Patent Publication Number: US-2007097358-A1

Title: System and method for obtaining multi-color optical intensity feedback

Description:
TECHNICAL FIELD  
      This invention relates to optical systems and more particularly to such systems where it is desired to determine multi-color optical intensity, and even more particularly to such systems in which the determined intensity levels for each color are used in a feedback system.  
     BACKGROUND OF THE INVENTION  
      Multi-color systems, such as are used for back-lighted displays, usually employ a tricolor system having red, green and blue light emitting diodes (LEDs) which can be mixed together to form a gamut of color. By proper stimulation, the LEDs can form white light as part of the gamut. Because the light intensity from each color LED is different, and because the intensity from each color can change differently over time it is often necessary to measure intensity during the operation of a display so as to be able to calibrate the resultant “mixed” color.  
      One method for determining light intensity from a multi-colored display is to place a photosensor in front of the display and measure the magnitude of the resultant output signal from the photosensor. In such a system, a filter can be placed between the display and the measuring photosensor so as to measure light intensity from only a specific LED. By selecting the filter properties, the light intensity from different colored LEDs can be measured. Such a system is cumbersome and not be readily adapted for use on a continuing basis for adjusting the LED driving signals in real time.  
     BRIEF SUMMARY OF THE INVENTION  
      In LED back-lighted display systems where the LED driving signals are pulse width modulated, brightness of the LED is determined by the duty cycle (length of the “on” pulse). In such systems, it is possible to, from time to time, skip an LED driving pulse without the human eye detecting the absence of (or change in) color during the “skipped” LED pulse. Using this approach, it is possible to measure the intensity of the light output from each LED of a multi-color LED display. Thus, in an embodiment having three colors, such as red, green and blue, it is possible to determine the light intensity from one of the LEDs (for example, the red LED) by skipping (blanking) the input driving pulses to the green and blue LEDs at a particular point in time. During that point in time the only light coming from the display would come from the unblanked LED which, in this case would be the red LED. It is then possible to measure the intensity of the red LED light using a photosensor without use of a filter. In similar manner, and at some later point in time, the light intensity from each of the other color LEDs can be measured using the same photosensor without using a filter. By spacing the blanking periods properly, the resulting change in color of the display will not be perceived by the human eye. 
    
    
     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 drawings, in which:  
       FIG. 1  shows one embodiment of a back-lighted device having colored LEDs grouped into pixels;  
       FIG. 2  shows one embodiment of a chart of timed power pulses and blanked pulses for light intensity measurement; and  
       FIG. 3  shows one embodiment of a process for determining brightness of an LED or group of LEDs. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 1  shows one embodiment of a back-lighted device  10  having therein a plurality of red, green, blue LEDs, such as LEDs  13 R,  13 G,  13 B, grouped into pixels to produce a color gamut depending upon the relative intensity of the individual LEDs. Associated with each grouping of LEDs is a sensor, such as sensor  12 - 1  through  12 -N,  11 - 1  through  11 -N. Each of the sensors can be, for example, a photodiode which measures the light intensity from the LEDs at that point. Note that while individual photosensors are shown for each group of LEDs a single photosensor could be utilized for the whole display if desired. Device  10  is controlled by processor  15  which creates the control for the light pulses in the manner to be discussed hereinafter.  
       FIG. 2  shows one embodiment  20  of a chart having individual lines and power pulses for the red, green, and blue LEDs and a line for common detector  12 - 1 . Note that with respect to  FIG. 2  only one grouping of red, green and blue LEDs are portrayed and the system discussed herein can be used simultaneously for all of the groupings for a display or can be used one at a time if desired.  
      The horizontal axis is time, starting at time T 0  and continuing with T 1 , T 2  etc., all the way out as shown on the graph to T 601 . These time spaces can be, for example, 1 millisecond apart. Thus, as shown at time T 0 , there is a power pulse in the red LED that turns on LED  13 R. At time T 0 , there is also a longer power pulse to turn on green LED  13 G and a power pulse to turn on blue LED  13 B. Note that the power pulse for LED  13 G is longer than the power pulses for LEDs  13 R and  13 B because green requires more brightness with respect to red and blue in order to make a color white by blending all three colors.  
      At time T=1, the pulses repeat and this repetition continues for a period of time with the various pulses going on or off so as to adjust the relative color desired at any one point in time.  
      At time T 200 , which, for example, can be 200 milliseconds after T 0 , the pulses for the green and blue are blanked and only the pulse for the red LED  13 R is on. This is shown at point T 200 R while points T 200 G and T 200 B are blank. Thus, at time T 200  the only light that comes on is the red light from LED  13 R. Sensor  12 - 1  at time T 200  is turned on and this sensor measures red light because it is the only light available at that time. Thus sensor  12 - 1  measures the light intensity without a filter since the system knows that at time T 200  only the red light is on.  
      At time T 201  all pulses continue in the normal fashion until time T 400  where the red and blue pulses are blanked, as shown at points T 400 R and T 400 B. At time T 400 , the only LED receiving power is LED  13 G and thus detector  12 - 1  measures the green color intensity at time T 400  again without a filter.  
      At time T 401 , all three LEDs are available to receive power which continues until time T 600 , where as shown at T 600 R and T 600 G, both red and green LEDs power inputs are blanked leaving power only to blue LED  13 B. Thus, detector  12 - 1  at time T 600  can only measure blue light intensity. Note that in the chart only three colors have been illustrated, but any number of different colors, can be measured in this fashion. Also note that the time spaces between the blinked pulses is a fixed time and the time pulses there between are evenly spaced. This need not be the case and, in fact, different spacing can be utilized from time to time, provided that they are spaced far enough apart so that the human eye will not detect the missing pulses. This minimum time space is approximately 10 milliseconds.  
      Also note that while only one cycle is shown in  FIG. 2  the pattern continues for as long as desired. Thus, the blanking pattern can continue indefinitely in this manner or can be run on a schedule, say each hour or each half day, such that the measurement is then designed to determine deterioration over a longer period of time as opposed to measuring changes on a relatively short basis.  
       FIG. 3  shows one embodiment  30  of a process for determining brightness of an LED or group of LEDs. This process would be run, for example, by processor  15 ,  FIG. 1  and would begin with process  301  determining that it is time to measure the brightness of one or more of the LEDs. If it is time, (for example, T 200  in  FIG. 2 ) process  302  selects the next color to be monitored. Since, in our example, at time T 200  red is being monitored, process  304  blanks the green and blue pulses. By the same token, as discussed above, if this were time T 400  then process  305  would be activated and the red and blue pulses would be blanked whereas if this were time T 600 , process  303  would be activated and the red and green pulses would be blanked.  
      Process  306  then measures the brightness for the non-blanked pulse at the measuring time. Process  307 , if desired, adjusts the proper LEDs intensity based on what the measurement level is and also, if desired, process  308  stores the new intensity reading. If desired, this transmitted intensity level is to a remote location for further processing.  
      Note that while multi-colored lights have been illustrated, the concepts taught herein can be used with multiple light sources having the same color, but placed in different locations. Thus, it would be possible to measure (and control) the brightness of one light segment while the other light segments are momentarily off.  
      Also note that while pulse width modulation is shown, the concepts discussed herein can be used with DC driven light. In such a situation, the DC would be broken into small time frames. When it is desired to measure a particular channel of light the other (different colors or different locations) light channels are blanked as discussed above.  
      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.