Abstract:
A LCD display comprises an LCD panel coupled to a backlight system. The backlight system comprises a direct view backlight comprising a first plurality of LEDs configured to emit white illumination that, at least in part, provides the required luminosity for the LCD panel. The backlight system further comprises an edge view backlight comprising a second plurality of LEDs configured to emit colored illumination that, at least in part, provides the required chromaticity for the LCD panel.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention generally relates to the field of LCD displays, and more particularly to a LED backlight system for LCD displays.  
       BACKGROUND OF THE INVENTION  
       [0002]     Liquid crystal display (LCD) monitors are replacing traditional cathode ray tube (CRT) monitors in many applications because of their light weight and superior performance. In a typical LCD monitor a backlight is placed behind an LCD panel to illuminate the LCD for viewing by a user. In an exemplary embodiment, a cold cathode fluorescent tube can be used to provide the illumination, although other sources of illumination can also be used.  
         [0003]     An exemplary pixel  100  of an LCD display is shown in  FIG. 1 . LCD pixel  100  includes a backlight section  101  and a LCD panel section  105 . The backlight section  101  comprises, in one exemplary embodiment, a backlight  102 , such as a fluorescent tube set into a backlight cavity  103 , and a diffuser  104  that is placed in front of the backlight cavity  103 .  
         [0004]     The LCD panel section  105  of pixel  100  comprises, in one exemplary embodiment, an addressing structure  108  including thin film transistors (a pixel typically comprises three transistors, which are not shown in  FIG. 1 ) processed onto glass plate  106 , the liquid crystal material  110 , a common electrode  112  adjacent to the liquid crystal material  110  and coated onto filters  116 - 120 , and a red filter  116 , green filter  118  and blue filter  120  processed onto glass plate  122 . Front polarizer  124  on the viewer side of color filter plate  122  and a rear polarizer  114  on the outside of the TFT glass plate  106  capture the inner panel structure. In a typical LCD panel, there is an active matrix array of many thousands of pixel structures.  
         [0005]     In operation, the perceived color and luminosity of a pixel in its white state is dependent upon the light energy emitted by backlight  102  over the visible wavelengths and the transmission characteristics of the color filter plate. The color can be tuned to a desired chromaticity target by selection of the backlight to match the characteristics of the LCD color filters. As discussed previously, backlight  102  can be comprised of a fluorescent tube. However, light emitting diodes (LEDs) can also be used as will be discussed in detail below. The light from the backlight  102  enters the backlight cavity  103  and exits through the diffuser  104  over the opening. Some light rays will bounce off the walls of backlight cavity  103  prior to passing through the diffuser  104  which spreads the light rays to reduce color and luminance non-uniformities on the display panel produced by the backlights whose radiation patterns are typically non-lambertian. Light exiting diffuser  104  is polarized by rear polarizer  114  and enters liquid crystal  110 . The thin film transistors contained in addressing structure  108  are used to govern the amount of charge between different regions of the addressing structure  108  and the electrode  112 . As the amount of charge changes, the liquid crystal  110  will act to change the polarization of the light passing through the liquid crystal  110 . The light is spectrally filtered by color filters  116 - 120  and is subsequently repolarized by front polarizer  124 . The amount of light exiting the front polarizer  124  depends on the degree of polarization of the light leaving the liquid crystal with respect to the front polarizer  124 . Since the addressing structure  108  can control the charge under each color filter  116 - 120  independently, the red, blue, and green colors each contain a different amount of light, which results in the pixel being perceived to be a particular color.  
         [0006]     In the backlight system illustrated in  FIG. 1 , the backlight is placed directly behind the liquid crystal  110  in what is known as a direct view system. The backlight can also be placed on the edge of a liquid crystal display, in an edge lit configuration. In an edge lit configuration, the light from the backlight propagates through a light guide placed behind the liquid crystal display.  
         [0007]     One drawback in the use of fluorescent tubes for backlights is that a relatively large amount of energy is required to operate them. Also, fluorescent tubes can be fragile, a drawback when the LCD displays are used in systems subject to vibrations and shock, such as aeronautical applications. One alternative to the use of fluorescent tubes is the use of light emitting diodes (LEDs). One type of LED suited for use in backlights is the white LED, which emits white light as opposed to the red or other color of typical well-known LEDs. Typically, white LEDs are LEDs that emit a blue light but have a special coating applied such that the LED emits a white light.  
         [0008]     White LEDs can be used in both edge lit and direct view applications. A concern in either application is providing a backlight that has both the desired luminosity and chromaticity. Luminosity is a measure of the relative brightness of a LCD display panel. Chromaticity is a measure of the quality of a color as determined by its “purity” and dominant wavelength. Purity in this context is roughly equivalent to the term “saturation” as used in general color theory. The dominant wavelength is roughly equivalent to the property “hue” as used in general color theory. Note that an LCD display is typically only able to display a subset, or color gamut, of a given range of visible color, or color space. The color gamut for a display can be adjusted with the addition of colored light to the backlight in order to achieve a desired chromaticity.  
         [0009]     When white LEDs are used in edge lit applications, a decrease in brightness can be experienced because the light must propagate throughout a light guide starting from the edge of the light guide. In many applications, the amount of light provided by edge lit applications with white LEDs may not meet required luminosity requirements for a given application. To alleviate this, either the LCD panel must be made more transmissive to light or more intense edge lighting needs to be used. Either alternative increases cost and design complexity.  
         [0010]     When LEDs are used in direct view applications, since each LED is a compact source of light, without proper diffusion, the lighting can be non-uniform. This can especially be a problem when white LEDs are mixed with color LEDs in order to achieve a certain chromaticity requirement. A less transmissive diffuser that has greater hiding power due to better diffusion characteristics could be used to help decrease the non-uniformity of lighting. However, less transmissive diffusers require more LED power and possibly more LEDs to achieve proper luminosity.  
         [0011]     Accordingly, it is desirable to provide a LED backlight system for LCD displays. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended, claims, taken in conjunction with the accompanying drawings and this background of the invention.  
       BRIEF SUMMARY OF THE INVENTION  
       [0012]     In an exemplary embodiment a backlight system for an LCD panel comprises a direct view backlight comprising a first plurality of LEDs configured to emit direct view illumination that, at least in part, provides the required luminosity for the LCD panel. The backlight system further comprises an edge lit backlight comprising a second plurality of LEDs configured to emit edge lit illumination that, at least in part, provides the required chromaticity for the LCD panel.  
         [0013]     In another exemplary embodiment, a method for illuminating an LCD display is disclosed. In a first step, direct view illumination is produced to achieve, at least in part, a luminosity requirement. In a second step, edge lit illumination is produced to achieve, at least in part, a chromaticity requirement. In one exemplary embodiment, the direct view illumination is produced from an array of white LEDs in a direct view backlight and the edge lit illumination is produced from colored LEDs mounted in an edge lit backlight.  
         [0014]     In yet another exemplary embodiment, a LCD display comprises an LCD panel coupled to a backlight system. The backlight system comprises a direct view backlight comprising a first plurality of LEDs configured to emit white illumination that, at least in part, provides the required luminosity for the LCD panel. The backlight system further comprises an edge lit backlight comprising a second plurality of LEDs configured to emit colored illumination that, at least in part, provides the required chromaticity for the LCD panel. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and:  
         [0016]      FIG. 1  is a cross sectional view of a prior art backlight system;  
         [0017]      FIG. 2  is a view of an LCD display in accordance with an exemplary embodiment of the present invention; and  
         [0018]      FIG. 3  is a cross sectional view of an LED backlight using a direct view LED system and an edge light color compensating system in accordance with an exemplary embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention.  
         [0020]      FIG. 2  is a view of an exemplary LCD display  200  comprising an LCD panel  202  contained within a housing  204 . The LCD display  200 , in one exemplary embodiment, comprises both a direct view backlight and an edge lit backlight. LCD display  200  can be used in any display application, including avionic displays.  
         [0021]      FIG. 3  is a cross sectional view of LCD display  200  taken along line A-A of  FIG. 2 . The cross sectional view of  FIG. 2  illustrates a backlight section  302  coupled to the LCD panel  202 . The backlight section  302  comprises a direct view backlight  304  and an edge lit backlight  306 , both of which are coupled to the housing  204 . A light guide  310  is optically coupled to the edge lit backlight  306 . Backlight section  302  further comprises a diffuser  312  coupled to the housing, with an area between the light guide and the diffuser  312  forming a light mixing region  311 ;  
         [0022]     The direct view backlight  304  produces illumination originating from directly behind the LCD panel  202 . In one exemplary embodiment, the direct view backlight comprises an array of LEDs  303 . In one exemplary embodiment, white LEDs can be used. White LEDs  303 , as discussed before, emit white light, which is a combination of light of differing wavelengths. The number of white LEDs  303  used in the direct view backlight can be based on the luminosity requirement for the intended use of the LCD display  200 . As an alternative, each white LED can be replaced by a red LED, a green LED and a blue LED whose outputs are merged together using an appropriate light merger to provide the equivalence of a white LED.  
         [0023]     Edge lit backlight  306  provides illumination originating along the edge of the light guide  310 . The light propagates throughout light guide  310 , providing illumination from behind the LCD panel  202 . The edge lit backlight  306 , in one exemplary embodiment, comprises a plurality of color LEDs  308  mounted in a cavity  307 . Cavity  307  includes a reflective surface  309  optically coupling the color LEDs  308  to the light guide  310 . The number and color of the color LEDs  308  can be selected based on the chromaticity requirements of the end user. Reflective surface  309 , in one embodiment, can be any metallic reflective surface, although any surface capable of reflecting light from a point source of light to a light guide  310  can be used. In one exemplary embodiment, the color LEDs  308  are mounted such that they provide illumination to the light guide  310  without the use of a reflective surface  309 .  
         [0024]     In operation, LEDs  303  of the direct view backlight  304  provide illumination originating directly behind the LCD panel  202 . In one exemplary embodiment, all of the LEDs in the direct view backlight  304  are white LEDs  303  and the number and distribution of the white LEDs is based upon the luminosity requirements of the LCD display  200 . The direct view illumination  320  produced by the direct view backlight impinges on the light guide  310  at approximately a ninety degree angle and passes through the light guide  310 . Edge lit illumination  322  can be produced using color LEDs, such as red LEDs, in order to assist in providing the correct chromaticity. The edge lit illumination  322  produced by the color LED  308  from the edge lit backlight  306  can reflect from reflective surface  309  into the light guide  310 . The edge lit illumination  322  can then propagate through the light guide  310 , exiting at one of a plurality of apertures or other light extraction features (not pictured) found at locations on or within the light guide  310 .  
         [0025]     The direct view illumination  320  and the edge lit illumination  322  mix with each other in mixer region  311 . Mixer region  311  helps to ensure the uniformity of a combined illumination  324  comprising the direct view illumination  320  and the edge lit illumination  322 , which in turn helps in achieving proper luminosity and chromaticity requirements. The mixed illumination  324  passes through a diffuser, which spreads the mixed illumination  324  for an even backlighting of the LCD panel  202 . While the positions and orientations of illumination on  324  and  322  are depicted in  FIG. 3  in a regular pattern, it is recognized that in general, such illumination will be diffuse in nature, that the apertures or other extraction features may be distributed throughout the light guide, and the light reflected back into the cavity by the diffuser will be further mixed and will contribute to the output of the backlight. It is further recognized that light extraction features on or in the light guide may act to partially diffuse direct view illumination passing through it.  
         [0026]     In the exemplary embodiment of  FIG. 3 , different amounts of luminosity can be achieved via varying the amount and arrangement of white LEDs in the direct view backlight. Different chromaticity requirements can be met by varying the intensity or color of the LEDs  308  of the edge lit backlight. Thus, the use of both a direct view backlight and an edge lit backlight can provide an LCD display that can provide an image with a required luminosity and achieve a desired chromaticity value.  
         [0027]     While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.