Abstract:
Apparatus and method for backlighting an electronic display with LEDs to control luminosity, radiometric power, and color levels by means of feedback control through a microprocessor, thereby maintaining white backlight at substantially constant levels, which can be chosen by an operator.

Description:
TECHNICAL FIELD 
     This invention relates in general to a backlight system for a liquid crystal (LCD) or other electronic display and, in particular, to controlling the color and lumen level of a red-green-blue (RGB) light-emitting diode (LED) backlight and the sensor(s) that control(s) such a backlight. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     Backlighting with white light generated by RGB LEDs is known to those skilled in the art. However, the characteristics of the LEDs vary with temperature, current, and aging. These characteristics also vary from one LED in a batch to another. Thus there is need for a feedback control to maintain within set limits the color and lumen level of such a backlighting system. For the feedback control to work satisfactorily, sensors must be placed properly to provide the necessary optical feedback. 
     The present invention provides apparatus and method for backlighting an electronic display with LEDs to control luminosity, radiometric power, and tristimulus levels by means of feedback control through a microprocessor, thereby maintaining the white backlight at substantially constant levels, which can be chosen by an operator. 
     In one embodiment of the invention, apparatus for backlighting an electronic display with white light comprises: a plurality of light-emitting diodes (LEDs), each of the LEDs effective for emitting light of a single color; at least one light source comprised of at least three of the LEDs arranged in a combination that produces white light; a light guide effective for illuminating the display with the white light; and circuitry effective for maintaining the white light at a substantially constant level of color and luminosity by controlling the at least one light source. This embodiment of the invention utilizes a method for backlighting an electronic display with white light comprising the steps of: driving a plurality of LEDs, each of the LEDs emitting light of a single color; combining light emitted from at least three of the LEDs to form white light; illuminating the display with the white light; and controlling the color and brightness of the white light by means of feedback circuitry. 
     In another embodiment of the invention there is provided apparatus for backlighting an electronic display with white light comprising means for driving a plurality of LEDs, each of the LEDs emitting light of a single color; means for combining light emitted from at least three of the LEDs to form white light; means for illuminating the display with the white light; means for controlling the color and brightness of the white light by feedback circuitry; and the means for controlling being subject to an operator&#39;s direction. The present invention addresses one or more of these concerns. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings, like reference numerals designate corresponding elements or parts throughout, wherein: 
     FIG. 1 illustrates the apparatus of the present invention for backlighting an LCD or other electronic display by means of RGB LEDs controlled by a microprocessor; 
     FIG. 2 illustrates the placement of photosensors in a light guide; and 
     FIG. 3 illustrates placement of photosensors in a light guide when only a single side light source is used. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, there is illustrated an apparatus for controlling white light for substantially uniform backlighting of an LCD  100  or similar display, utilizing a power supply  110 , which obtains power from an alternating current source  115 . Power supply  110  further comprises a plurality of LED drivers  120 ,  130 ,  140 , one each for red, green, and blue drivers, respectively. Each of LED drivers  120 ,  130 ,  140  is connected to a plurality of LEDs of the same color, connected in suitable series and parallel combinations, that comprise each of a plurality of light sources  150 ,  160 . 
     Light sources  150 ,  160  are each embedded in a heat sink.  190 ,  200  to avoid overheating of LEDs and maximize uniformity of color. Light sources  150 ,  160  are in turn mounted on the edges of a light guide  170 . Uniformity of color is maintained by forming a unit white cell on each of light sources  150 ,  160  in a suitable combination of LEDs, such as R-G-B, R-G-B-G, G-R-B, etc., that maximize uniformity of color. Optical arrangements couple the light from the LEDs of light sources  150 ,  160  to light guide  170 . 
     LED drivers  120 ,  130 ,  140  supply current, suitably converted within power supply  110 , to the LEDs in light sources  150 ,  160 . A microprocessor  180 , programmed with the functions necessary to control color and lumen level in light guide  170 , provides signals that control the currents from LED drivers  120 ,  130 ,  140 . A plurality of photo sensors  210  send feedback via a circuit  230  to permit microprocessor  180  to vary the signals sent to LED drivers  120 ,  130 ,  140 . These signals may take the form of amplitude modulation, PWM signals, or other suitable values. A controller  240  feeds to microprocessor  180  signals that determine color and brightness levels of an LCD or other electronic display (not shown) backlit by light guide  170 . 
     Feedback control is required to maintain color and brightness in light guide  170 . Without such control, variations in the characteristics of the individual LEDs in light sources  150 ,  160  will cause the color and brightness in light guide  170  to vary within unacceptable limits. The feedback control required depends on taking appropriate samples by sensing. 
     In a first embodiment of the present invention, temperatures of light sources  150 ,  160  are sensed within heat sinks  190 ,  200 . Microprocessor  180  is programmed to compensate for temperature-related variations in color and brightness in light guide  170  caused by variations in the characteristics of the LEDs in light sources  150 ,  160 . This compensation is effected by adjusting the currents sent by LED drivers  120 ,  130 ,  140  to the LEDs. This first embodiment has no mechanism to overcome aging effects in the individual LEDs. 
     In a second embodiment of the present invention, photo diodes  210  measure at least one of either the lumen level and the radiometric power level in light guide  170  by unfiltered photo diodes, photo diodes with Y filters, or other suitable means. Microprocessor  180  is programmed to compensate for variations in color and brightness in light guide  170 , caused by variations in the characteristics of the LEDs in light sources  150 ,  160 , by adjusting the currents from LED drivers  120 ,  130 ,  140  to the desired levels of lumen and/or radiometric power. This second embodiment cannot overcome variations in color caused by variations in temperature. 
     In a third embodiment, both the temperatures in heat sinks  190 ,  200  and at least one of either the lumen level or the radiometric power level in light guide  170  are sensed as described in the first and second embodiments and fed to microprocessor  180 . By programming microprocessor  180  to adjust the currents from LED drivers  120 ,  130 ,  140  in response to both sets of feedback stimuli, this embodiment of the present invention compensates for both aging and temperature variations in the LEDs in light sources  150 ,  160 . 
     In a fourth embodiment, photo diodes  210  are fitted with appropriate filters to sense the tristimulus values of the white light in light guide  170 . These tristimulus values (or another measure of color), fed back to a suitably programmed microcomputer  180 , adjust the currents of LED drivers  120 ,  130 ,  140  to match the tristimulus values for the light in light guide  170  to match reference values. 
     In a fifth embodiment, temperatures in heat sinks  190 ,  200  are measured to add temperature compensation to the adjusted tristimulus values referred to in the fourth embodiment. 
     In all of the above embodiments, the color and lumen level of the white light from light guide  170  can be manually set by an operator or automatically by the control circuitry. 
     To insure uniformity of color, the sensors must be placed appropriately to provide the necessary feedback components for uniform color control. Referring again to FIG. 1, each of heat sinks  190 ,  200  has three temperature sensors  250 . The placement of temperature sensors  250  on heat sinks  190 ,  200  depends on the latter&#39;s temperature profile. Feedback control is based on a weighted average of the outputs of temperature sensors  250 . 
     A minimum of one pair of photo diodes  210  is required by the present invention, but their placement can vary. Referring again to FIG. 1, a first embodiment places each of a pair of photo diodes  210  in the middle of each of two sides of light guide  170 . 
     Referring to FIG. 2, a second embodiment places photo diodes  210  on the underside of light guide  170 , between its body and the reflector below. The light in light guide  170  is sensed by at least one set of photo diodes  210 , and the average from all of them is used by microprocessor  180 . FIG. 2 shows three sets of photo sensors  260 ,  270 , and  280 . They are placed in a row substantially in the middle of a planar light guide  170 , with photo sensors  270  in the middle of the row and photo sensors  260 ,  280  each placed approximately one-quarter of the distance from the side. 
     Referring to FIG.  3 ,in this embodiment only a single source can illuminate light guide  170 , i.e., light source  150  is embedded in heat sink  190  for single-sided illumination of light guide  170 , and there is no light source  160  as in FIG.  1 . When light source  150  is alone, photo sensors  260 ,  270 ,  280  may be placed at the opposite edge of light guide  170  from light source  150 . They are placed in a row with photo sensors  270  in the middle of the row and photo sensors  260 ,  280  each placed approximately one-quarter of the distance from the side. 
     Many other positions and numbers of photo diodes  210  and temperature sensors  250  are possible within the present invention. 
     Functional Description 
     Having described preferred embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims. Other aspects and features of the present invention can be obtained from a study of the drawings, the disclosure, and the appending claims.