PATENT DOCUMENT

Publication Number: US-9076376-B2
Application Number: US-201313861175-A
Country: US
Kind Code: B2

Title: Subtractive color based display white point calibration

Abstract:
Systems, methods, and devices for white point calibration using subtractive color measurements are provided. Specifically, a white point of a display may be calibrated using subtractive color measurements rather than merely additive color measurements. In one example, a display having red, green, and blue pixels may measure the responses in a subtractive color space (e.g., CMY) rather than additive color space (e.g., RGB). Measurements of the display response using subtractive color space may involve providing image data to two or more color channels at once. Thus, any crosstalk effect between channels may be accounted for, even though the same crosstalk effect might not be apparent using additive color measurements in which only a single channel color channel were measured.

Claims:
What is claimed is: 
     
       1. A method for calibrating an electronic display comprising:
 measuring a first optical response of an electronic display, wherein the first response occurs when the electronic display is programmed to display a first mixed color due to image data supplied to a first color subpixel and a second color subpixel but not a third color subpixel; 
 measuring a second optical response of the electronic display, wherein the second response occurs when the electronic display is programmed to display a second mixed color due to image data supplied to the first color subpixel and the third color subpixel but not the second color subpixel; 
 measuring a third optical response of the electronic display, wherein the third response occurs when the electronic display is programmed to display a third mixed color due to image data supplied to the second color subpixel and the third color subpixel but not a first color subpixel; and 
 calibrating a white point of the electronic display based at least in part on the first, second, and third optical responses. 
 
     
     
       2. The method of  claim 1 , wherein:
 the first color subpixel comprises a blue color subpixel; 
 the second color subpixel comprises a green color subpixel; 
 the third color subpixel comprises a red color subpixel; 
 the first mixed color comprises cyan; 
 the first mixed color comprises magenta; and 
 the first mixed color comprises yellow. 
 
     
     
       3. The method of  claim 1 , wherein the white point is calibrated based at least in part on a 3×3 XYZ transformation matrix linking XYZ tristimulus values to a color space associated with the first color subpixel, the second color subpixel, and the third color subpixel. 
     
     
       4. The method of  claim 3 , wherein the white point is calibrated based at least in part on the 3×3 XYZ transformation matrix, wherein the each of the components of the 3×3 XYZ transformation matrix are determined based on coupled values of the colors of the color space, wherein each of the coupled values of the colors of the color space are determined by subtracting one of the three optical responses from a sum of the remaining two responses. 
     
     
       5. The method of  claim 1 , wherein the white point is calibrated without considering any other optical responses of the electronic display occurring when the electronic display is programmed to display any color due to image data supplied only to a single one of the first, second, or third color subpixel. 
     
     
       6. A system for calibrating a white point of an electronic display, comprising:
 one or more light sensors configured to detect light emitted by the electronic display; and 
 a calibration test controller configured to cause the electronic display to display a series of subtractive colors and program white point calibration parameters of the electronic display based at least in part on the resulting light detected by the one or more light sensors while the electronic display is displaying the series of subtractive colors; 
 wherein the calibration test controller is configured to program the white point calibration parameters based at least in part on the resulting light detected by the one or more light sensors while the electronic display is displaying the series of subtractive colors when the electronic display comprises driving circuitry formed using low temperature polysilicon (LTPS) and when the electronic display comprises driving circuitry formed using amorphous silicon (a-Si). 
 
     
     
       7. The system of  claim 6 , wherein the calibration test controller is configured to cause the electronic display to display the series of subtractive colors, wherein the series of subtractive colors comprises cyan, magenta, and yellow. 
     
     
       8. The system of  claim 6 , wherein the calibration test controller is configured to cause the electronic display not to display any single additive colors. 
     
     
       9. The system of  claim 6 , wherein the calibration test controller is configured to isolate additive colors from the resulting light detected by the one or more light sensors while the electronic display is displaying the series of subtractive colors and to program the white point calibration parameters based at least in part on the isolated additive colors. 
     
     
       10. A method for manufacturing an electronic device, comprising:
 providing an enclosure; 
 providing an electronic display having red, green, and blue subpixels, wherein a white point of the electronic display has been calibrated by: 
 measuring a first optical response of the electronic display when the electronic display is programmed to display cyan using only the green and blue subpixels; 
 measuring a second optical response of the electronic display when the electronic display is programmed to display magenta using only the blue and red subpixels; 
 measuring a third optical response of the electronic display when the electronic display is programmed to display yellow using only the red and green subpixels; and 
 calibrating the white point of the electronic display based at least in part on the first, second, and third optical responses; and 
 installing the electronic display into the enclosure. 
 
     
     
       11. The method of  claim 10 , wherein the electronic display that is provided exhibits crosstalk between the red, green, and blue subpixels. 
     
     
       12. The method of  claim 10 , wherein the electronic display that is provided uses a demultiplexer to demultiplex image data signals provided to the red, green, and blue subpixels, wherein the demultiplexer does not completely separate the image data signals provided to the red, green, and blue subpixels. 
     
     
       13. The method of  claim 10 , wherein the white point has been calibrated using only the first, second, and third optical responses. 
     
     
       14. An electronic display comprising:
 a display panel comprising a plurality of subpixels of different colors; and 
 display driver circuitry configured to provide image data signals to the display panel, wherein the display driver circuitry comprises white point calibration parameters configured to cause the display driver circuitry to provide the image data signals to substantially achieve a desired white point on the display panel, wherein the white point calibration parameters are based on a transformation matrix relating the different colors of the plurality of subpixels to a color space, wherein coefficients of the transformation matrix are based exclusively on subtractive color measurements of the plurality of subpixels of the different colors; 
 wherein:
 the color space comprises an XYZ color space; and 
 each coefficient of the transformation matrix comprises a value relating one of the components of the XYZ color space to a particular one of the different colors of the plurality of subpixels via a subtraction of one subtractive color measurement from other subtractive color measurements. 
 
 
     
     
       15. The electronic display of  claim 14 , wherein:
 the different colors of the plurality of subpixels consist of red, green, and blue; 
 the transformation matrix comprises a 3×3 transformation matrix; and 
 each coefficient of the transformation matrix comprises a value that relates one of the components of the XYZ color space to a particular one of the different colors of the plurality of subpixels while taking into account crosstalk from image data provided to the other of the different colors. 
 
     
     
       16. The electronic display of  claim 14 , wherein:
 the different colors of the plurality of subpixels consist of red, green, and blue; 
 the transformation matrix comprises a 3×3 transformation matrix; and 
 the transformation matrix is configured to relate the different colors of the plurality of subpixels (RGB) to the color space (XYZ) according to the following relationship: 
 
       
         
           
             
               
                 
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       17. The electronic display of  claim 14 , wherein:
 the different colors of the plurality of subpixels consist of red, green, and blue; 
 the transformation matrix comprises a 3×3 transformation matrix; and 
 the transformation matrix is configured to relate the different colors of the plurality of subpixels (RGB) to the color space (XYZ) according to the following relationship: 
 
       
         
           
             
               
                 
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         where each coefficient (X r′ , X g′ , X b′ , Y r′ , Y g′ , Y b′ , Z r′ , Z g′ , Z b′ ) is based on coupled measurements of red, green, and blue. 
       
     
     
       18. The electronic device of  claim 14 , wherein
 the different colors of the plurality of subpixels consist of red, green, and blue; and 
 the transformation matrix is configured to relate the different colors of the plurality of subpixels (RGB) to the color space (XYZ) according to the following relationships: 
 
       
         
           
             
               
                 
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         where: 
         the coefficient X y  represents the X component of the XYZ color space occurring while the electronic display is displaying a yellow color due to image data supplied to the red and green subpixels; 
         the coefficient Y y  represents the Y component of the XYZ color space occurring while the electronic display is displaying the yellow color due to image data supplied to the red and green subpixels; 
         the coefficient Z y  represents the Z component of the XYZ color space occurring while the electronic display is displaying the yellow color due to image data supplied to the red and green subpixels; 
         the coefficient X m  represents the X component of the XYZ color space occurring while the electronic display is displaying a magenta color due to image data supplied to the blue and red subpixels; 
         the coefficient Y m  represents the Y component of the XYZ color space occurring while the electronic display is displaying the magenta color due to image data supplied to the blue and red subpixels; 
         the coefficient Z m  represents the Z component of the XYZ color space occurring while the electronic display is displaying the magenta color due to image data supplied to the blue and red subpixels; 
         the coefficient X c  represents the X component of the XYZ color space occurring while the electronic display is displaying a cyan color due to image data supplied to the blue and green subpixels; 
         the coefficient Y c  represents the Y component of the XYZ color space occurring while the electronic display is displaying the cyan color due to image data supplied to the blue and green subpixels; and 
         the coefficient Z c  represents the Z component of the XYZ color space occurring while the electronic display is displaying the cyan color due to image data supplied to the blue and green subpixels.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/699,782, “Subtractive Color Based Display White Point Calibration,” filed 11 Sep. 2012, which is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     This disclosure relates generally to white point calibration for an electronic display and, more particularly, to white point calibration using subtractive color measurements. 
     This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present techniques, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
     Electronic displays appear in many electronic devices. One type of electronic display, known as a liquid crystal display (LCD), modulates light passing through pixels of various colors using a liquid crystal material to generate images. An LCD may include a number of mass-produced components with characteristics that can vary from display to display. To provide a few examples, a backlight unit of the LCD may have light emitting diodes (LEDs) that emit light of different wavelengths and may have variable phosphor concentration; or a cell gap of the display panel and/or a color filter thickness may vary slightly. Such variations may cause a white point—the color emitted when the display is programmed to the color white—to vary slightly from LCD to LCD. 
     To account for these variations, LCDs may be calibrated to produce a white point within a desired color range. Such white point calibration may rely on the color additivity properties of red, green, and blue pixel channels of the LCDs. The assumption of linearity may not hold, however, for all types of LCDs. Indeed, when an LCD exhibits a crosstalk phenomenon, the color additivity of red, green, and blue channels may not hold. As a result, the white point calibration may not reliably produce properly calibrated displays. Moreover, techniques relating to accounting for crosstalk may involve complex or inefficient calculations or color channel characterizations. 
     SUMMARY 
     A summary of certain embodiments disclosed herein is set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of these certain embodiments and that these aspects are not intended to limit the scope of this disclosure. Indeed, this disclosure may encompass a variety of aspects that may not be set forth below. 
     Embodiments of this disclosure relate to systems, methods, and devices for white point calibration using subtractive color measurements. Specifically, since additive color measurements may not account for crosstalk that may occur in some displays, a white point of a display instead may be calibrated using subtractive color measurements. For example, a display having red, green, and blue pixels may measure the responses in a subtractive color space (e.g., CMY) rather than additive color space (e.g., RGB). Measurements of the display response using subtractive color space may involve providing image data to two or more color channels at once. Thus, any crosstalk effect between channels may be accounted for, even though the same crosstalk effect might not be apparent using additive color measurements in which only a single channel color channel were measured. In an example involving a display with red, green, and blue pixels, the subtractive color space measurements may be cyan (blue and green), magenta (blue and red), and yellow (red and green). The various systems, methods, and devices described herein may be used effectively to calibrate both electronic displays that exhibit a crosstalk phenomenon and electronic displays that do not exhibit a crosstalk phenomenon. 
     Various refinements of the features noted above may exist in relation to various aspects of the present disclosure. Further features may also be incorporated in these various aspects as well. These refinements and additional features may exist individually or in any combination. For instance, various features discussed below in relation to one or more of the illustrated embodiments may be incorporated into any of the above-described aspects of the present disclosure alone or in any combination. The brief summary presented above is intended only to familiarize the reader with certain aspects and contexts of embodiments of the present disclosure without limitation to the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects of this disclosure may be better understood upon reading the following detailed description and upon reference to the drawings in which: 
         FIG. 1  is a block diagram of an electronic device having a display calibrated based on subtractive color measurements, in accordance with an embodiment; 
         FIG. 2  is a perspective view of the electronic device of  FIG. 1  in the form of a notebook computer, in accordance with an embodiment; 
         FIG. 3  is a front view of the electronic device of  FIG. 1  in the form of a handheld device, in accordance with an embodiment; 
         FIG. 4  is a perspective exploded view of components of the display, in accordance with an embodiment; 
         FIG. 5  is a circuit diagram of a pixel matrix of the display, in accordance with an embodiment; 
         FIG. 6  is a block diagram of a component of driver circuitry of the display that could produce crosstalk between different color channels, in accordance with an embodiment; 
         FIGS. 7 and 8  are histograms of the results of color additivity evaluations performed on display panels of amorphous silicon (a-Si) and display panels of low temperature polysilicon (LTPS), respectively, in accordance with embodiments; 
         FIG. 9  is a diagram of a color cube showing a relationship between additive red, green, and blue (RGB) colors and subtractive cyan, magenta, and yellow (CMY) colors; 
         FIG. 10  is a block diagram of a white point calibration system, in accordance with an embodiment; 
         FIG. 11  is a flowchart of a method for calibrating the white point of the display using measurements of subtractive colors, in accordance with an embodiment; 
         FIG. 12  is a plot of several electronic display white points before and after calibration using additive color measurements; 
         FIG. 13  is a plot of electronic display white points before and after calibration using subtractive color measurements, in accordance with an embodiment; 
         FIG. 14  is a comparative box plot of white point calibration using additive and subtractive color space measurements, respectively, measured in a Y direction of the color space, in accordance with an embodiment; and 
         FIG. 15  is a comparative box plot of white point calibration using additive and subtractive color space measurements, respectively, measured in an X direction of the color space, in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     One or more specific embodiments of the present disclosure will be described below. These described embodiments are only examples of the presently disclosed techniques. Additionally, in an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but may nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     When introducing elements of various embodiments of the present disclosure, the articles “a,” “an,” and “the” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Additionally, it should be understood that references to “one embodiment” or “an embodiment” of the present disclosure are not intended to be interpreted as excluding additional embodiments that also incorporate the recited features. 
     As mentioned above, this disclosure relates to calibrating an electronic display for white point using subtractive color measurements rather than exclusively additive color measurements. As will be discussed below, using subtractive color measurements may permit white point calibration that may be accurate whether or not the display exhibits crosstalk behavior between color channels. Specifically, rather than calibrate the electronic display for white point using additive color measurements (e.g., separate red, blue, and green channel measurements), the display&#39;s white point may be calibrated based on subtractive color measurements (e.g., cyan (G+B), magenta (R+B), and yellow (R+G)). Measuring subtractive colors rather than additive colors may account for crosstalk occurring when one color channel interferes with another color channel. Based on such subtractive color measurements, any suitable white point calibration technique may be used to determine white point calibration parameters. The white point calibration parameters may represent, for example, values of a transformation matrix linking International Commission on Illumination (CIE) tristimulus values and RGB color space when the display includes red, green, and blue color channels. 
     With the foregoing in mind, many suitable electronic devices may employ electronic displays calibrated using subtractive color measurements. For example,  FIG. 1  is a block diagram depicting various components that may be present in an electronic device suitable for use with such a display.  FIGS. 2 and 3  respectively illustrate perspective and front views of suitable electronic devices. Specifically,  FIGS. 2 and 3  illustrate a notebook computer and a handheld electronic device, respectively. 
     Turning first to  FIG. 1 , an electronic device  10  according to an embodiment of this disclosure may include, among other things, one or more processor(s)  12 , memory  14 , nonvolatile storage  16 , a display  18  calibrated to have a white point according to white point calibration parameters  20 , input structures  22 , an input/output (I/O) interface  24 , network interfaces  26 , and/or a power source  28 . The various functional blocks shown in  FIG. 1  may include hardware elements (including circuitry), software elements (including computer code stored on a computer-readable medium) or a combination of both hardware and software elements. It should be noted that  FIG. 1  is merely one example of a particular implementation and is intended to illustrate the types of components that may be present in the electronic device  10 . 
     By way of example, the electronic device  10  may represent a block diagram of the notebook computer depicted in  FIG. 2 , the handheld device depicted in  FIG. 3 , or similar devices. In the electronic device  10  of  FIG. 1 , the processor(s)  12  and/or other data processing circuitry may be operably coupled with the memory  14  and the nonvolatile memory  16  to execute instructions. For instance, the processor(s)  12  may generate image data to be displayed on the display  18 . The display  18  may be a touch-screen liquid crystal display (LCD). In some embodiments, the electronic display  18  may be a Multi-Touch™ display that can detect multiple touches at once. The display  18  may operate according to white point calibration parameters  20  programmed based on subtractive color measurements. 
     The input structures  22  of the electronic device  10  may enable a user to interact with the electronic device  10  (e.g., pressing a button to increase or decrease a volume level). The I/O interface  24  may enable electronic device  10  to interface with various other electronic devices, as may the network interfaces  26 . The network interfaces  26  may include, for example, interfaces for a personal area network (PAN), such as a Bluetooth network, for a local area network (LAN), such as an 802.11x Wi-Fi network, and/or for a wide area network (WAN), such as a 3G or 4G cellular network. The power source  28  of the electronic device  10  may be any suitable source of power, such as a rechargeable lithium polymer (Li-poly) battery and/or an alternating current (AC) power converter. 
     The electronic device  10  may take the form of a computer or other suitable type of electronic device. Such computers may include computers that are generally portable (such as laptop, notebook, and tablet computers) as well as computers that are generally used in one place (such as conventional desktop computers, workstations and/or servers). In certain embodiments, the electronic device  10  in the form of a computer may be a model of a MacBook®, MacBook® Pro, MacBook Air®, iMac®, Mac® mini, or Mac Pro® available from Apple Inc. By way of example, the electronic device  10 , taking the form of a notebook computer  32 , is illustrated in  FIG. 2  in accordance with one embodiment of this disclosure. The depicted computer  32  may include a housing  34 , a display  18 , input structures  22 , and ports of an I/O interface  24 . In one embodiment, the input structures  22  (such as a keyboard and/or touchpad) may be used to interact with the computer  32 , such as to start, control, or operate a GUI or applications running on computer  32 . The display  18  may have a white point controlled by the white point calibration parameters  20 . The white point calibration parameters  20  may be determined using subtractive color measurements. As such, the white point calibration parameters  20  may cause the white point of the display  18  to more closely approach a target white point than were the calibration parameters  20  determined using exclusively additive color measurements. 
       FIG. 3  depicts a front view of a handheld device  36 , which represents one embodiment of the electronic device  10 . The handheld device  36  may represent, for example, a portable phone, a media player, a personal data organizer, a handheld game platform, or any combination of such devices. By way of example, the handheld device  36  may be a model of an iPod® or iPhone® available from Apple Inc. of Cupertino, Calif. In other embodiments, the handheld device  36  may be a tablet-sized embodiment of the electronic device  10 , which may be, for example, a model of an iPad® available from Apple Inc. 
     The handheld device  36  may include an enclosure  38  to protect interior components from physical damage and to shield them from electromagnetic interference. The enclosure  38  may surround the display  18 . The I/O interfaces  24  may open through the enclosure  38  and may include, for example, a proprietary I/O port from Apple Inc. to connect to external devices. User input structures  40 ,  42 ,  44 , and  46 , in combination with the display  18 , may allow a user to control the handheld device  36 . For example, the input structure  40  may activate or deactivate the handheld device  36 , the input structure  42  may navigate a user interface to a home screen, a user-configurable application screen, and/or activate a voice-recognition feature of the handheld device  36 , the input structures  44  may provide volume control, and the input structure  46  may toggle between vibrate and ring modes. A microphone  48  may obtain a user&#39;s voice for various voice-related features, and a speaker  50  may enable audio playback and/or certain phone capabilities. A headphone input  52  may provide a connection to external speakers and/or headphones. 
     One example of the display  18  of the electronic device  10  appears in exploded-view form in  FIG. 4 . The display  18  generally includes an LCD panel  60  and a backlight unit  62 , which may be assembled within a frame  64 . As may be appreciated, the LCD panel  60  may include numerous pixels that selectively modulate the amount and color of light passing from the backlight unit  62  through the LCD panel  60 . The LCD panel  60  may employ any suitable liquid crystal display architecture, such as twisted nematic (TN), in-plane switching (IPS), fringe-field switching (FFS), and/or vertical alignment (e.g., multi-domain vertical alignment (MVA) or patterned vertical alignment (PVA)). The backlight unit  62  supplies the light that illuminates the LCD panel  60 . This light derives from a light source  66 , which is routed through portions of the backlight unit  62  before being emitted toward the LCD panel  60 . The light source  66  may include a cold-cathode fluorescent lamp (CCFL), one or more light emitting diodes (LEDs), or any other suitable source of light. 
     The display  18  may operate by activating and programming a number of picture elements, or pixels. These pixels may be generally arranged in a pixel array  100  of the LCD panel  60 , as shown in  FIG. 4 . The pixel array  100  of the display  18  may include a number of unit pixels  102  disposed in a pixel array or matrix. In such an array, each unit pixel  102  may be defined by an intersection of gate lines  104  (also referred to as scanning lines) and source lines  106  (also referred to as data lines). Although only six unit pixels  102  are shown ( 102 A- 102 F), it should be understood that in an actual implementation, the pixel array  100  may include hundreds or thousands of such unit pixels  102 . Each of the unit pixels  102  may represent one of three subpixels that respectively filter only one color (e.g., red, blue, or green) of light. For purposes of this disclosure, the terms “pixel,” “subpixel,” and “unit pixel” may be used largely interchangeably. 
     In the example of  FIG. 4 , each unit pixel  102  includes a thin film transistor (TFT)  108  for switching a data signal supplied to a respective pixel electrode  110 . The potential stored on the pixel electrode  110  relative to a potential of a common electrode  112  may generate an electrical field sufficient to alter the arrangement of a liquid crystal layer of the display  18 . When the arrangement of the liquid crystal layer changes, the amount of light passing through the pixel  102  also changes. A source  114  of each TFT  108  may connect to a source line  106  and a gate  116  of each TFT  108  may connect to a gate line  104 . A drain  118  of each TFT  108  may be connect to a respective pixel electrode  110 . Each TFT  108  may serve as a switching element that may be activated and deactivated by a scanning or activation signal on the gate lines  104 . 
     When activated, a TFT  108  may pass the data signal from its source line  106  onto its pixel electrode  110 . As noted above, the data signal stored by the pixel electrode  110  may be used to generate an electrical field between the respective pixel electrode  110  and a common electrode  112 . This electrical field may align the liquid crystal molecules within the liquid crystal layer to modulate light transmission through the pixel  102 . Thus, as the electrical field changes, the amount of light passing through the pixel  102  may increase or decrease. In general, light may pass through the unit pixel  102  at an intensity corresponding to the applied voltage from the source line  106 . 
     These signals and other operating parameters of the display  18  may be controlled by integrated circuits (ICs) of the display  18 . These driver ICs of the display  18  may include a processor, microcontroller, or application specific integrated circuit (ASIC). The driver ICs may be chip-on-glass (COG) components on a TFT glass substrate, components of a display flexible printed circuit (FPC), and/or components of a printed circuit board (PCB) that is connected to the TFT glass substrate via the display FPC. Further, the driver ICs of the display  18  may include the source driver  120  may include any suitable article of manufacture having one or more tangible, computer-readable media for storing instructions that may be executed by the driver ICs. 
     For instance, a source driver integrated circuit (IC)  120  may receive image data  122  from the processor(s)  12  and send corresponding image signals to the unit pixels  102  of the pixel array  100 . The source driver  120  may also couple to a gate driver integrated circuit (IC)  124  that may activate or deactivate rows of unit pixels  102  via the gate lines  104 . As such, the source driver  120  may provide timing signals  126  to the gate driver  124  to facilitate the activation/deactivation of individual rows (i.e., lines) of pixels  102 . In other embodiments, timing information may be provided to the gate driver  124  in some other manner. 
     The source driver IC  120  may, in some examples, calibrate the image data  122  using the white point calibration parameters  20 . The white point calibration parameters  20  may be stored (e.g., on a boot sector of read only memory (ROM) of the electronic device  10  or the display  18 ) and used to transform the image data  122  before or after the image data  122  is distributed by a multiplexer  128  to the source lines  106 . In other examples, the white point calibration parameters  20  may be encoded into the display circuitry of the display  18  (e.g., voltage supply values and so forth). In still other examples, the white point calibration parameters  20  may be programmed onto other components of the electronic device  10  (e.g., the storage  16 ) and the processor(s)  12  may first adjust the image data  122  before it is provided to the display  18 . 
     The white point calibration parameters  20  may be determined from any suitable image transformation parameters to adjust the image data  122  to account for individual differences from display  18  to display  18 . In some examples, the white point calibration parameters  20  may be determined from a transformation matrix linking International Commission on Illumination (CIE) tristimulus values and RGB color space. Other similar transformation matrices may be employed when the display  18  includes additional or alternative color channels than red, green, and blue. One example of such a transformation matrix appears as Equation 1 below: 
     
       
         
           
             
               
                 
                   
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     In Equation 1, the initial XYZ matrix represents color values in the CIE XYZ color space. The initial XYZ matrix is equal to a product of an XYZ transformation matrix and an RGB matrix, representing a color value in the RGB color space. Each of the coefficients of the XYZ transformation matrix (X r , X g , X b , Y r , Y g , Y b , Z r , Z g , and Z b ) relate the individual contribution of a particular color channel (subscripts r, g, or b) to a particular CIE XYZ color space component (X, Y, or Z). In effect, the nine coefficients of the transformation matrix appear to be composed by the measurement (X, Y, Z) of the full red, green, and blue channels for a particular display  18 . When R=G=B=1, the X, Y, Z matrix on the left-hand side of the equation should equate to the sum of the tristimulus value of the full red, green, and blue channels, which represents white color. As mentioned above, however, such color modeling is based on an assumption that the display  18  has good color additivity. Not all displays  18  exhibit such color additivity. Thus, the coefficients of the XYZ transformation matrix may be determined not by additive color measurements, but rather may be determined based on subtractive color measurements, as will be discussed further below. 
     The use of subtractive color measurements rather than additive color measurements may account for crosstalk that may occur between the various color channels. Indeed, when the multiplexer  128  of the source driver IC  120  is fabricated using certain materials (e.g., low temperature polysilicon, or LTPS), the multiplexer  128  may provide the image data  122  to red, green, and blue pixels  102  with slight but potentially noticeable crosstalk. As seen in  FIG. 6 , a source driver pin  140  may receive the image data  122  which may be, for example, red data  142 , green data  144 , and blue data  146 . The multiplexer  128  may include switches  148 ,  150 , and  152  that switch on and off to pass the image data  122  to a particular source line  106 . In an amorphous silicon (a-Si) implementation, when a particular switch (e.g.,  148 ) is closed, the remaining open switches (e.g.,  150  and  152 ) will not permit any of the image data to pass to the respective source lines  106 . When the multiplexer  128  is implemented in low temperature polysilicon (LTPS), however, some coupling may occur as the switches  148 ,  150 , and  152  may not completely open and/or close. 
     This relationship appears when amorphous silicon (a-Si) displays  18  and low temperature polysilicon (LTPS) displays  18  were tested for color additivity errors in white point. Histograms of such errors appear in  FIG. 7  (a-Si displays  18 ) and  FIG. 8  (LTPS displays  18 ). First considering the color additivity errors in displays  18  of amorphous silicon (a-Si), three plots  160 ,  162 , and  164  of  FIG. 7  respectively represent histograms of color additivity errors in the X, Y, and Z tristimulus values for a number of a-Si displays  18 . As can be seen in the histograms  160 ,  162 , and  164 , cumulative error totals  166  (X),  168  (Y), and  170  (Z) tend to center on an area of substantially zero error. Indeed, though some errors become apparent from each of the histograms,  160 ,  162 , and  164 , the error is generally mild and appears to extend not much farther than a maximum of 1%. 
     For displays  18  of low temperature polysilicon (LTPS), however, the presence of crosstalk may increase the amount of additive color error. In  FIG. 8 , histograms  180 ,  182 , and  184  respectively represent histograms of color additivity errors in X, Y, and Z tristimulus values for a number of LTPS displays  18 . In contrast to the histograms,  160 ,  162 , and  164  of  FIG. 7  that relate to a-Si displays  18 , the histograms  180 ,  182 , and  184  of  FIG. 8  that relate to LTPS displays  18  show a marked increase in color error. Indeed, the cumulative totals  186  (X),  188  (Y), and  190  (Z) of appear to extend to as much as around 3% in the various X, Y, and Z components for some displays  18 , which could significantly impact the accuracy of white point calibration. 
     A white point calibration based on subtractive color measurements, rather than exclusively additive color measurements, may account for some of these errors. Such subtractive color based white point calibration may compensate for crosstalk-induced color non-linearity. Moreover, the subtractive color based white point calibration of this disclosure may effectively calibrate displays  18  that may exhibit substantial crosstalk (e.g., LTPS displays  18 ) and those that may not (e.g., a-Si displays). 
     For a display  18  with red, green, and blue channels, white point calibration may take place using subtractive colors of cyan, magenta, and yellow. As mentioned above, in Equation 1 above, the nine coefficients of the transformation matrix are composed by the measurement (X, Y, Z) of the full red, green, and blue channels for a particular display  18 . When R=G=B=1, the X, Y, Z matrix on the left-hand side of the equation should equate to the sum of the tristimulus value of the full red, green, and blue channels, which represents white color. As also mentioned above, however, such color modeling is based on an assumption that the display  18  has good color additivity. Not all displays  18  exhibit such color additivity. For example, an LTPS display  18  may have an inaccurate white point calibration when the coefficients of the XYZ transformation matrix are determined exclusively from additive color measurements. 
     As such, subtractive colors of RGB (cyan, magenta, and yellow) may be used instead. These subtractive colors of the RGB color space are described in a color cube  200  shown in  FIG. 9 . In the color cube  200 , the color black  202  represents an absence of light of any color. Extending from the color black  202  are three channels of colored light: red  204 , green  206 , and blue  208 . Subtractive colors yellow  210 , magenta  212 , and cyan  214  may be formed by the combination of two of the three channels red  204 , green  206 , and blue  208 . Specifically, yellow  210  may represent a combination of red  204  and green  206 ; magenta  212  may represent a combination of red  204  and blue  208 ; and cyan  214  may represent a combination of green  206  and blue  208 . White  216  may represent the combination of red  204 , green  206 , and blue  208  or, accordingly, the combination of cyan  214 , magenta  212 , and yellow  210 . 
     As will be noted below, the subtractive RGB colors cyan, magenta, and yellow may be measured and recomposed into a new 3×3 matrix made up of coupled red, green, and blue. The new XYZ transformation matrix, which may take the place of the XYZ transformation matrix of Equation 1, appears below relating various measurements of cyan, magenta, and yellow: 
     
       
         
           
             
               
                 
                   
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     In Equation 2 above, the 3×3 XYZ transformation matrix on the left-hand side of the equation is made up of coupled red, green, and blue (thus, its coefficients include subscripts r′, g′, and b′). This 3×3 XYZ transformation matrix may take the place of the 3×3 XYZ transformation matrix shown in Equation 1. Because the 3×3 XYZ transformation matrix is made up of coupled color values, color activity may be re-established while taking into account the effect of crosstalk between the color channels. Namely, since subtractive color is, by definition, the combination of neighbor subpixels  102 , measuring subtractive color already accounts for crosstalk components without any complex or inefficient calculations or characterizations. For example, a yellow color output by the display  18  may not be composed of pure red and green, but rather coupled red and green. Likewise, magenta output by the display  18  is composed of coupled red and blue, and cyan is composed of coupled blue and green. 
     Each subtractive color based coefficient (X r′ , X g′ , X b′ , Y r′ , Y g′ , Y b′ , Z r′ , Z g′ , Z b′ ) of the 3×3 XYZ transformation matrix of Equation 2 may be determined from three subtractive color measurements, as apparent in Equation 2. Namely, to obtain the coefficients relating X, Y, and Z to respective RGB channels, a subtractive color measurement that does not include an RGB color channel of interest may be subtracted from the two other measurements that do include the RGB color channel of interest. For example, to obtain the coefficient relating the X color component and the red channel (X r′ ), a value of the X color component from a measurement of cyan (B+G) may be subtracted from a sum of the measurements of yellow (R+G) and magenta (R+B), leaving only the value of the X color component relating to the red channel—though with the crosstalk effects of the other channels thereby included. 
     Using these subtractive color measurements, a calibration system  230  may calibrate a display  18 , as shown in  FIG. 10 . A calibration test controller  232  may receive an indication of various responses from light sensor(s)  234 , which may measure light  236  emitted by the display  18 . The calibration test controller  232  may represent any suitable computing equipment that may perform a white point calibration based on subtractive color measurements. For instance, the calibration test controller  232  may carry out instructions stored on memory used by the calibration test controller  232 . The instructions running on the test controller  232  may cause the display  18  to display certain test data to emit the light  236 . The light sensor(s)  234  may represent any suitable light sensors (e.g., an imaging device such as a camera and/or photodiodes) that can perform measurements of subtractive colors. In some examples, the light sensor(s)  234  may specifically measure red, green, and blue light. In other examples, the light sensor(s)  234  may specifically measure cyan, magenta, and yellow light. In others, the light sensor(s)  234  may measure other colors of light and/or white light. The light sensor(s)  234  may, in some embodiments, include spectroscopic sensors. 
     The calibration test controller  232  may calibrate the white point of the display  18  using any suitable method. One such method may be the iterative approach described by U.S. patent application Ser. No. 13/477,680, “Method and Apparatus for Display Calibration,” filed on May 22, 2012, which is assigned to Apple Inc. and incorporated by reference herein in its entirety. Regardless of the particular manner of determining the white point calibration parameters  20 , the calibration test controller  232  generally may base the determination of white point calibration parameters  20  on subtractive color measurements rather than exclusively additive color measurements. 
     Specifically, regardless of the number of times measurements are obtained, subtractive color measurements may be obtained for each subtractive color at least once, as shown by a flowchart  240  of  FIG. 11 . Namely, the calibration test controller  232  may cause the display  18  to emit light of a first subtractive color (e.g., cyan), measuring the response of the light sensor(s)  234  (block  242 ). The calibration test controller  232  may cause the display  18  to emit light of a second subtractive color (e.g., magenta), measuring the response of the light sensor(s)  234  (block  244 ). Finally, the calibration test controller  232  may cause the display  18  to emit light of a third subtractive color (e.g., yellow), measuring the response of the light sensor(s)  234  (block  246 ). 
     As described above in Equation 3, three subtractive color measurements may be used to obtain an XYZ transformation matrix for an RGB color space. From the relationships shown in Equations 1 and 2, the calibration test controller  232  may determine the white point calibration parameters  20  based on the subtractive color measurements of blocks  242 ,  244 , and  246 , (block  248 ). Indeed, the calibration test controller  232  may ascertain appropriate white point calibration parameters  20  for each color RGB channel using the relationships discussed above in relation to Equations 1 and 2. In essence, the calibration test controller  232  may select white point calibration parameters  20  that are expected, based on the subtractive color measurements of blocks  242 ,  244 , and  246  and the relationships of Equations 1 and 2, to cause the display  18  to have a white point near to a target white point (e.g., within an acceptable range of white points). As such, the calibration test controller  232  may subtract subtractive color measurements that do not include an RGB color channel of interest from the two other measurements that do include the RGB color channel of interest to isolate the individual color channels (while still effectively measuring the effect of crosstalk with other color channels). For example, to obtain responses particularly associated with to the red color channel, a measurement of cyan (B+G) may be subtracted from a sum of the measurements of yellow (R+G) and magenta (R+B), leaving only the value relating to the red channel—though with the crosstalk components of the other channels thereby included. In determining the white point calibration parameters  20 , the calibration test controller  232  may employ any suitable technique, including those that may involve additional measurements from the display  18 . In most cases, however, at least the subtractive color measurements indicated at blocks  242 ,  244 , and  246  may be performed to account for crosstalk that may occur when two or more channels are active. 
     When the calibration test controller  232  has determined the white point calibration parameters  20 , the calibration test controller  232  may store the white point calibration parameters  20  into the display  18  (block  250 ). It should be appreciated that the flowchart  240  of  FIG. 11  is intended to be illustrative and should not be taken as representing the only manner of calibrating the white point of the display  18 . The purpose of the flowchart  240  of  FIG. 11  is to convey that the display  18  may be calibrated based on subtractive color measurements (e.g., cyan, magenta, and yellow) rather than only additive measurements (e.g., red, green, and blue). 
     The impact of using subtractive rather than additive color to calibrate white point may be significant. The effects may be especially noticeable when used with displays  18  of low temperature polysilicon (LTPS), which may be prone to crosstalk between color channels.  FIGS. 12 and 13  describe a comparison between performing white point calibration on a number of displays by measuring additive colors ( FIG. 12 ) and measuring subtractive colors ( FIG. 13 ). In a plot  260  shown in  FIG. 12 , an ordinate  262  represents Y component values of color and an abscissa  264  represents X component values of color in the CIE XYZ color space. A white point specification tolerance  266  represents a range of acceptable white points. The white points of displays  18  are represented by circles  268  before calibration and as x&#39;s  270  after calibration. As indicated by  FIG. 12 , before calibration, most of the displays  18  fall outside of the calibration white point specification range  266 . After calibration using additive color measurements, most of the displays  18  are successfully calibrated to within the acceptable white point specification range  266 , but not all. Moreover, while most of the displays  18  are shown to have been successfully calibrated to within the acceptable white point specification range  266 , the white points of the displays  18  fall within a relatively wide range. 
     In comparison, as shown by a plot  280  of  FIG. 13 , when the displays  18  are calibrated using subtractive color measurements instead, the calibration may place the white points of the displays  18  more squarely within the white point specification range  266 . In the plot  260  shown in  FIG. 13 , the ordinate  262  represents Y component values of color and the abscissa  264  represents X component values of color in the CIE XYZ color space. As indicated above, the white point specification tolerance  266  shown in  FIG. 13  represents the range of acceptable white point colors, and the white points of displays  18  are represented by circles  268  before calibration and as x&#39;s  270  after calibration. In the example of  FIG. 13 , the displays  18  calibrated using subtractive color measurements have white points that are much more likely to remain within white point specification range  266 . Moreover, the displays  18  are more likely to have white points that are much more close to the center of the white point specification range  266 . 
       FIGS. 14 and 15  similarly support calibrating white points using subtractive color measurements rather than exclusively additive color measurements. In  FIG. 14 , a comparative box plot  290  illustrates white point distributions for samples of displays  18  before and after calibration using either additive or subtractive color measurements. In the comparative box plot  290 , an abscissa  292  represents a range of X components of the XYZ color space. A nominal target white point X value  294  (an X value of 0.308), a high limit of acceptable white point X values  296  (an X value of 0.311), and low limit of acceptable white point X values  298  (an X value of 0.305) are shown. A white point distribution  300  denotes a range of initial white point values for a sample of displays  18 . After a white point calibration using additive color values, the same sample of displays  18  has a calibrated white point distribution  302 . The calibrated white point distribution  302  is much closer to the nominal target white point X value  294  (0.308), having an average X value of 0.307 with a standard deviation of 0.0007. 
     Yet a white point calibration using subtractive color measurements yields even better results. In the comparative box plot  290 , a white point distribution  304  denotes another range of initial white point values for a sample of displays  18 . Despite this wider range of white points, after a white point calibration using subtractive color values, the same sample of displays  18  has a calibrated white point distribution  306 . The calibrated white point distribution  306  is even closer to the nominal target white point X value  294  (0.308), having an average X value of 0.308 with a standard deviation of 0.0004. 
     Similarly,  FIG. 14  illustrates a comparative box plot  310  showing white point distributions in the Y color component for samples of displays  18  before and after calibration using either additive or subtractive color measurements. In the comparative box plot  310 , an abscissa  312  represents a range of Y components of the XYZ color space. A nominal target white point Y value  314  (a Y value of 0.326), a high limit of acceptable white point Y values  316  (a Y value of 0.329), and low limit of acceptable white point Y values  318  (a Y value of 0.323) are shown. A white point distribution  320  denotes a range of initial white point values for a first sample of displays  18 . After a white point calibration using additive color values, the same sample of displays  18  has a calibrated white point distribution  322 . The calibrated white point distribution  322  is closer to the nominal target white point Y value  314  (0.326), having an average Y value of 0.326 with a standard deviation of 0.0007. These results are similar to those shown in the plot  290  of  FIG. 14 . 
     Like the comparative box plot  290  of  FIG. 14 , the comparative box plot  310  also shows that a white point calibration using subtractive color measurements may yield even better results. In the comparative box plot  310 , a white point distribution  324  denotes another range of initial white point values for a sample of displays  18 . Despite this wider range of white points, after a white point calibration using subtractive color values, the same sample of displays  18  has a calibrated white point distribution  326 . The calibrated white point distribution  326  is even closer to the nominal target white point Y value  314  (0.326), having an average X value of 0.326, but with a much smaller standard deviation of 0.0003. 
     Technical effects of this disclosure include a more effective manner of calibrating the white point of an electronic display. Indeed, even when the electronic display exhibits crosstalk characteristics, the use of subtractive color measurements may substantially improve the resulting white point behavior after white point calibration. Moreover, measuring subtractive color rather than merely additive color may be effective regardless of the degree to which the electronic display exhibits crosstalk between color channels (e.g., LTPS displays vs. a-Si displays). It may also be noted that, because the use of subtractive color measurements may result in tighter tolerances, using subtractive color measurements with an iterative color white point calibration may result in faster convergence to an acceptable white point within a particular white point specification range. This may save valuable time and may result in more displays that become acceptably calibrated within a maximum period of time allotted to calibrate a display. 
     The specific embodiments described above have been shown by way of example, and it should be understood that these embodiments may be susceptible to various modifications and alternative forms. It should be further understood that the claims are not intended to be limited to the particular forms disclosed, but rather to cover all modifications, equivalents, and alternatives falling within the spirit and scope of this disclosure. For example, although some of the particular examples discussed above relate to displays with red, green, and blue color channels, subtractive color measurements may benefit displays with any other suitable color channels. Moreover, while this disclosure has described white point calibration of a liquid crystal display, essentially any other form of display (e.g., organic light emitting diode (OLED) display, plasma display, cathode ray tube (CRT) display) may benefit from white point calibration according to this disclosure, particularly if such displays exhibit crosstalk between color channels.

Metadata:
Filing Date: 20130411
Publication Date: 20150707
Grant Date: 20150707
Priority Date: 20120911
Inventors: CHU CHIA-CHING
MARCU GABRIEL G.
WU JIAYING
Assignee: APPLE INC
CPC Classifications: [{"code": "G06T11/001", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G2310/0297", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04N1/6058", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G2320/0666", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G3/3648", "inventive": true, "first": true, "tree": "[]"}, {"code": "G09G3/3648", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G2340/06", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/0666", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2340/06", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G3/2003", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06T3/4007", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2310/0297", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2340/06", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2310/0297", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/0666", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G3/3648", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G3/2003", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06T3/4007", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06T11/001", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N1/6058", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 50232844