Abstract:
A display system includes a light modulator divided into an array of individually controllable pixels and an input-driven illumination device. The illumination device is adapted to receive a variable input and is configured to direct light of variable intensity onto the modulator, depending on the input. The display system further includes a calibrating arrangement for establishing the input to the illumination device to produce a desired intensity level of light. The calibrating arrangement includes a light sensing mechanism, which senses the light from the illumination device while the illumination device is driven by an initial input. The calibration arrangement is configured to determine a comparison between the sensed light and a value representative of the desired-intensity level. The calibration arrangement further includes a control arrangement responsive to the comparison for varying the input so as to provide light of the desired intensity level.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to methods and arrangements for calibrating illumination assemblies to obtain desired white-point, color balance and/or intensity. More specifically, the invention relates to using electronic storage devices and/or photodetectors and electronic circuitry to vary the current supplied to illumination devices such as light-emitting diodes, thus providing a calibrated light source for display applications. 
     In micro-display applications utilizing tri-color RGB (red, green and blue) light-emitting diode (LED) assemblies to illuminate a display panel, LED part-to-part illumination variation results in inconsistent brightness, white-point and color balance. Every LED&#39;s illumination output as a function of current is different, and each LED&#39;s illumination response to current across its entire current-controlled operating range may be non-linear. Manufacturing LEDs within tighter tolerances and more closely matching the three LED colors in a single assembly, thereby providing a more stable white-point and/or color balance, would be unnecessarily expensive, and would nevertheless provide unsatisfactory results. 
     Referring initially to FIG. 1, a prior art display system  100  providing a partial solution to the above-described problem will be described. Display system  100  includes a light modulating display  102 , an illumination device  104 , which provides the light source for display  102 , and an adjustable current source  106  electrically connected to illumination device  104 . Adjustable current source  106  is manually adjusted during manufacturing in order to cause illumination device  104  to provide calibrated light. The adjustment takes place by comparing the illumination output of illumination device  104  to a reference intensity and adjusting current source  106  until the illumination output of illumination device  104  matches the reference intensity. If illumination device  104  contains more than one light source, the process is repeated for each light source. 
     Display system  100  further includes a controller  108  and a display information input  110 . During operation of display system  100 , controller  108  receives display information via input  110  and determines the current to be supplied to illumination device  104 . The setting made during manufacturing to adjustable current source  106  causes the current to vary proportionally to the setting, thereby providing partially calibrated light. Because the adjustment made to adjustable current source  106  during manufacturing calibrates the illumination output of illumination device  104  for only a single intensity, this system does not correct the non-linear illumination response to current of illumination device  104  across the device&#39;s entire current-controlled operating range. 
     Display system  100  includes the additional limitation that adjustable current source  106  must be manually set during manufacturing. Having to manually calibrate the current source increases the cost of producing such a device. FIG. 2 illustrates a display system that overcomes this particular limitation. 
     Referring now to FIG. 2, a second prior art display system  120  will be described using like reference numbers for like components. Display system  120  includes a voltage source  122  and an adjustable resistor  124 . Adjustable resistor  124  may be a laser trim resistor that is capable of being adjusted during manufacturing using an automated process to provide the desired intensity for a specific voltage. While this method overcomes one limitation of display system  100  by allowing the calibration to be accomplished by automated means during manufacturing, display system  120  similarly fails to correct the non-linear illumination response to current of illumination device  104  across its entire current-controlled operating range. Further, neither display system  100  nor display system  120  is capable of correcting illumination device variations that occur after manufacturing, such as illumination device aging. 
     The present invention discloses arrangements and methods for calibrating illumination devices to reduce both pre- and post-manufacturing variations, including non-linear illumination output as a function of current across the current-controlled operating range and illumination device aging. 
     SUMMARY OF THE INVENTION 
     As will be described in more detail hereinafter, a display system including an arrangement for calibrating an input-driven illumination device is disclosed. The display system includes a spatial light modulator divided into an array of individually controllable pixels and an input-driven illumination device which is adapted to receive a variable input and which is configured to direct light of variable intensity onto the modulator, depending on the input. The display system further includes an arrangement adapted for connection with the illumination device for providing to the illumination device a specific input for a desired intensity level of the light, the specific input being provided from calibration information particular to the illumination device. The arrangement further includes a memory device for storing the calibration information. 
     A method of operating a display system as described above includes determining calibration information for an input driven illumination device which is adapted to receive a variable input and which is configured to direct light of variable intensity onto a light modulator, depending on the input. The method further includes storing the calibration information in a memory device and establishing a specific input for a desired intensity level of the light from the calibration information. The method further includes providing the specific input to the illumination device, and directing the light of the desired intensity level onto the light modulator. 
     As will be described in more detail hereinafter, an illumination assembly, including calibration information is also disclosed. The illumination assembly includes an input-driven illumination device which is adapted to receive a variable input and which is configured to produce light of variable intensity depending on the input. The illumination assembly further includes an arrangement including a memory device for storing calibration information and generating from the information a specific input for causing the illumination device to produce light of a particular intensity. The arrangement is adapted to be connected with the illumination device such that the latter receives the specific input. 
     In another embodiment of a display system, the display system includes a light modulator and an input-driven illumination device which has been pre-calibrated to provide light of a given intensity in response to a particular input and which is configured to direct the light onto the modulator. The display system further includes an electronic storage arrangement for storing a value which corresponds to the particular input, and an arrangement responsive to the value in the electronic storage means for generating the particular input and using it to drive the illumination device in a way which provides light of the given intensity. 
     A method of operating a display system as described above includes determining a particular value for controlling the input to an input-driven illumination device and electronically storing the particular value. The method further includes driving the illumination device in response to the particular value in a way which produces light of a desired intensity level, and directing the light of the desired intensity level onto a light modulator. 
     In a preferred embodiment, the display system includes a light modulator divided into an array of individually controllable pixels and an input-driven illumination device which is adapted to receive a variable input and which is configured to direct light of variable intensity onto the modulator, depending on the input. The display system further includes a calibrating arrangement for establishing the input for a desired intensity level of the light. The arrangement includes a light sensing mechanism, which senses the light from the illumination device while the illumination device is driven by an initial input. The calibration arrangement is configured to determine a comparison between the sensed light and a value representative of the desired intensity level. The calibration arrangement further includes a control arrangement responsive to the comparison for varying the input so as to provide light of the desired intensity level. The light sensing mechanism may form part of the light modulator. 
     The input-driven illumination device in either of the aforementioned display systems or the aforementioned illumination assembly may contain one, and only one, light source. Alternatively, the illumination device may include a plurality of light sources, wherein the calibration arrangement is designed to establish the input for a desired intensity level for each light source, so as to produce combined light of a desired color. The particular intensity of light produced by each light source may be different. The desired color may be white. The illumination device may consist of red, green and blue light-emitting diodes. 
     In the aforementioned display system, the sensing mechanism may be a photodetector. The sensing mechanism may be configured to sense only light within the visible spectrum. The sensing mechanism may be configured to have photopic spectral response substantially similar to the human eye. 
     A method of operating the immediately aforementioned display system includes providing an input-driven illumination device which is adapted to receive a variable input and which is configured to direct light of variable intensity onto a light modulator depending on the input. The method further includes sensing the light from the illumination device while the illumination device is driven by an initial input and comparing the sensed light to a value representative of the desired intensity. The method further includes establishing the input for a desired intensity level of the light in response to the comparison and directing the light of the desired intensity level onto the light modulator. 
     In another embodiment similar to the immediately preceding embodiment of a display system, the spectral response of the photodetector may vary from photodetector to photodetector, and the value representative of the desired intensity level is pre-calibrated to vary proportionally with the photodetector spectral response variation. Also, the sensing mechanism may include a plurality of photodetectors, each configured to sense light of a specific range of wavelengths and wherein each range of wavelengths is different. 
     In another embodiment, a color display includes a light modulator and a plurality of different colored lights, each of which are pre-calibrated to provide light of a given intensity in response to an input of a particular value. The lights are configured to direct the light onto the modulator. This embodiment includes an improvement that includes an electronic storage arrangement for storing the particular value and a control arrangement responsive to the particular value in the electronic storage arrangement for driving the light sources in a way which provides light of the given intensity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings. 
     FIG. 1 is a diagrammatic illustration of a first prior art display system. 
     FIG. 2 is a diagrammatic illustration of an alternative prior art display system. 
     FIG. 3 is a diagrammatic illustration of a first embodiment of a display system designed in accordance with the present invention. 
     FIG. 4 is a diagrammatic illustration of a calibration arrangement for calibrating a display system designed in accordance with the present invention. 
     FIG. 5 is a diagrammatic illustration of a second embodiment of a display system designed in accordance with the present invention. 
     FIG. 6 is a diagrammatic illustration of a third embodiment of a display system designed in accordance with the present invention. 
     FIG. 6 a  is a flow diagram illustrating the various steps of a method of operating a display system in accordance with the invention. 
     FIG. 7 is a diagrammatic illustration of a fourth embodiment of a display system designed in accordance with the present invention. 
     FIG. 8 is a diagrammatic illustration of a fifth embodiment of a display system designed in accordance with the present invention. 
     FIG. 9 is a diagrammatic illustration of a sixth embodiment of a display system designed in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An invention is herein described for providing methods and arrangements for calibrating the illumination output of illumination devices used, for instance, in display applications. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, in view of this description, it will be obvious to one skilled in the art that the present invention may be embodied in a wide variety of specific configurations. In order not to unnecessarily obscure the present invention, known manufacturing processes will not be described in detail. Also, the various components used to produce illumination devices and display systems, other than the novel circuitry, will not be described in detail. These components are known to those skilled in the art of display systems and their associated illumination devices. 
     Referring to FIG. 3, a first embodiment of a display system  200  designed in accordance with the present invention will be described. Display system  200  includes an illumination assembly  202  and a light modulating display  204  having an array of pixels  205 . One such novel display system is disclosed in U.S. Pat. No. 5,748,164, entitled ACTIVE MATRIX LIQUID CRYSTAL IMAGE GENERATOR, and issued May 5, 1998, which patent is incorporated herein by reference. A display system of this type is further described in U. S. Pat. No. 5,808,800, entitled OPTICS ARRANGEMENTS INCLUDING LIGHT SOURCE ARRANGEMENTS FOR AN ACTIVE MATRIX LIQUID CRYSTAL IMAGE GENERATOR, and issued Sep. 15, 1998, which patent is also incorporated herein by reference. Illumination assembly  202  provides the light source for light-modulating display  204 . Those skilled in the art of micro-displays understand that images are displayed on display system  200  by switching pixels  205  between various optical states in response to image data supplied at the display information input I, thereby forming a pattern of modulated light. The system is operated by displaying image frames at a certain frame rate in order to produce a viewable image. In the case of a sequential color system, each frame is typically divided into subframes or fields for sequentially displaying each of the different primary-color separations of the image. These color fields are displayed at a rate faster than the critical flicker frequency of the human eye. Therefore, the color fields of the different colors are integrated by the viewer&#39;s eye. The color sensed by the eye of a person viewing the display depends on the ratio of intensities of the primary colors in any given portion of the image displayed. The relative intensities of the light sources at different brightness levels are therefore important to producing the correct colors in the final image. It is sufficient, however, to calibrate the light source to produce white light at a desired color and intensity, because the system is thereby calibrated to produce other colors correctly when the system is operated as described above. 
     Continuing to refer to FIG. 3, illumination assembly  202  further includes a memory device  206  and an illumination device  208 . Memory device  206  may be any standard electronic memory device. Preferably memory device  206  is a semiconductor memory such as an SRAM (static random access memory) or DRAM (dynamic random access memory). Even more preferably, memory device  206  is a non-volatile semiconductor memory such as a programmable read-only memory (PROM), EEPROM (electrically erasable programmable read-only memory), or “flash” memory device. Illumination device  208  may be LEDs, laser diodes, incandescent lamps, fluorescent lamps, or any other illumination device capable of being calibrated. Calibration methods may include adjusting the illumination device drive current, voltage, and/or any other parameter(s) that changes the illumination intensity. Although specific examples have been given for memory device  206  and illumination device  208 , the present invention is not limited to these specific examples; other devices may be used that nevertheless remain within the scope of the present invention. Memory device  206  may store one or more calibrating values, each representing the current required to provide light of a specific intensity from illumination device  208 . In this embodiment the calibrating values are determined and placed in memory device  206  utilizing the calibration arrangement such as calibration arrangement  210  of FIG.  4 . 
     Referring to FIG. 4, one possible arrangement, calibration arrangement  210 , for calibrating illumination assembly  202  will be described. Calibration arrangement  210  includes a current source  212 , a light sensing device  214  to measure the intensity of light from illumination device  208 , a calibration controller  216  connected electrically to light sensing device  214 , and a reference value storage device  218  connected electrically to calibration controller  216 . Light sensing device  214  may be a photodetector or any other device capable of converting an optical signal into an electrical signal representative of the illumination intensity of the optical signal. 
     During a manufacturing calibration process, current source  212  supplies a specific current to illumination device  208 . Light sensing device  214  measures the intensity of the light produced by illumination device  208 , and calibration controller  216  compares the measured intensity unique to illumination device  208  to a reference value representing the desired intensity, the reference value being stored in reference value storage device  218 . The reference value may be obtained by exposing the same light sensing device  214  to a reference or standard light source  219  and causing the value of the measured light level to be stored in the reference value storage device  218 . Based on the comparison, calibration controller  216  causes current source  212  to vary the current supplied to illumination device  208  until the intensity of light provided by illumination device  208  matches the reference intensity. Once illumination device  208  is providing the desired intensity of light, calibration controller  216  causes a calibrating value unique to illumination device  208  to be stored in memory device  206 . The calibrating value may be the specific current required to produce light of the desired intensity, or any other calibrating value capable of allowing a controller  220  of FIG. 3 to determine the correct current to provide to illumination device  208  in order to produce light of the desired intensity. The process may be repeated for a plurality of desired intensity levels. Thus, memory device  206  may store a plurality of calibrating values representing the current required to produce light of various specific intensities. 
     Returning again to FIG. 3, display system  200  further includes controller  220  electrically connected to memory device  206  and a current source  222 . In this embodiment, current source  222  is also electrically connected to illumination device  208 . The current source provides electrical drive appropriate to the illumination device and can be of any of the types well known in the art associated with the various types of illumination devices. In particular, if the illumination device is made from LEDs, in may be preferred to provide electrical drive whose drive current does not depend on the LED forward voltage drop. Electronic circuits of this capability are well known in the art. Furthermore, it may be desired to have current source  222  respond to a digital input from controller  220 , in which case current source  222  may incorporate a digital-to-analog converter (DAC) giving it the capability of providing an output current that varies in response to a digital input from the controller. As is known in the art, light modulating display  204  can be implemented on a silicon integrated circuit. In this case, controller  220  and current source  222  may also be implemented on the same integrated circuit. 
     During operation of display system  200 , controller  220  receives display information via input I. Controller  220  uses the display information in combination with the calibrating value stored in memory device  206  to cause current source  222  to provide the particular amount of current to illumination device  208  in order to produce light of a desired intensity. The desired intensity of light to be produced at any particular time may be the same as or different from the intensities for which calibrating values are stored in memory device  206 . For example, if the desired intensity is the same intensity for which a calibrating value is stored in memory device  206 , then controller  220  causes current source  222  to provide current corresponding to that value. If, however, the desired intensity is different from any intensity for which calibrating values are stored in memory device  206 , then controller  220  interpolates between values to determine the correct current to produce light of the desired intensity. If only one calibrating value is stored in memory device  206 , then controller  220  interpolates between that calibrating value and zero current, which represents zero intensity, to determine the current necessary to produce light of the desired intensity. Controller  220  then causes current source  222  to provide that current to illumination device  208 . This method of interpolating between multiple calibrating values stored in memory device  206  provides the advantage that illumination assembly  202  may be calibrated to correct the non-linear response illumination device  208  has to current. 
     Referring now to FIG. 5, an alternative illumination device  224  and its operation will be described. In this embodiment, illumination device  224  includes a plurality of light sources, specifically red, green and blue light-emitting diodes (LEDs) indicated by reference numbers  226   a-c . Memory device  206  stores one or more calibrating values for each light source, each value representing the current required to provide light of a particular intensity for the associated light source. Ideally, memory device  206  stores the calibrating values for each light source representing the current required for each light source to produce light that, when combined, produces light of a chosen color, color temperature, and/or white point. Further, if memory device  206  is configured to store more than one value for each light source, the stored values represent the current required to produce white light at various specific brightness levels. As a result, illumination assembly  202 , when operated as described above, is calibrated to provide a stable white-point for various brightness levels. In this embodiment the calibrating values stored in memory device  206  for each light source of illumination device  224  are determined and placed in memory device  206  using a calibration arrangement such as calibration arrangement  210  of FIG. 4 as described above. 
     The calibration process is carried out in a way similar to that described above with reference to FIG.  4 . Current source  212  supplies current to each light source  226   a-c  in sequence. As each light source is illuminated, light sensing device  214  measures the intensity of light produced, and calibration controller  216  compares the measured intensity to a reference value from reference value storage device  218 . Calibration controller  216  then causes current source  212  to vary the current until the light source is producing light of the desired intensity. 
     Calibration controller then causes calibration information unique to the light source to be stored in memory device  206 . The process is repeated for each light source  226   a-c  and for all desired brightness levels of each light source. Thus memory device  206  ultimately contains values unique to each light source  226   a-c.    
     The reference values stored in device  218  may preferably have been obtained in sequence by exposing the same light sensing device  214  to a reference light source that produces a sequence of red, green, and blue illuminations. In this case it is desirable that light sensing device  214  have a spectral response that mimics that of the human eye (i.e. that it have a “photopic” response). In this way the effect of output spectral variation from the light sources in one illumination device  208  to those in the next illumination device on the achieved white point can be minimized. Alternately, it is desirable that the spectra of the red, green, and blue illuminations provided by the reference light source match the spectra of the red, green, and blue LEDs of light source  224 . 
     Although the present embodiment has been described having RGB LEDs, it should be understood that the present invention is not limited to RGB LEDs or even LEDs. The present invention may be used to calibrate any light source, combination of light sources and/or combination of colors of light sources. Also although illumination device  224  has been described as being configured to produce white light with a stable white-point, this is not a requirement. Instead, light sources with a wide variety of colors may be mixed in a wide variety of manners to produce any desired color when combined. Also, as described previously with reference to FIG. 3, the controller and/or current source can be fabricated with display  204  as a single integrated circuit. 
     Returning to FIGS. 3 through 5, one additional advantage provided by illumination assembly  202  will be described. Often in manufacturing operations, components such as display  204 , illumination assembly  202 , current source  222  and controller  220  are not assembled into a combined product until late in the manufacturing process. By providing memory device  206  and either illumination device  208  or illumination device  224  as an integrated sub-assembly, the calibration process of FIG. 4 may take place early in the manufacturing process. This is because the particular illumination device contained on the sub-assembly remains coupled throughout the manufacturing process with memory device  206  and the calibrating value stored therein that is unique to that specific illumination device. Further, illumination assembly  202  may be integrated with any combination of controller  220 , current source  222  and display  204 , without requiring further calibration, again because the unique calibrating value for the illumination device remains coupled with the illumination device. However, this advantage requires that memory device  206  is capable of maintaining the calibrating values without requiring an external power source. One particular example of a memory device capable of maintaining stored information without the need for external power is programmable read-only memory (PROM). However, the present invention is not limited to PROM; any memory device capable of maintaining its stored value without external power may be used. Alternatively as illustrated in FIGS. 3 and 5, illumination assembly  202  may include an appropriate power supply  228  such as a battery or capacitor to power the memory device, and allow it to retain its calibration values during the interval between the calibration operation and the use of the display. 
     Turning now to FIG. 6, another embodiment of the present invention will be described. FIG. 6 illustrates a display system  230  designed in accordance with the present invention. Display system  230  includes an illumination device  232  electrically connected to a controller  233  via a current source  234 . Display system  230  further includes a display backplane  236 , which is illuminated by illumination device  232 . Display system  230  further includes a light modulating display  240  and a light sensing device  242 . Light modulating device  240  operates to form images, as previously described. Light sensing device  242  may be a photodetector or any other device capable of converting an optical signal into an electrical signal representative of the illumination intensity of the optical signal. As mentioned previously, display backplane  236  can be implemented as a silicon integrated circuit. In this case light sensing device  242  can easily be implemented on the same integrated circuit, for example as a photodiode or phototransistor, using techniques well known in the integrated circuit art. Controller  233  and current source  234  may also be implemented on the same integrated circuit. 
     During operation of display system  230 , illumination device  232  illuminates display backplane  236  in response to current supplied by current source  234 . Current source  234  provides current in response to control information provided by controller  233 . Controller  233  determines the control information to supply to current source  234  based on information supplied by light sensing device  242  in combination with display information from a display information input I. The display information supplied via display information input I includes information directing a desired intensity level of light to be supplied by illumination device  232 . Controller  233  compares this desired intensity level with the output from light sensing device  242 , which represents the intensity of light being sensed. Controller  233  then varies the control information supplied to current source  234  so as to adjust the intensity of light from illumination device  232  until it matches the desired intensity. In this embodiment, the calibration arrangement of display system  230  acts as a servomechanism with continuous feedback for adjusting the light output of illumination device  232  to achieve and maintain the desired intensity of light. 
     Referring now to FIG. 6 a  in combination with FIG. 6, a method of operating display system  230  will be described. As mentioned previously, display system  230  includes illumination device  232  and display  240 . In this embodiment, the method includes the step of causing the illumination device to illuminate the display by driving it with an initial input as indicated by block  246 . As indicated by block  247  of FIG. 6 a , this method further includes the step of sensing the light from the illumination device. Block  248  includes the step of comparing the signal representative of the intensity of the sensed light to a signal representative of the desired intensity of light. Finally, block  249  includes the step of determining a new input for the illumination device, based on the comparison from the previous step, for causing the illumination device to produce light of the desired intensity. 
     Illumination device  232  of FIG. 6 could contain multiple light sources of different colors, in the same manner as was described with reference to FIG. 5, to create a sequentialcolor display system. In this case, the calibration servomechanism, with single light sensing device  242  can function nevertheless according to the above method. Controller  233  switches each different-colored light source within illumination device  232  on one at a time. Light sensing device  242  then measures in turn the intensity of each light source, and controller  233  acts on current source  234  to bring the measured intensity to its desired value. 
     Turning now to FIG. 7, another embodiment of the present invention will be described. FIG. 7 illustrates a display system  250  designed in accordance with the present invention and containing many of the same elements of display system  230  of FIG.  6 . Like reference numbers are used for like elements between FIGS. 6 and 7. However, the display backplane  256  of display system  250  further includes a comparator  264 , a reference value storage device  266  and a calibrating value storage device  267 . In this embodiment reference value storage device  266  may be non-volatile programmable read-only memory, or may be conventional SRAM or DRAM circuitry, or circuitry designed to represent a specific value or values. Additionally, display system  250  includes input I 2 , for selecting specific memory locations within reference value storage device  266 . Comparator  264  is electrically connected to both light sensing device  242  and reference value storage device  266 . Comparator  264  is configured for comparing values representing sensed light intensity received from light sensing device  242 , to reference intensities provided by reference value storage device  266  and selected from reference value storage device  266  using information from input  12 . Comparator  264  is also electrically connected to calibrating value storage device  267 . Calibrating value storage device  267  may be any programmable memory capable of being reprogrammed with new information following a calibration process. 
     During operation of display system  250 , illumination device  232  illuminates display backplane  256  in response to current supplied by current source  234 . Current source  234  provides current in response to control information provided by controller  233 . Controller  233  determines the control information to supply to current source  234  based on information supplied from calibrating value storage device  267  in combination with display information from a display information input I. The information supplied from calibrating value storage device  267  is calibration information determined during a calibration process. 
     In a first embodiment of a calibration process in accordance with the present invention, illumination device  232  is driven by a reference current from current source  234 . Light sensing device  242  senses the light from illumination device  232  and provides light intensity information to comparator  264 . Comparator  264  compares the sensed light with a reference value from reference value storage device  266 . This reference value may be derived from an earlier exposure of light sensing device  242  to a reference light source, as described previously. Comparator  264  then causes a calibrating value that is unique to illumination device  232  to be stored in calibrating value storage device  267 . Controller  233  later uses the comparison to appropriately adjust the control information supplied to current source  234 , thereby varying the current supplied to illumination device  232  in proportion to the comparison. 
     The calibration process described above may be repeated for various brightness levels and for multiple light sources included in illumination device  232 . By determining calibrating values for various brightness levels, display system  250  is capable of correcting the light source&#39;s non-linear response to current in the same manner as previously described for display system  202  of FIG.  3 . Further, by determining calibrating values for multiple light sources included in illumination device  232 , display system  250  is able to provide a stable white-point and color balance. Finally, by determining calibrating values for various brightness levels and multiple light sources, display system  250  is capable of providing a stable white-point and color balance across the system&#39;s current-controlled operating range. It should be understood that calibrating value storage device  267  must be capable of storing values representing calibration information for all light sources and all brightness levels. For example, if three light sources are included in illumination device  232  and values are stored for two brightness levels, then calibrating value storage device  267  must contain six memory locations. 
     If in the embodiment described, display backplane  256  contains no internal power source and calibrating value storage device  267  is a volatile memory device, then calibrating value storage device  267  is not capable of maintaining its stored values without external power. As a result, the calibration process described above must be repeated following each external power interruption. However, this configuration provides the advantage that the calibration process corrects post-manufacturing variations, such as LED aging, that result in light source intensity differences. Alternatively, calibrating value storage device  267  could be non-volatile memory, such as flash, or a readily providable power source could be easily incorporated into display backplane  256  of display system  250  as demonstrated by power source  270  of FIG.  7 . This would allow calibration to take place during manufacturing and negate the need to recalibrate the system following each power interruption. 
     The present embodiment functions best if the part-to-part spectral response variation of light sensing device  242  is small. The following embodiment provides a display system that functions correctly even with large spectral variation. 
     Referring now to FIG. 8, another embodiment designed in accordance with the present invention will be described. FIG. 8 illustrates a display system  300  that functions in a similar manner to display system  250  of FIG. 7, except that display system  300  includes a reference value storage device  302  and a display backplane  304 . Reference value storage device  302  of display system  300  need not be located on display backplane  304  as in display system  250 . Reference value storage device  302  is made of a non-volatile memory type, or is provided with a power supply. Further, although reference value storage device  302  contains reference intensity information as described in the previous embodiment, the reference value(s) for the present embodiment is/are adjusted during a sensing device calibration manufacturing process to account for spectral response variation of light sensing device  242 . The sensing device manufacturing calibration process takes place as follows. 
     In one embodiment of a sensing device calibration manufacturing process, display backplane  304  is illuminated by light of a reference intensity and color. Light sensing device  242  measures the intensity of the light, and the intensity reference value that is unique to light sensing device  242  is stored in reference value storage device  302 . The process is repeated for each light source within illumination device  232  (for example, different colored LEDs) and all desired brightness levels for each of those light sources. 
     During operation of display system  300 , the reference value is provided to comparator  264  during a calibration process as described for display system  256  of FIG.  7 . Thereby, this embodiment corrects the spectral response variation of light sensing device  242 . 
     Referring now to FIGS. 6 through 8 an additional potential problem that may be encountered during operation of display system  230 , display system  250  or display system  300  will now be described. Some light sources that may be included in illumination device  232  have the potential for emitting light with a wavelength outside the visible spectrum. Because light sensing device  242  is not necessarily limited to sensing light of wavelengths within the visible spectrum, light emitted by illumination device  232  outside the visible spectrum will be sensed, and the calibration process may provide inaccurate calibration information. In order to overcome the aforementioned potential problem, a filter  306  may be positioned over light sensing device  242 . Filter  306  may be designed to pass only light having a wavelength within the visible spectrum, thereby preventing any light from outside the visible range being measured by light sensing device  242 . 
     Alternatively, filter  306  may be designed to solve yet another potential problem that may arise in display system applications. Part-to-part spectral output variation for a typical light source used in display system applications may produce unacceptable color balance and white-point stability, even when calibrated in accordance with the present invention. This occurs because the typical light sensing device measures light intensity irrespective of the wavelength of light being measured. Therefore, a light source may produce light of an undesired wavelength, yet this fact would go undetected by the previously described display systems. To solve this problem filter  306  may be a photopic response filter having the same wavelength variation sensitivity as a human eye. As a result, the light sensing device will have the same response to light source spectral variations as the human eye, and desired white-point calibration will be obtained. 
     Referring to FIG. 9, another embodiment of the present invention, display system  320 , will be described that also solves the potential problem of light source spectral response output variation. Display backplane  322  of display system  320  includes a plurality of light sensing devices  324   a-c , each configured to measure only light of a specific range of wavelengths, and each configured to measure a different range of wavelengths of light. For example, display system  320  could include three light sensing devices for measuring the three primary colors, red, green and blue. Light sensing devices  324   a-c  may have filters  326   a-c  positioned so as to filter the light being sensed by devices  324   a-c . Alternatively, light sensing devices  324   a-c  may be photodetectors specifically designed with a particular spectral response variation so as to be more sensitive to light within specific wavelength ranges. For light sensing devices implemented as photodetectors on an integrated circuit, spectral sensitivity can be tailored by the design of the photodetector, for example whether or not the photodetector is implemented directly in the silicon substrate or is alternately implemented in a CMOS well. During a calibration process similar to that described above for display system  250  of FIG. 7, light sensing devices  324   a-c  measure the intensity of light from individual light sources contained in illumination device  232 . Comparator  264  compares the measured intensities to reference values for the specific wavelengths of sensed light and causes the comparison information to be stored in calibrating value storage device  267 . Controller  233  later uses the comparison to appropriately adjust the control information supplied to current source  234 , thereby varying the current supplied to illumination device  232  in proportion to the comparison. 
     Although only a few embodiments of an illumination device and a display system designed in accordance with the present system have been described in detail, it should be understood that the present invention may take on a wide variety of specific configurations and still remain within the scope of the present invention. For example the invention embodied in display system  320  of FIG. 9 may be embodied in a display system similar to display system  230  of FIG. 6 (i.e., without elements comparator  264 , calibration value storage device  267 , and reference value storage device  302 ). Therefore, the present examples are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claims.