  132  having a color temperature of 3,221 K (FIG.  8 B). Curve  282  represents the luminance in Cd/sq m over the given range of color temperatures. Peak luminance point  286  represents the maximum brightness condition (4,800 K) when both lamps  136  and  132  are fully on. That portion of the curves to the right of point  286  represents the condition when tube  136  is fully on and tube  132  is dimmed down to increase the color temperature. That portion of the curves to the left of point  286  represents the condition when tube  132  is fully on and tube  136  is dimmed down to decrease the color temperature. 
     It is appreciated that many of the layers within an LCD flat panel display system tend to “yellow” shift light passing there through, e.g., the acrylic in the light pipes, the ultra-violet cured extraction patterns, the DBEF and BEF films, the polarizers, the LCD layer and the color filters. Therefore, to compensate for this yellow shift, the red and/or blue light sources  132  and  136  selected can be slightly blue shifted. 
     Additional Multi-Light Source Embodiments 
     FIG. 10A illustrates a cross section of an alternate embodiment  310  of a color balancing system in accordance with the present invention that utilizes four light sources. Two red light sources  312  and  314  are positioned on opposite sides of a planar light pipe  130  and are parallel with each other. The brightness of these two red light sources  312  and  314  are varied in tandem. Two blue light sources  316  and  318  are positioned on opposite sides of a planar light pipe  134  and are parallel with each other. The brightness of these two blue light sources  316  and  318  are varied in tandem independently of the red light sources  312  and  314 . It is appreciated that the positions of the blue and red light sources can be switched in accordance with the present invention. Light pipe  134  is positioned under light pipe  130 . An air gap  133  is positioned between the light pipes  130  and  134  but is optional in this embodiment because the locations of the red and blue light source pairs are symmetrical with respect to both light pipes  130  and  134 . As with embodiment  160 , CCFL tubes or HCL tubes can be used as the light sources with particular red, green, blue phosphor contributions to differentiate the blue from the red light sources. 
     Within embodiment  310 , because the brightness of the light sources that are on opposite sides of a same light pipe are varied in tandem, there is a uniform decrease or increase in brightness on both sides of the light pipe (e.g., light pipe  130  or  134 ). In this case, the extraction pattern  144   c  applied to the underside of each light pipe  130  and  134  utilizes extraction dots that vary in size with respect to their closest distance from the two light sources. That, is, along the sides having the light sources, the extraction dots are smaller and they increase in size (from both sides) toward the middle. An extraction pattern  144   c  fitting this description is described in U.S. Pat. No. 5,696,529, issued Dec. 9, 1997 by Evanicky, et al., and assigned to the assignee of the present invention. 
     In accordance with the embodiment  310  of FIG. 10A, to vary the color temperature of the display, the voltage driving the red light sources  312  and  314  is varied to vary their brightness. With the blue light sources  316  and  318  at maximum brightness, the color temperature can be increased from mid-range by dimming down the red light sources  312  and  314  in tandem. Conversely, to vary the color temperature of the display, the voltage of the inverter supply driving the blue light sources  316  and  318  is varied to vary their brightness. With the red light sources  312  and  314  at maximum brightness, the color temperature can be decreased from mid-range by dimming down the blue light sources  316  and  318  in tandem. 
     Embodiment  310  provides increased brightness through the color temperature variation because more light sources are utilized. Therefore, this embodiment  310  has a larger pool of red/blue light source candidates which allow good color temperature range variation while also providing adequate brightness through the color temperature range. However, since more light sources are used in embodiment  310  over the dual light pipe embodiment  160 , embodiment  310  consumes more power. 
     FIG. 10B illustrates a cross section of an embodiment  330  of a color balancing system in accordance with the present invention that is a variation of embodiment  310 . Embodiment  330  includes a single planar light pipe  340  having a red/blue pair of light sources located on two opposite sides. On the left are located a red light source  312  and a blue light source  316  and on the right are located a red light source  314  and a blue light source  318 . Like embodiment  310 , the red light sources  312  and  314  of embodiment  330  are varied in tandem and the blue light sources  316  and  318  are varied in tandem, independently from the red light sources  312  and  214 . An extraction pattern  144   c , as described above, is applied to the underside of light pipe  340 . 
     Color temperature variation is performed for embodiment  330  in the same manner as described with respect to embodiment  310 . The advantage of embodiment  330  is that a single light pipe  340  can be used. Since the brightness of the red and blue light sources are varied in tandem (for a given color), only two inverters  175   a  and  175   b  (FIG. 4) are required for embodiment  310  and for embodiment  330 . In other words, the voltage signal on line  180   a  of FIG. 4 can be coupled to control both red light sources  312  and  314  and the voltage signal on line  180   b  can be coupled to control both blue light sources  316  and  318 . 
     FIG. 11 illustrates a cross section of another embodiment  350  of a color balancing system in accordance with the present invention that utilizes two blue light sources  316  and  318  and a single red light source  314 . The blue light sources  316  and  318  are positioned along opposite edges of a first light pipe  130 . An extraction pattern  144   c , as described above, is applied to the underside of light pipe  130 . Positioned under light pipe  130  (with an air gap  133  in between) a second light pipe  134 . A single red light source  314  is positioned along one edge of light pipe  134  (e.g., on the right or left side). When light source  314  is positioned on the right side, as shown, extraction pattern  144   b  is used with light pipe  134  and when light source  314  is positioned on the left side, extraction pattern  144   a  is used. 
     In operation, the blue light sources  316  and  318  are maintained at (or slightly above) a color temperature above a predetermined color temperature range (e.g., at or above 6,500 K). The blue light sources  316  and  318  are maintained at or near their full brightness to provide the required luminance for the display and the red light source  314  is adjusted in brightness to provide a varying degree of down-shifted color temperature. Embodiment  350  provides the advantage that the color temperature of the display can effectively be adjusted without affecting the backlight luminance. That is to say, if the emission spectra of the red lamp is in the deep red region (e.g., 658 nm), then even at its full brightness it would only contribute about 5 percent of the backlight luminance because of the human eye&#39;s insensitivity to that color region. It is appreciated that two inverters  175   a  and  175   b  are required for embodiment  350  even though the brightness of light sources  316  and  318  is held constant. Power consumption for the red light source  314  is within the region of 0.5 watt. 
     FIG. 12 illustrates a cross section of an embodiment  370  of a color balancing system in accordance with the present invention. Embodiment  370  is similar to embodiment  160  except the planar light tubes  372  and  374  are wedge-shaped in cross section. The light tubes  372  and  374  are positioned as shown in FIG. 12 so that they have a lower profile in cross section. That is, the light pipes  130  and  134  of embodiment  160 , in one implementation, are roughly 3 mm thick so their total width is just over 6 mm when stacked with an air gap  133  in between. However, because the wedge-shaped light pipes  372  and  374  can be positioned as shown in FIG. 12, the overall height of light pipes  372  and  374  is only 3 mm since they are inter-crossed (e.g., cross-nested) together. As shown, a modification of extraction pattern  144   a  is applied to the underside of light pipe  372  and a modification of extraction pattern  144   b  is applied to the underside of light pipe  374 . The functionality of embodiment  370  similar to embodiment  160  however embodiment  370  offers a much lower profile and reduced weight. A modification of the extraction pattern is necessary to compensate for the influence on extraction resulting from the angle of the wedge of each light pipe  372   374 . 
     Color Balancing User Interface 
     There are several mechanisms in which the user can adjust the color balance of the display in accordance with the present invention. In one embodiment, the user can adjust a slider between two extreme mechanical positions in which the position of the slider (or knob  2   b ) represents a particular color temperature within the predetermined color temperature range. The particular color temperature selected is then translated into a dimming configuration by which one or more tubes are dimmed to achieve the color temperature. 
     In another embodiment, the slider is provided but the display also contains a chromaticity measuring device (e.g., a calorimeter) with gives the user immediate feedback as to the color temperature of the display. The user then monitors the measuring device while adjusting the slider mechanism until the desired color temperature is reached. Alternatively, the white balance can be set via a feedback loop from a colorimeter positioned so that it analyzes the color temperature of the display and feeds that information to the host computer through a serial port and the host computer then automatically adjusts the white balance. 
     Improved Light Pipe Assembly Construction for Enhanced Display Brightness 
     In prior art flat panel LCD systems, almost 50% of the luminance from the backlight is lost. This is due to the fact that the rear polarizer film of the LCD only accepts light with a specific orientation and rejects the rest. The present invention recognizes that, if this rejected light could somehow be “recycled,” the net display brightness could be increased. FIG. 13A illustrates on embodiment of the present invention in which backlight is recycled to increase luminance of the LCD. 
     Particularly, as illustrated in FIG. 13A, portions of a flat panel LCD  410  including a backlight distributor  430 , which may comprise various configurations of light pipes and light sources, such as the embodiments of the dual light pipes illustrated in FIGS. 2A-2B and FIGS. 3B-3C. In addition, according to the present embodiment, flat panel LCD  410  further includes a rear reflector layer  438  positioned on the back side of the backlight distributor  430  for recycling light. On the other side of the backlight distributor  430 , a diffuser layer  428  is followed by one or more brightness enhancement layers (BEFS)  426 , followed by a dual brightness enhancement film (DBEF) layer  424  which is followed by a special cover layer  422 . The LCD panel includes a first (rear) polarizer layer  120  followed by a selectively energized transistor-layer  110  and a LCD layer  118 , followed by red/green/blue color filter layers  114 , a front glass layer  115  followed by a second polarizer layer  116 . A glass or acrylic protection layer  112  is then used. 
     According to the present embodiment, DBEF layer  424  of FIG. 13A, in combination with the rear reflector  438 , increases brightness of flat panel LCD  410  by first “pre-polarizing” the light emitted from the backlight distributor  430  to the same orientation as the rear polarizer  120 . Any light having the wrong orientation is reflected by DBEF layer  424  to the rear reflector  438  where much of it is rephased and reflected back to the display or lost through absorption. A large portion of the rephased and reflected light from the reflector  438  can then pass through the rear polarizer layer  120 . 
     Naturally, the amount of light that can be recycled depends upon the reflectivity of rear reflector  438 . Conventional reflective materials currently employed by the industry only reflect about 92% of the light. According to one embodiment of the present invention, a white rear reflector  438  made of a Teflon-like material developed by W.I. Gore and Associates, Inc., and sold under the name of PTFF, is used. In this embodiment, a near 100% increase in luminance is attainable. In one embodiment, rear reflector  438  comprises a layer of PTFF  437  coated onto plastic film substrate  441 , and is illustrated in FIG.  13 C. It should be apparent to those ordinarily skilled in the art, upon reading the present disclosure, that other materials displaying similar light-rephasing and reflectivity properties may also be used. 
     LCD brightness may be further enhanced by the using a brightness enhancement film (BEF) in between the backlight distributor  430  and the DBEF layer  424 . As illustrated in FIG. 13A, flat panel LCD  410  includes optional BEF layer(s)  426 . In one embodiment where a single layer of BEF is used, LCD brightness may be increased by 50%. An even higher luminance gain may be attained when two layers of BEF are aligned at an angle of 90 to each other, a forming a “crossed BEF” layer. In some systems, a luminance gain of 75% may be attained when crossed BEFs are used. An additional benefit of the crossed BEF is that the viewing angle of the LCD is significantly increased. 
     However, one drawback of the DBEF is that off-axis visual artifacts such as color pattern or color stripes, may appear on the LCD screen if viewed at large angles from the normal direction. In furtherance of one embodiment of the present invention, a special cover sheet  422  may be used to eliminate such off-axis visual artifacts. According to the present embodiment, special cover sheet  422  comprises a thin light diffusing film placed between the DBEF and the rear polarizer of the LCD such that a small portion of the backlight is scattered at large off-axis angles. As a result, the contrast of the color stripe pattern is greatly reduced. However, it should be noted that special cover sheet  422  should not cause significant de-polarization because excess de-polarization would reduce the efficiency of the light-recycling process. In the present embodiment, special cover sheet  422  is made of an Lexan  8 A 35  material available from General Electric. Further, in the present embodiment, the special cover sheet  422  is cut with its optical axis matching the transmission axis of the polarizer and the DBEF. 
     In furtherance of the present invention, FIG. 13B illustrates another embodiment of a rear reflector  440  that may be used in place of rear reflector  438  of FIG.  13 A. As shown, rear reflector  440  comprises a layer of barium sulfate  439  deposited on substrate  441  such as white plastic film(s). According to the present embodiment, barium sulfate layer  439  may be deposited on substrate  441  by first mixing barium sulfate powder with an organic binder to form a paste, and then screen printing the paste on the substrate  441 . Significantly, barium sulfate layer  439  should be at least 0.01″ thick. It should be apparent to those of ordinary skill in the art, upon reading the present disclosure, that numerous well known organic binders and coating techniques may also be used to manufacture rear reflector  440 . 
     Selecting Appropriate Percentage Composition of Phosphors for Use in Cold Cathode Fluorescent Tubes 
     Portions of the present invention comprise computer-readable and computer-executable instructions which reside in, for example, computer-usable media of a computer system. FIG. 14 illustrates an exemplary computer system  500  upon which one embodiment of the present invention may be practiced. It is appreciated that system  500  of FIG. 14 is exemplary only and that the present invention can operate within a number of different computer systems and/or electronic device platforms. 
     System  500  of FIG. 14 includes an address/data-bus  510  for communicating information and a central processor unit  520  coupled to bus  510  for processing information and instructions. System  500  also includes data storage features such as computer-usable volatile memory  530 , e.g. random access memory (RAM), coupled to bus  510  for storing information and instructions for central processor unit  520 ; computer usable non-volatile memory  540 , e.g. read only memory (ROM), coupled to bus  510  for storing static information and instructions for the central processor unit  520 ; a data storage unit  550  (e.g., a magnetic or optical disk and disk drive) coupled to bus  510  for storing information and instructions; and a network interface unit  590  (e.g. ethernet adapter card, modem, etc.) for receiving data from and transmitting data to a computer network. System  500  also includes optional devices such as an optional alphanumeric input device  570  coupled to bus  510  for communicating information and command selections to central processor unit  520 ; an optional cursor control device  580  coupled to bus  510  for communicating user input information and command selections to central processor unit  520 ; and an optional display device  560  coupled to bus  510  for displaying information. 
     Unless specifically stated otherwise as apparent from the following discussions, it is appreciated that throughout the present invention, discussions utilizing terms such as “receiving,” “determining,” “indicating,” “transmitting,” “repeating,” or the like, refer to the actions and processes of a computer system or similar electronic computing device. The computer system or similar electronic device manipulates and transforms data, represented as physical (electronic) quantities within the computer system&#39;s registers and memories, into other data, similarly represented as physical quantities within the computer system memories, into other data similarly represented as physical quantities within the computer system memories, registers, or other such information storage, transmission, or display devices. 
     In the embodiments described above, LCD color temperature is altered by selectively dimming the brightness of one or the other of the light sources so that their overall contribution matches the desired LCD color temperature. LCD color temperature is also dependent upon the percentages of different phosphors that are used within the CCFLs. For instance, a CCFL tube that includes 33% of R-phosphor, 33% of G-phosphor, and 33% of B-phosphor will have different color temperature, chromaticity, and brightness than another one that includes 40% R-phosphor, 40% G-phosphor, and 20% B-phosphor. Therefore, it is necessary to select an appropriate percentage composition of R, G, and B phosphors such that the color temperature, luminance, and chromaticity of the LCD may be accurately controlled. 
     According to one embodiment of the present invention, an appropriate percentage composition of phosphors is selected such that, within a predetermined range of color temperature, the brightness of the LCD is not reduced below a given threshold minimum (e.g., 70 percent of the maximum brightness). In addition, in the present embodiment, the appropriate mix of R, G, B phosphors is selected such that, within the predetermined color temperature range, the color temperature of the display is held close to the black body curve  200  of the CIE chromaticity diagram  190  (FIG.  5 ). Finally, in the present embodiment, the appropriate mix of R, G, B phosphors in the light sources is selected such that their maximum brightness point is set to be near the middle of the predetermined color temperature range. 
     FIG. 15 is a flow diagram  600  illustrating a process (executed on system  500 ) of selecting appropriate light source candidates such that, within a predetermined range of color temperature, the brightness of the LCD is not reduced below a given threshold minimum (e.g., 70 percent of the maximum brightness). For simplicity, in the present embodiment, the selection is made for a “blue” light source  136 , and a “red” light source  132  of a color balancing embodiment  160  of FIG.  3 B. Further, a target color temperature range is predetermined to be between 3,400 K to 8,250 K. 
     At step  610 , high-resolution (1 nm) spectral files for particular types of R, G, and B phosphors are input into computer system  500  and stored at RAM  530  or data storage unit  550 . The high-resolution spectral files for R, G, and B phosphors may be obtained from manufacturers of the phosphors. Particularly, each spectral file contains energy data of the emission spectrum of one type of phosphor (e.g. R-phosphors, G-phosphors or B-phosphors). Then, the energy data of the emission spectra of the R, G, and B phosphors are converted to luminance (brightness) data using the human eye sensitivity data over the visible emission spectrum. For simplicity, in the following discussion, the luminance data for R, G, and B phosphors are labeled R(λ), G(λ) and B(λ), respectively. Significantly, each light source candidate is essentially represented by the percentages of R-phosphors, G-phosphors, and B-phosphors present in the light source candidate. 
     At step  620 , a pool of light source candidates are generated by varying the amount of different phosphor types. According to the present embodiment, a 5%-increment scheme in the relative amounts of different phosphor types is adopted. For example, one light source candidate may have a percentage composition of 25% R-phosphors, 35% G-phosphors, and 40% B-phosphors. Another light source candidate may have a percentage composition of 40% R-phosphors, 40% G-phosphors, and 20% B-phosphors. Further, in the present embodiment,  400  (20×20) light source candidates are available. 
     At step  630 , the luminance spectrum W(λ) of each of the light source candidates is computed. According to the present embodiment, a “bluish.” luminance spectrum W 1 (λ) for “blue” light source  136  is calculated according to the following equation: 
       W   1 (λ)= a   1 * R (λ)+ b   1 * G (λ)+ c   1 * B (λ), 
     where a 1 , b 1 , c 1  correspond to the percentages of red phosphors, green phosphors, and blue phosphors, respectively, selected for the “blue” light source  136 , and where a 1 +b 1 +c 1 =1. According to the present embodiment, a total number of  400  calculations have to be made for each of the 400 light source candidates. 
     Similarly, a “reddish” luminance spectrum W 2 (λ) for “red” light source  132  is calculated according to the following equation: 
     
       
           W   2 (λ)= a   2 * R (λ)+ b   2 * G (λ)+ c   2 * B (λ), 
       
     
     where a 2 , b 2 , c 2  correspond to the percentages of red phosphors, green phosphors, and blue phosphors selected for the “red” light source  132 , and where a 2 +b 2 +c 2 =1. 
     At step  640 , a light source candidate is matched up with another light source candidate to form a candidate pair. In the present embodiment, the total number of light source candidate is 20 for each of the two light sources  132  and  136 . Therefore, a total number of possible candidate pairs is 400. 
     At step  650 , a combined luminance spectrum, W 3 (λ), is computed for the selected candidate pair. The combined luminance spectrum results from contributions from “blue” light source candidate and from “red” light source candidate, and can be calculated according to the equation: 
     
       
           W   3 (λ)= L   1 * W   1 (λ)+ L   2 * W   2 (λ), 
       
     
     where L 1  and L 2  represent brightness levels of “blue” light source  136 , and “red” light source  132 , respectively. According to the present embodiment, the brightness level Li of the “blue” light source  136 , and the brightness level L 2  of the “red” light source  132  may be selectively and independently adjusted to modify the color temperature of the LCD. Further, according to the present embodiment, a 5% increment/decrement scheme in the intensity levels L 1  and L 2  is adopted. Thus, in the present embodiment, after discarding redundancy, a total number of 200 combined luminance spectrums are calculated for the selected candidate pair. 
     At step  660 , for each of the combined luminance spectrums generated at step  650  for the selected candidate pair, a luminance value and a color temperature is calculated. Methods for calculating luminance values and color temperatures from luminance spectrums are well known in the art. Therefore, details of such methods are not described herein to avoid obscuring aspects of the present invention. In one embodiment, a maximum luminance value, Lmax, corresponding to the maximum brightness levels (L 1 =L 2 =100%) of the “blue” light source candidate and the “red” light source candidate, is also calculated. 
     At step  670 , a table for storing the luminance values and color temperatures associated with the selected candidate pair is constructed, thus forming a color temperature-luminance relationship for the selected candidate pair. 
     At step  680 , candidate pairs having luminance values (L) smaller than a minimum luminance threshold (e.g. 70% of Lmax) between the predetermined color temperature range (e.g. between 3,400 K to 8,250 K) are rejected. In this way, a significant number of candidate pairs are rejected and the simulation time for any subsequent steps is reduced. 
     At step  690 , it is determined whether all the possible combinations of candidate light sources have been processed. If it is determined that all possible combinations of the candidate light sources have been processed, candidate pairs that do not meet the requisite criterion are rejected, and the processed ends. However, if there are possible combinations of candidate light sources that have not been processed, steps  610  through  690  are repeated. 
     FIG. 16 is a flow diagram  700  illustrating a process of selecting appropriate light source candidates such that within the predetermined color temperature range, the color temperature is held close to the black body curve  200  of the CIE chromaticity diagram  190 . 
     At step  710 , a candidate pair is selected from the pool of candidate pairs that have not been rejected. According to the one embodiment, the process illustrated in flow diagram  700  is performed after candidate pairs that do not meet the luminance requirement are rejected. 
     At step  720 , the chromaticity values (x, y) for a candidate pair is determined from the combined luminance spectrums calculated in step  660 . In one embodiment, a different chromaticity value (x, y) is calculated for each luminance spectrum for each candidate pair. Methods for calculating chromaticities from luminance spectrums are well known in the art. Therefore, details of such methods are not described herein to avoid obscuring aspects of the present invention. 
     At step  730 , the chromaticity values over the luminance spectrums for a candidate pair are compared to the black body curve  200  of chromaticity diagram  190 . In one embodiment, a relationship of color temperature and chromaticity is built, and the relationship is compared to the black body radiation curve. Methods for performing statistical comparison for two set of data are well known in the art. Therefore, it would be apparent to those of ordinary skill in the art, upon reading the present disclosure, that numerous statistical analysis algorithms may be implemented herein. In one embodiment, the sum of the derivatives are computed and compared to other sums of other candidate pairs. 
     At step  740 , if it is determined that the chromaticity values of the candidate pair is significantly deviated from the black body curve  200 , then the candidate pair is rejected. 
     At step  750 , if it is determined that the chromaticity does not significantly deviate from the black body curve  200 , then the candidate pair remains in the pool. 
     At step  760 , a new candidate pair is selected from the remaining pool of candidate pairs, and steps  710 - 750  are repeated until all the candidate pairs have been processed. Thereafter, at step  770 , when all candidate pairs have been processed, the process returns, and the number of candidate pairs is further reduced. 
     FIG. 17 is a flow diagram  800  illustrating a process of selecting appropriate light source candidates such that a maximum luminance occurs approximately in the middle of a given color temperature range. 
     At step  810 , a pair of light source candidates are selected from the pool of candidate pairs that have passed the luminance threshold requirement and that closely follow the black body curve  200 . 
     At step  820  of FIG. 17, color temperature values of the selected candidate pair are plotted against luminance values of the selected candidate pair to provide a color temperature and luminance diagram. Exemplary color temperature and luminance diagrams are illustrated in FIGS. 9B and 9C. 
     At step  830 , it is determined whether a peak luminance point, or maximum luminance point, i.e. when both “blue” and “red” light source candidates are turned on at a maximum intensity, occurs approximately at the middle of the given color temperature range. In the exemplary color temperature and luminance diagram of FIG. 9B, peak luminance point  266  occurs at roughly 4700 K. In FIG. 9C, the peak luminance point  276  occurs at roughly 5050 K. 
     At step  840 , if it is determined that the peak luminance point does not occur approximately at the middle of the given color temperature range, then the candidate pair is rejected. For instance, in the example as illustrated in FIG. 9B, for a predetermined range of color temperature between 3,400 K to 8,250 K, the peak luminance point should occur at approximately 5825 K. The peak luminance point, however, occurs at approximately 4700 K for the example as illustrate in FIG.  9 B. Therefore, the example as illustrated in FIG. 9B does not satisfy this requirement, and will be rejected. 
     At step  850 , if it is determined that the maximum luminance occurs roughly in the middle of the predetermined color temperature range, then the candidate pair remains in the pool of potential candidates. Thereafter, the process returns. 
     At step  860 , a new candidate pair is selected from the remaining pool of candidate pairs, and steps  810 - 850  are repeated until all the candidate pairs have been processed. Thereafter, at step  870 , when all candidate pairs have been processed, the process returns, and the number of candidate pairs is further reduced. 
     According to one embodiment of the present invention, an offset value may be added to the luminance spectrum so as to compensate for the yellow shift caused by many of the layers, e.g., the acrylic in the light pipes, the ultra-violet cured extraction patterns, the DBEF and BEF films, the polarizers, the LCD layer and the color filters, within an LCD flat panel display system. The offset value, however, is largely determined by the experience and skill of the light tube designer, and by empirical experimentation. In addition, light tube designers may adjust the values of the percentage compositions of the R-phosphors, G-phosphors, and B-phosphors to produce a pair of CCFL tubes that have the ideal “look and feel.” 
     Rear Backlighting Embodiments 
     FIG. 18A illustrates a backlighting embodiment  910  of the present invention that positions an array of light sources  132   a - 132   b  and  136   a - 136   b  directly under the display layer  912  thereby obviating the need for any light pipes. A diffuser layer  914  is used to diffuse the light emitted from the light sources to promote light uniformity. If the light sources are placed near the diffuser layer  914  (e.g., less than one inch away), then hiding lines may be required. These hiding lines are typically etched on the diffuser layer and are more numerous near the body of the light sources. The relative intensities of the light sources (red and blue) can be controlled to perform color temperature balancing as described above in the numerous edge lit embodiments. A rear reflector layer  138  is also used in embodiment  910 . 
     The information display layer  912  (“display layer  912 ”) is generally a non-emissive modulated display layer. The layer  912  is non-emissive meaning that it does not generate any original light but rather modules the light of another light source (e.g., sources  132   a-b  and  136   a-b ). The display layer  912  modules light that layer  912  does not generate. Modulation is used to form images on layer  912  thereby conveying information. In one embodiment, display layer  912  is a liquid crystal display layer of the technology shown in FIG. 2A including the layers between layer  126  and layer  112 . Alternatively, the display layer  912  an electrophoretic display layer using ion migration for modulation. Layer  912  can also be a reflective display layer or a layer having a fixed modulated design printed or otherwise laid thereon. Layer  912  can also be an electromechanical display using shuttered pixels (“windows”) for modulation. Layer  912  can also be a ferroelectric display. 
     FIG. 18B illustrates another backlighting embodiment  922  similar to embodiment  910  except the rear reflector  920  contains scallops cut therein and each light source  132   a - 132   b  and  136   a - 136   b  is positioned with its own scallop to increase directed reflection. FIG. 18C illustrates another backlighting embodiment  930  that is similar to embodiment  922  except that within the reflector layer  936  a pair of light sources  132   a  and  136  can be positioned within a single scallop. 
     Conclusion 
     The preferred embodiment of the present invention, a color balancing system for a flat panel LCD unit applying variable brightness to multiple light sources of varying color temperature, is thus described. While the present invention has been described in particular embodiments, it should be appreciated that the present invention should not be construed as limited by such embodiments, but rather construed according to the below claims.