Patent Publication Number: US-2007103934-A1

Title: System and method for constructing a backlighted display using dynamically optimized light source

Description:
TECHNICAL FIELD  
      This invention relates to backlighted displays and more particularly to light sources that are dynamically adjustable to achieve a desired output brightness.  
     BACKGROUND OF THE INVENTION  
      It has become common to use backlighted displays for a variety of purposes. One such usage is in cellular phones, PDAs, cameras and other handheld devices. In many of these applications it is desired to have white back light and thus such displays typically use white (phosphor converted) light emitting diodes (LEDs) or cold crystal fluorescent lamps (CCFL) in conjunction with a liquid crystal display (LCD) to form the backlighted device.  
      Because a wider color gamut is often required than is available with CCFL or white LEDs, backlighted devices are beginning to use red, green and blue (RGB) LEDs. The light output of these red, green and blue LEDs are mixed to produce the required color. However, because their light output (primarily the green LED) is not as efficient in lumens/watt as CCFLs or white diodes more RGB diodes are required. In some situations, red diodes are used in combination with other green and blue light sources, such as CCFLs, to produce white light.  
     BRIEF SUMMARY OF THE INVENTION  
      A combination of light source types is used to form a dynamically adjusted backlighted display with the particular combination dependant upon the desired light output at a given point in time. In one embodiment, RGB diodes are used for low brightness situations and as the brightness requirement increases white light sources are added. In another embodiment, different light source types are used to produce the different color components and optical feedback is used to control the power levels of the various diode types.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which:  
       FIG. 1  shows one embodiment of a backlighted display;  
       FIG. 2  shows one embodiment of a backlighted display using feedback for color control; and  
       FIGS. 3A, 3B , and  3 C are charts showing the composite light from different light types. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 1  shows one embodiment of backlighted display  10  having light guide  13  and having two independently controllable light sources  111  and  12 . In this embodiment, light source  11  ( 11 - 1 ,  11 - 2 ) is one or more RGB LED arrays and light source  12  ( 12 - 1 ,  12 - 2 ) is a CCFL or white LED source. Note that the white light source can be a single diode array or could be a plurality of red, green, blue (or other colors) CCFLs or phosphor covered LEDs biased to achieve white light.  
      Optical sensor  14  can determine both the color of the light and the intensity of the light. In this embodiment the intensity is important such that backlight controller  17  will use readings from sensor  14  to increase RGB LED driver  15  until a point is reached where further power to RGB LED driver  15  will not increase the lumen output. As shown in  FIG. 3A , line  30  represents the increase in lumen output of RGB LED until point  301 . When point  301  is reached, the lumen output goes essentially flat.  
      At that point backlight controller  17  ( FIG. 1 ) begins to increase the lumen output of the white light source under control of driver  16 . As shown in  FIG. 3B , point  302  is the point at which the backlight controller begins to have CCFL (or white light) driver  16  turn on. Line  31  shows the white light increase from elements  12 - 1 ,  12 - 2 .  FIG. 3C  shows the composite from  FIGS. 3A and 3B  where the light output, which is measured by optical sensor  14 , is shown as line  32 .  
      In one embodiment, the optical sensor can be implemented by letting the RGB LEDs go on open loop when the white LED is turned on, since the addition of white will interfere with the RGB LED backlight. The sensor could then be turned off. Color point measurement is performed only at the beginning. For example, the system would turn on RGB LEDs and bring the backlight to the desired color level. Then the RGB LEDs would go on open loop (turned off) while the while LEDSs remain on. Alternatively, the system could selectively blank the white LEDs for a brief moment. During that brief moment, any intensity correction can be made. For example, the RGB LEDs are turned on and the backlight is brought to the desired color point. Then put the RFB LEDS on open loop and turn on the white LEDs. Then turn off the white LEDs and put the RGB LEDs back on closed LEDs loop for a very brief period. Then put RGB LEDs back on open loop and turn on white LEDs. Another system would be to assume that the sensor has a standard CIE output or a sensor that can be calibrated to the CIE standard (human eye response). In this case, RGB+white LEDs can be turned on at the same time and the feedback system can maintain the color point and brightness in a closed loop.  
       FIG. 2  shows one embodiment of device  20  where the two different light sources are RED LED  21 - 1 ,  21 - 2  and CCFL or phosphor converted LED (or CCFL)  22 - 1 ,  22 - 2 . If phosphor converted LEDs are used then the output would be biased towards blue and green which would then match with the red from light source  21 - 1 ,  21 - 2  to provide essentially white light through light guide  13  of backlighted device  20 . In this embodiment, optical sensor  14  detects the relative color and intensity and acts as an input to backlight controller  17  which in turn drives RED LED driver  24  to provide LIGHT in the red spectrum and CCFL or phosphor converted LED driver  25  to provide the remainder of the color balance. In this way the output through light guide  13  comprises light from two different source types, namely  21 - 1 ,  21 - 2 ,  22 - 1 , and  22 - 2 .  
      Using this technique, device  20  provides high efficiency in terms of lumens per watt for a wide color gamut while also compensating for color drift and degradation over time.  
      Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.