Abstract:
A display has a screen which incorporates a light modulator. The screen may be a front projection screen or a rear-projection screen. The screen is illuminated with light from an illuminator comprising an array of individually-controllable light sources. The light sources and elements of the light modulator may be controlled to adjust the intensity and frequency of light emanating from corresponding areas on the screen. The display may be calibrated to compensate for differences in intensities of the light sources.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional application Ser. No. 60/899,098 filed Feb. 1, 2007 and entitled CALIBRATION OF DISPLAYS HAVING SPATIALLY-VARIABLE BACKLIGHT which is hereby incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The invention relates to calibration of displays for displaying digital images. 
     BACKGROUND 
     Some types of displays for displaying digital images comprise a plurality of individually-controllable light sources. Examples of such displays include those described in International Application No. PCT/CA03/00350, which is hereby incorporated by reference herein. Such displays may use light emitting diodes (LEDs) as the individually-controllable light sources, for example. 
     One problem with using LEDs as light sources is that the amount of light emitted at a specific driving current level can vary significantly between individual LEDs. This variation can result from manufacturing process variations. Further, the amount of light that an individual LED will produce for any given driving current tends to slowly decrease in an unpredictable manner as the LED ages. 
     It may therefore be desirable to provide a mechanism for calibrating a display which employs individually-controllable light sources to compensate for differences in brightness between different ones of the light sources. Some such calibration mechanisms are described in the above-noted International Application No. PCT/CA03/00350. 
     Another problem associated with some LEDs is that the color spectrum of the emitted light can vary between individual LEDs. For example, some types of white LEDs comprise a blue LED which illuminates a yellow phosphor. Individual ones of such LEDs may, when driven to emit white light, emit light having a color spectrum (also referred to as a “color temperature”) ranging from “blue white” to “yellow white”. Such variation in the color temperature among LEDs is undesirable in many situations. 
     There exists a need for further methods and systems for calibrating a display comprising a plurality of individually-controllable light sources. 
     SUMMARY OF THE INVENTION 
     One aspect of the invention provides a method for calibrating a display comprising an array of individually-controllable light sources coupled to a controller. The individually-controllable light sources are configured to emit light when supplied with electrical current under control of the controller in response to image data. The method comprises collecting at least a portion of light emitted by one light source of the plurality of light sources, receiving a collected light signal representative of the collected light, comparing the collected light signal to expected light characteristics, and, if the comparison indicates that an intensity of the collected light is different from an expected intensity indicated by the expected light characteristics, determining an intensity correction for the one light source, the intensity correction comprising an indication to alter a duty cycle of pulses of electrical current supplied to the one light source by the controller. 
     Further aspects of the invention and features of specific embodiments of the invention are described below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In drawings which illustrate non-limiting embodiments of the invention, 
         FIGS. 1A and 1B  show displays having calibration mechanisms which collect forward-emitted light; 
         FIGS. 2A-C  show example calibration mechanisms which collect stray light; 
         FIG. 3A  shows a calibration mechanism which detects stray light from one light source using one or more nearby light sources as light detectors; 
         FIG. 3B  is a block diagram of an example circuit for selectively causing a light source to emit light or detect light; 
         FIG. 4  is a flowchart illustrating steps of a method for calibrating a display according to one embodiment of the invention; 
         FIG. 5A  illustrates an uncalibrated pulse of electrical power supplied to drive a light source; 
         FIG. 5B  illustrates an calibrated pulse of electrical power supplied to drive a light source; and, 
         FIG. 6  shows an example arrangement of light sources. 
     
    
    
     DESCRIPTION 
     Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense. 
     This invention provides systems and methods for calibrating a display comprising an illuminator comprising a plurality of individually-controllable light sources. The illuminator may backlight a screen. In some embodiments, the light output of each of the individually-controllable light sources is measured and compared with an expected light output. When a measured output of one light source differs from the expected output of the light source, a correction may be determined for that light source. The determined correction may comprise an indication to adjust illuminator control signals to be provided to that light source and/or neighboring light sources. The illuminator control signals may, for example, directly or indirectly control one or more of driving current, driving waveform, duty cycle of a driving waveform, wave shape of a driving waveform, or the like. 
     In some embodiments, the screen may comprise a modulator comprising a plurality of individually-controllable pixel elements. In such embodiments, the determined correction may additionally or alternatively comprise an indication to adjust modulator control signals for portions of the modulator which are backlit by that light source. The modulator control signals may, for example, control a transmissivity of one or more pixel elements, times at which the one or more pixel elements are made to pass light from the illuminator, or the like. 
     Once the corrections have been determined, they may be applied by adjusting illuminator and/or modulator control signals generated from image data. The corrections may also be applied when generating the illuminator and/or modulator control signals from the image data. Alternatively, the corrections may be stored in an electronic memory or other suitable storage system for future application. 
       FIG. 1A  shows a display  10  comprising a modulator  12  which is backlit by an illuminator  14  comprising an array of individually-controllable light sources  16 . Modulator  12  comprises a plurality of pixel elements  13 . Pixel elements  13  may be individually controlled to selectively modulate light from light sources  16 . In the illustrated embodiment, light sources  16  comprise light-emitting diodes (LEDs). In the following description, light sources  16  are referred to as LEDs  16  and modulator  12  is referred to an LCD panel. Other suitable light sources could be used in place of LEDs  16 . Other suitable modulators could be used in place of LCD panel  12 . The light output of each LED  16  and the modulation of each pixel element  13  may be controlled individually as described, for example, in International Application No. PCT/CA03/00350. 
     A controller  19  generates illuminator control signals  17  and modulator control signals  18  to display a desired image. The desired image may be specified by image data  11  which directly or indirectly specifies luminance values (and, if the image is a color image, color values) for each pixel. Image data  11  may have any suitable format and may specify luminance and color values using any suitable color model. For example, image data  11  may specify:
         red, green and blue (RGB) color values for each pixel;   YIQ values wherein each pixel is represented by a value (Y) referred to as the luminance and a pair of values (I, Q) referred to as the chrominance;   CMY or CMYK values;   YUV values;   YCbCr values;   HSV values; or   HSL values.
 
Image data  11  may have any suitable image data format.
       

     In some embodiments, light sources  16  may comprise LEDs of different colors, or may comprise tri-color LEDs which each include red, green and blue LEDs all encapsulated within a single housing. In such embodiments, illuminator control signals  17  may cause suitable driving circuits to separately control the brightness of LEDs  16  of different colors and, within a particular color, to separately control the brightness of LEDs  16  in different locations. This permits illuminator  14  to project onto modulator  12  a pattern of light that has different mixtures of colors at different locations on modulator  12 , or to sequentially project red, green and blue color patterns onto modulator  12  in a time-interleaved manner. 
     In the embodiment of  FIG. 1A , controller  19  receives image data  11  and generates illuminator control signals  17  which control the intensities of LEDs  16  based on image data  11 . Controller  19  also generates modulator control signals  18  which control the amounts of light passed by each of pixel elements  13 . Modulator control signals  18  may also control the spectrum of light passed by each of pixel elements  13  in some embodiments. 
     Modulator control signals  18  may be generated, for example, based on the intensities and spread functions of LEDs  16 . The spread function of an LED  16  represents a pattern of light from that LED  16  which is incident on modulator  12 . The intensities and spread functions of LEDs  16  may be used in a light field simulation to obtain an expected illumination pattern created by illuminator  14  on modulator  12 . The light field simulation may then be used to determine the amount of light which should be passed by each of pixel elements  13  to display the desired image. Where the desired image is a color image, the light field simulation may also be used to determine the amount of color filtration (if any) which should be applied by each of pixel elements  13  to display the desired image. 
     In the embodiment of  FIG. 1A , a light detector  20  detects light emitted by LEDs  16  and provides light detector signals  21  to controller  19 . Light detector signals  21  may indicate the intensity of light emitted by LEDs  16  that is detected at light detector  20 . Light detector  20  may additionally or alternatively comprise a spectrometer, in which case light detector signals  21  may indicate the spectral characteristics of light emitted by LEDs  16 . 
     In the embodiment of  FIG. 1A , a single light detector  20  is provided which may be moved into different positions for capturing forward-emitted light from different LEDs  16 . In the alternative, multiple light detectors may be provided, or a suitable optical system may be provided to direct light from LEDs  16  to light detector  20 . For example,  FIG. 1B  shows an embodiment similar to that of  FIG. 1A  wherein a planar optical waveguide  22  collects a small fraction of the forward-emitted light emitted by LEDs  16  and carries that light to light detector  20 . The embodiment of  FIG. 1B  also comprises a grid  23  of reflective-walled channels for increasing the uniformity with which each LED  16  illuminates modulator  12 , as described, for example, in International Application No. PCT/CA03/00350. 
       FIGS. 1A and 1B  are schematic in nature. The components of modulator  12  and light sources  16  may be arranged in any suitable two dimensional arrangements, not necessarily the arrangements shown. 
       FIGS. 2A-C  show embodiments wherein light detector  20  detects stray light emitted by LEDs  16 . In the  FIG. 2A  embodiment, optical waveguides  24  carry stray light from LEDs  16  to light detector  20 . Only a small fraction of the light emitted by each LED  16  is captured by waveguides  24 . As long as the coupling between a waveguide  24  and the corresponding LED  16  does not change, the proportion of the light emitted by an LED  16  which is captured by waveguide  24  remains constant. One light detector  20  or a few light detectors  20  may be located at convenient locations such as at edges of illuminator  14 . 
     In the embodiment of  FIG. 2B , individual optical waveguides  24  are replaced by a planar optical waveguide  26 . Power leads for LEDs  16  pass through holes in waveguide  26 . One or more light detectors  20  are located at edges of optical waveguide  26 . Light emitted in the rearward direction by any of LEDs  16  is trapped within optical waveguide  26  and detected by light detector(s)  20 . In the embodiment of  FIG. 2C , a planar optical waveguide  28  collects light emitted by LEDs  16  in sideways directions and carries that light to one or more light detectors  20 . 
       FIG. 3A  shows an embodiment wherein stray light from one LED  16  is collected by nearby LEDs  16 . When stray light from one LED  16  which is emitting light in response to illuminator control signals  17  is incident on an LED  16  which is not emitting light, an electrical potential is induced in that non-emitting LED  16 . 
     Each LED  16  may be connected to a circuit  32 . Only circuits  32  connected to non-emitting LEDs  16  are shown in  FIG. 3 . The electrical potential induced by light incident on a non-emitting LED  16  may cause a current which is proportional to the intensity of the light incident thereon to flow in the connected circuit  32 . The current flowing in circuits  32  may be measured by current detectors  33  which provide feedback signals  31  to controller  19 . Controller  19  may determine the light output of the one LED  16  based on feedback signals  31  from other LEDs  16 . Alternatively, circuits  32  may be connected to controller  19 , and controller  19  may comprise one or more built-in current detector(s) for measuring current produced by non-emitting LEDs  16 . In such embodiments, controller  19  may determine the light output of the one LED  16  based on the current measurements. Such current measurements may be made at times when only one LED  16  is emitting light which is incident on the non-emitting LEDs  16  for which current is measured, or when a known set of two or more LEDs  16  are emitting light, such that the contribution from each of the emitting LEDs  16  may be individually determined. The contribution from each of the emitting LEDs  16  may be individually determined, for example, by triangulation, using a plurality of non-emitting LEDs  16  having a known geometric relationship to the emitting LEDs  16  to sense light from the emitting LEDs  16 . A separate light detector  20  is not required in the  FIG. 3A  embodiment. 
       FIG. 3B  shows an embodiment wherein a switch  34  is provided for selectively connecting LED  16  to a driving circuit  35  or a measuring circuit  36 . Switch  34  may be operated between a driving position and a measuring position by controller  19  by means of a switch control line  37 . When switch  34  is in the driving position, LED  16  is driven to emit light by driving circuit  35  in response to control signals  38  from controller  19 . Measuring circuit  36  may provide a reverse bias to LED  16 , and may be configured such that current drawn by LED  16  varies with the amount of light incident on LED  16 . When switch  34  is in the measuring position, current flow through LED  16  may be measured by measuring circuit  36 , which provides measurement signals  39  to controller representative of light incident on LED  16 . 
       FIG. 4  is a flowchart illustrating a method  40  for calibrating a display according to one embodiment of the invention. Method  40  may be carried out by a controller of a display which is backlit by a plurality of individually-controllable light sources, such as, for example, a display according to any of the embodiments of  FIGS. 1A-B ,  2 A-C or  3 A-B. Method  40  may also have application to other types of displays which comprise a plurality of individually-controllable light sources. 
     At block  41 , the controller causes one of the light sources, which is referred to herein as a source-under-test, to emit light. The source-under-test may emit light in the course of displaying an image, or in response to a calibrating illuminator control signal. 
     In some situations, the controller may cause only the source-under-test to emit light. In such situations the emitted light may be detected by a light detector upon which the emitted light is incident, or may be collected by any suitable optical system and provided to a light detector. Alternatively, in embodiments wherein the light sources comprise LEDs, the emitted light may be detected by neighboring LEDs. 
     In other situations, the controller may cause one or more light sources other than the source-under-test to emit light. In such situations, light emitted by the source-under-test may be detected by a light detector positioned such that only light from the source-under-test is incident thereupon, or may be collected by an optical system configured to collect only light emitted by the source-under-test and provided to a light detector. 
     At block  42 , the controller receives a collected light signal. The collected light signal may comprise one or more light detector signals received from one or more light detectors. Alternatively or additionally, the collected light signal may comprise one or more feedback signals received from LEDs. The collected light signal may indicate the intensity of light emitted from source-under-test. In some embodiments, the collected light signal also indicates the color temperature of light emitted from the source-under-test. 
     The collected light signal may represent light collected during a calibration cycle wherein the source-under-test is provided with a calibrating illuminator control signal. Alternatively, the collected light signal may represent light collected while the display is displaying an image wherein the source-under-test is provided with an illuminator control signal determined by image data. 
     At block  44  the controller determines expected light characteristics for the collected light represented by the collected light signal. Determining the expected light characteristics may comprise, for example, looking up stored reference values for the source-under-test. The expected light characteristics may comprise, for example, intensity levels and/or spectral characteristics expected for given illuminator control signals. The reference values may be stored, for example, in a memory accessible by the controller. 
     At block  46  the controller compares the collected light signal with the expected light characteristics. If the collected light signal indicates that the light emitted by the source-under-test has the expected characteristics (block  46  YES output), then no correction is required. Method  40  may then return to block  41  in order to calibrate other light sources, or may end if all light sources have been calibrated. 
     If the collected light signal indicates that the light emitted by the source-under-test does not have the expected characteristics (block  46  NO output), then a correction may be required. Method  40  then proceeds to block  48 . 
     At block  48 , the controller determines a correction to be applied based on the results of the comparison of block  46 . For example, if the comparison indicates that the intensity of the light emitted by the source-under-test is different from the expected intensity, the controller may determine an intensity correction for the source-under-test and store the intensity correction in a data structure located in a memory accessible by the controller. Likewise, if the comparison indicates that the color temperature of the source-under-test differs from the expected color temperature, the controller may determine a color correction for the source-under-test and store the color correction in a data structure located in a memory accessible by the controller. 
     If the comparison indicates that the intensity of the light emitted by the source-under-test is less than the expected intensity, the intensity correction may comprise, for example, an indication to adjust the illuminator control signals such that an increased current is provided to the source-under-test. Alternatively or additionally, the intensity correction may comprise an indication to adjust the illuminator control signals such that an increased voltage is provided to the source-under-test. 
     In some embodiments the light sources are provided with pulses of electrical power, rather than provided with a continuous supply of power. For each light source, the duty cycle of the pulses determines the perceived intensity of light emitted from that light source. The term “duty cycle” is used herein to refer to the proportion of time during which electrical power is supplied to a light source.  FIG. 5A  shows example illuminator control signals for providing pulses of electrical power to a light source wherein the light source emits light at full intensity for 50% of the time, which corresponds to a duty cycle of 50%. The time scale of the pulses is such that the human eye perceives the light source to be continuously emitting light at 50% intensity. In such embodiments, the intensity correction may comprise an indication to adjust the illuminator control signals such that the electrical pulses provided to the source-under-test have increased or decreased duty cycles.  FIG. 5B  shows an example of such adjusted illuminator control signals for a situation wherein the source-under-test is determined to have a 33% reduction in intensity, and the illuminator control signals have been adjusted to increase the duty of the pulses by 33%, resulting in an adjusted duty cycle of 66.5%. 
     Instead of or in addition to an indication to adjust the illuminator control signals for the source-under-test, the intensity correction may comprise an indication to adjust the illuminator control signals for other light sources in an area surrounding the source-under-test.  FIG. 6  shows an example arrangement of light sources which comprises a portion of a rectangular array. The columns and rows of the light sources shown in  FIG. 6  have been labelled with reference letters a-e and numbers  1 - 5 , respectively. In the  FIG. 6  embodiment, if the intensity of light source c 3  is less than the expected intensity, the intensity correction may comprise, for example, an indication to increase the current, voltage and/or duty cycle of electrical power provided to light sources c 2 , c 4 , b 3  and d 3 . The intensity correction may also comprise an indication to adjust the illuminator control signals for light sources b 2 , b 4 , d 2  and d 4 , or for light sources farther away from light source c 3 . 
     In some embodiments, the intensity correction comprises an indication to adjust the illuminator control signals for light sources in an area surrounding the source-under-test in a non-uniform manner. For example, the illuminator control signals for surrounding light sources may be non-uniformly adjusted according to a weighting function. The weighting function may be based, for example, on the intensities of the surrounding light sources, or the similarity of the intensities of the surrounding light sources to the expected intensity of the source-under-test. One factor which may be included in the weighting function is the spatial distribution of light from the source-under-test. The intensity correction may be generated based on weighting the measured intensity by the spatial distribution. The spatial distribution may be, for example, a point-spread-function used in the image processing for the display. 
     For example, in the  FIG. 6  embodiment, if the intensity of light source c 3  is less than the expected intensity, the intensity correction may comprise, for example, an indication to increase the current, voltage and/or pulse width of electrical power provided to one or more light sources within a predetermined proximity to light source c 3  which have the highest intensity. For example, if light source al has a relatively high intensity as compared to the other light sources surrounding light source c 3 , the intensity correction may comprise an indication to increase the current, voltage and/or pulse width of electrical power provided to light source a 1  without adjusting the illuminator control signals for light sources located closer to light source c 3 . Alternatively, the intensity correction may comprise, for example, an indication to increase the current, voltage and/or pulse width of electrical power provided to one or more light sources within a predetermined proximity to light source c 3  which have an intensity value closest to the expected intensity of light source c 3 . 
     In embodiments where the light sources comprise an array of evenly spaced LEDs, the intensity correction may comprise an indication to adjust the control signals so that for sources at the same distance away from a non-emitting LED connected to a measuring circuit, the non-emitting LED senses the same intensity. Multiple sources the same distance from the non-emitting LED can be calibrated to emit uniformly. Then another non-emitting LED can detect the intensities of these calibrated LEDs, and use the detected intensities as reference intensities. That other non-emitting LED may then be used to detect intensities from other sources at the same distance from it as the calibrated LEDs, and calibrate those other sources based on the reference intensities. This process can be carried out over the entire LED array to make the LEDs emit uniformly without calibrating for the sensitivity of each LED as a detector. An analogous process can be used to calibrate for the sensitivity of each LED as a detector once LEDs are calibrated to emit uniformly. Thus subsequently the sensitivity of LEDs as detectors can be used without repeating the aforementioned process. 
     In addition to or instead of an intensity correction, at block  48  the controller may determine that a color correction is required for the source-under-test. The determination that a color correction is required may be made, for example, by providing illuminator control signals to drive the source-under-test to emit white light, measuring the spectrum of the emitted light, and comparing the measured spectrum to an expected spectrum. The expected spectrum may comprise, for example, a predefined spectrum such as the D65 white point specified by ITU Recommendation BT.709. 
     In embodiments where the light sources comprise color light sources, the color correction may comprise an indication to adjust the color values used to generate the illuminator control signals for the source-under-test to compensate for any deviation from the expected color temperature. Alternatively or additionally, the color correction may comprise an indication to adjust the color values used to generate the modulator control signals for portions of the modulator on which light from the source-under-test is incident. Such adjustment of the modulator control signals may be determined, for example, by substituting the measured color temperature for the source-under-test for the expected color temperature to calculate a color-calibrated spread function for the source-under-test. The color-calibrated spread function may then be included in the light field simulation, such that the modulator applies color filtration to correct the color temperature perceived by a viewer of the displayed image. In embodiments which use RGB color values, the adjustment to the color values may be determined, for example, by normalizing the measured spectrum by the minimum of the red, green and blue color channels. 
     After the correction has been determined at block  48 , the correction may be applied at block  50 . Applying the correction may comprise adjusting the illuminator and/or modulator control signals as indicated by the correction. The correction may also be stored at block  50 . Storing the correction may comprise storing the correction in an electronic memory accessible by the controller. The controller may apply the corrections as they are determined, or may store a plurality of corrections and apply the stored corrections at a subsequent time. 
     Method  40  may be carried out sequentially for each of the plurality of light sources. For example, when the display is being driven to display a series of frames specified by the image data, method  40  may be carried out for one of the light sources during each frame until every light source has been calibrated. Alternatively, method  40  may be simultaneously carried out for more than one of the light sources. For example, a plurality of collected light signals may be received at block  42  which are representative of light collected from a subset of the light sources, or all of the light sources. In embodiments wherein the collected light signals are received for a subset of the light sources, method  40  may be repeated for every other subset of the light sources. 
     Method  40  may be automatically carried out periodically, or may be carried out in response to a calibration command received by the controller. Alternatively or additionally, data from the display may be continually or periodically measured, and method  40  may be carried out in response to the measured data exceeding the threshold. The measured data may comprise, for example, thermal data. 
     As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. For example:
         Instead of receiving the collected light signals at the controller which provides the illuminator and modular control signals, a separate calibration controller may be provided to receive the collected light signals and determine any corrections to be applied.       

     As will be apparent to those skilled in the art in light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.