Patent Application
20070097358
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.
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.
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.
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:
The horizontal axis is time, starting at time T0 and continuing with T1, T2 etc., all the way out as shown on the graph to T601. These time spaces can be, for example, 1 millisecond apart. Thus, as shown at time T0, there is a power pulse in the red LED that turns on LED 13R. At time T0, there is also a longer power pulse to turn on green LED 13G and a power pulse to turn on blue LED 13B. Note that the power pulse for LED 13G is longer than the power pulses for LEDs 13R and 13B 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 T200, which, for example, can be 200 milliseconds after T0, the pulses for the green and blue are blanked and only the pulse for the red LED 13R is on. This is shown at point T200R while points T200G and T200B are blank. Thus, at time T200 the only light that comes on is the red light from LED 13R. Sensor 12-1 at time T200 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 T200 only the red light is on.
At time T201 all pulses continue in the normal fashion until time T400 where the red and blue pulses are blanked, as shown at points T400R and T400B. At time T400, the only LED receiving power is LED 13G and thus detector 12-1 measures the green color intensity at time T400 again without a filter.
At time T401, all three LEDs are available to receive power which continues until time T600, where as shown at T600R and T600G, both red and green LEDs power inputs are blanked leaving power only to blue LED 13B. Thus, detector 12-1 at time T600 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
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.
