Patent Publication Number: US-8970468-B2

Title: Dynamic color adjustment for displays

Description:
BACKGROUND 
     This relates generally to electronic devices with displays and, more particularly, to electronic devices with calibrated displays. 
     Electronic devices such as portable computers, media players, cellular telephones, set-top boxes, and other electronic equipment are often provided with displays for displaying visual information. 
     Displays are often capable of displaying color images. However, the color response of a display may change as the display operates. For example, changing operating conditions such as changing display temperature may affect the color response of a display. Some displays depict white as somewhat yellowish when initially powered on and cold. As the display warms, the white point of the display shifts toward a more neutral white, such as that defined by the standard illuminant, D65. Other display colors such as skin tone colors may also experience shifts within a color space as the temperature of the display changes. Similarly, other parameters of the display may shift as a function of temperature such as luminance, black level, contrast, or electro-optical transfer function, which may be referred to as the “native gamma” of the display. This set of parameters may be referred to as the color profile of the display. 
     The shift in the color profile due to temperature changes in the display generally causes each pixel of the display to change color until a stable operating temperature is achieved, at which point the pixel colors may likewise be stable. That is, although a pixel may be instructed to display the same color at an initial temperature and a stable operating temperature, the actual color displayed, as objectively measured by its chromaticity and luminance, may vary. 
     Displays are sometimes calibrated to account for temperature induced white point shifts. Conventional methods include applying temperature dependent adjustment values to input pixel values to obtain adapted input pixel values. However, the temperature dependent adjustment values are typically optimized for neutral colors such as white. Applying these temperature dependent adjustment values to non-neutral colors leads to an undesirable color shift for the non-neutral colors. 
     It would therefore be desirable to be able to provide improved ways of calibrating electronic devices with color displays. 
     SUMMARY 
     An electronic device may include a display and display control circuitry. The display may be calibrated during manufacturing using a calibration system. The calibration system may include calibration computing equipment coupled to a light sensor and may be used to gather display performance information from the display. The display performance information may be recorded as a function of one or more input parameters such as display temperature. Gathering display performance information may include measuring display luminance and chromaticity values. 
     Display performance information may be used to calculate color-specific and temperature-dependent adjustment values. For example, a table of adjustment values may be derived for each color in a number of different colors. Each table of adjustment values may include a number of different adjustment values corresponding respectively to different display temperatures. The tables of adjustment values may be stored in the electronic device. 
     Display control circuitry in the electronic device may use the stored adjustment values to adjust display colors in order to compensate for temperature related shifts in display colors. 
     The display control circuitry may receive incoming pixel values for a display pixel. The display control circuitry may also receive display temperature information from a temperature sensor in the electronic device. 
     The display control circuitry may determine the color associated with the incoming pixel values. This may include determining a ratio of a red pixel value to a green pixel value to a blue pixel value. The display control circuitry may determine adjustment values to apply to the incoming pixel values based on the display temperature information and the color associated with the incoming pixel values. 
     If the color associated with the incoming pixel values does not exactly match any of the colors for which adjustment values have been stored, methods such as a combination of Inverse Distance Weighting and Delaunay Triangulation may be used to interpolate adjustment values for the incoming pixel values. 
     If the color associated with the incoming pixel values matches one of the colors for which adjustment values have been stored, the stored adjustment values may be directly applied to the incoming pixel values. 
     The display control circuitry may apply the adjustment values to the incoming pixel values to obtain adapted pixel values. The display control circuitry may provide the adapted pixel values to the display pixel. 
     The display control circuitry may determine whether a color associated with incoming pixel values falls within a predetermined subset of colors. The predetermined subset of colors may, for example, be a subset of skin tone colors, a subset of grass tone colors, or a subset of sky tone colors. 
     If the color associated with the incoming pixel values does not fall within one of the predetermined subsets of colors, the display control circuitry may apply adjustment values of a first type to the incoming pixel values. The adjustment values of the first type may be values that compensate for temperature related white point shifts. 
     If the color associated with the incoming pixel values falls within one of the predetermined subsets of colors, the display control circuitry may apply adjustment values of a second type to the incoming pixel values. The adjustment values of the second type may be values that compensate for temperature related shifts in an associated one of the predetermined subsets of colors. 
     Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of an illustrative electronic device such as a portable computer having a calibrated display in accordance with an embodiment of the present invention. 
         FIG. 2  is a diagram of an illustrative electronic device such as a cellular telephone or other handheld device having a calibrated display in accordance with an embodiment of the present invention. 
         FIG. 3  is a diagram of an illustrative electronic device such as a tablet computer having a calibrated display in accordance with an embodiment of the present invention. 
         FIG. 4  is a diagram of an illustrative electronic device such as a computer monitor with a built-in computer having a calibrated display in accordance with an embodiment of the present invention. 
         FIG. 5  is a schematic diagram of an illustrative electronic device having a calibrated display in accordance with an embodiment of the present invention. 
         FIG. 6  is a diagram of a portion of an illustrative display showing how colored display pixels may be arranged in rows and columns in accordance with an embodiment of the present invention. 
         FIG. 7  is a chromaticity diagram showing how changes in display temperature may cause colors to shift within a color space. 
         FIG. 8  is a chromaticity diagram showing illustrative colors for which tables of adjustment values may be calculated in accordance with an embodiment of the present invention. 
         FIG. 9  is a diagram of an illustrative calibration system for performing display calibration including calibration computing equipment and a test chamber having a light sensor in accordance with an embodiment of the present invention. 
         FIG. 10  is an illustrative table of color-specific adjustment values optimized for neutral colors in accordance with an embodiment of the present invention. 
         FIG. 11  is an illustrative table of color-specific adjustment values optimized for yellowish colors in accordance with an embodiment of the present invention. 
         FIG. 12  is an illustrative table of color-specific adjustment values optimized for greenish blue colors in accordance with an embodiment of the present invention. 
         FIG. 13  is a flow chart of illustrative steps involved in obtaining tables of adjustment factors for multiple colors in accordance with an embodiment of the present invention. 
         FIG. 14  is a flow chart of illustrative steps involved in performing display color correction using color-specific and temperature-dependent adjustment factors in accordance with an embodiment of the present invention. 
         FIG. 15  is a flow chart of illustrative steps involved in performing display color correction using first and second types of temperature-dependent adjustment factors in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Electronic devices such as cellular telephones, media players, computers, set-top boxes, wireless access points, and other electronic equipment may include displays. Displays may be used to present visual information and status data and/or may be used to gather user input data. 
     Displays may be configured to display color images. For example, displays may include color display pixels configured to create colored light. Individual pixels of a display may receive a red, green, and blue value that together define the color to be created by the pixel. These red, green, and blue values are referred to herein in the aggregate as an “RGB value,” as understood to those of ordinary skill in the art. 
     If care is not taken, display colors may shift within a color space as the temperature of a display varies. To account for changes in display operating temperature, display colors may be adjusted using adjustment values. For example, an adjustment value may be applied to an input RGB value to obtain an adapted RGB value that accounts for changes in display temperature. The adjustment value may be based on the color associated with the RGB value and on the display temperature. The adjustment value may be found in a look-up table or may be computed by interpolating from the values found in the table. The adjustment value may be applied, depending on the type of display, to an RGB value that may be supplied to a display pixel or to the gain of the red channel, green channel, and blue channel to adjust the colors of the display. 
     Display colors may be corrected as the display warms up and changes temperature. Display performance information such as luminance and chromaticity values may be recorded for different RGB input values and for every temperature in a set of temperatures. The display may produce a color range that may be referred to herein as the “display color gamut.” The display color gamut may determined based on the recorded data using either a matrix multiplication and gamma correction based model (called the matrix model) or a look-up table and optional interpolation based model, called the “LUT model.” Generally, a color model is a way of representing the correspondence between colors as measured by an instrument on the display and the RGB values that produce these colors on the display. The table based model may be created, for example, by empirically measuring luminance and chromaticity for a variety of pixel colors expressed in RGB values and comparing them to desired or perceived luminance and chromaticity values. 
     These desired values generally correspond to the luminance and chromaticity that are set as the luminance and chromaticity target values for that display. The target may correspond to the luminance and chromaticity of the displayed color when the electronic display has achieved its stable operating temperature. Alternatively, the target may correspond to a different set of luminance and chromaticity values. For example, the target values may be those recommended by a certain standard or selected by the user according to particular needs. As another example, a fixed luminance and D65 reference white point may be used as a target for white. Also, the target may be specified by a luminance and chromaticity value that varies according to a precise function selected by the user. In short, the target luminance and chromaticity for a given color can be an arbitrary set. At various temperature values, certain color models may be more suitable than others for coding the colors produced by that device. There may be multiple color models such that each individual color model corresponds to a specific temperature. Thus, as the temperature of the display increases, the color model of the display (or its component pixels) may change. 
     A target state of the display may be defined as a chromaticity value and a luminance value of the display. For a specific temperature and color for which the parameters of the color model have been measured, the adjustment values for each R, G, and B components may be computed using the color models and the target luminance and chromaticity value. The RGB adjustment values may be organized into tables such that each line in a given table provides the RGB adjustment values corresponding to specific temperature and a specific color. For an arbitrary color that is not included in the set of tables, the corresponding RGB adjustment values may be computed by interpolating the RGB adjustment values among two or more tables. For an arbitrary temperature value that is not included in a given table, the corresponding RGB adjustment values may be computed by interpolating the RGB adjustment values in that table. These tables may sometimes be referred to herein as RGB adjustment value tables. 
     An illustrative electronic device of the type that may be provided with a calibrated display is shown in FIG.  1 . Electronic device  10  may be a computer such as a computer that is integrated into a display such as a computer monitor, a laptop computer, a tablet computer, a somewhat smaller portable device such as a wrist-watch device, pendant device, or other wearable or miniature device, a cellular telephone, a media player, a tablet computer, a gaming device, a navigation device, a computer monitor, a television, or other electronic equipment. 
     As shown in  FIG. 1 , device  10  may include a display such as display  14 . Display  14  may be a touch screen that incorporates capacitive touch electrodes or other touch sensor components or may be a display that is not touch-sensitive. Display  14  may include image pixels formed from light-emitting diodes (LEDs), organic light-emitting diodes (OLEDs), plasma cells, electrophoretic display elements, electrowetting display elements, liquid crystal display (LCD) components, or other suitable image pixel structures. Arrangements in which display  14  is formed using liquid crystal display pixels are sometimes described herein as an example. This is, however, merely illustrative. Any suitable type of display technology may be used in forming display  14  if desired. 
     Device  10  may have a housing such as housing  12 . Housing  12 , which may sometimes be referred to as a case, may be formed of plastic, glass, ceramics, fiber composites, metal (e.g., stainless steel, aluminum, etc.), other suitable materials, or a combination of any two or more of these materials. 
     Housing  12  may be formed using a unibody configuration in which some or all of housing  12  is machined or molded as a single structure or may be formed using multiple structures (e.g., an internal frame structure, one or more structures that form exterior housing surfaces, etc.). 
     As shown in  FIG. 1 , housing  12  may have multiple parts. For example, housing  12  may have upper portion  12 A and lower portion  12 B. Upper portion  12 A may be coupled to lower portion  12 B using a hinge that allows portion  12 A to rotate about rotational axis  16  relative to portion  12 B. A keyboard such as keyboard  18  and a touch pad such as touch pad  20  may be mounted in housing portion  12 B. 
     In the example of  FIG. 2 , device  10  has been implemented using a housing that is sufficiently small to fit within a user&#39;s hand (e.g., device  10  of  FIG. 2  may be a handheld electronic device such as a cellular telephone). As show in  FIG. 2 , device  10  may include a display such as display  14  mounted on the front of housing  12 . Display  14  may be substantially filled with active display pixels or may have an active portion and an inactive portion. Display  14  may have openings (e.g., openings in the inactive or active portions of display  14 ) such as an opening to accommodate button  22  and an opening to accommodate speaker port  24 . 
       FIG. 3  is a perspective view of electronic device  10  in a configuration in which electronic device  10  has been implemented in the form of a tablet computer. As shown in  FIG. 3 , display  14  may be mounted on the upper (front) surface of housing  12 . An opening may be formed in display  14  to accommodate button  22 . 
       FIG. 4  is a perspective view of electronic device  10  in a configuration in which electronic device  10  has been implemented in the form of a computer integrated into a computer monitor. As shown in  FIG. 4 , display  14  may be mounted on a front surface of housing  12 . Stand  26  may be used to support housing  12 . 
     A schematic diagram of electronic device  10  is shown in  FIG. 5 . As shown in  FIG. 5 , electronic device  10  may include a display such as display  14 . Display  14  may include light-emitting components  32 , touch-sensitive circuitry  30 , display control circuitry  28  for operating light-emitting components  32 , and other display components. 
     Light-emitting components  32  may include display pixels formed from reflective components, liquid crystal display (LCD) components, organic light-emitting diode (OLED) components, or other suitable display pixel structures. To provide display  14  with the ability to display color images, light-emitting components  32  may include display pixels having color filter elements. Each color filter element may be used to impart color to the light associated with a respective display pixel in the pixel array of display  14 . 
     Display touch circuitry such as touch-sensitive circuitry  30  may include capacitive touch electrodes (e.g., indium tin oxide electrodes or other suitable transparent electrodes) or other touch sensor components (e.g., resistive touch technologies, acoustic touch technologies, touch sensor arrangements using light sensors, force sensors, etc.). Display  14  may be a touch screen that incorporates display touch circuitry  30  or may be a display that is not touch-sensitive. 
     Display control circuitry  28  may include a graphics controller (sometimes referred to as a video card or video adapter) that may be used to provide video data and control signals to display  14 . Video data may include text, graphics, images, moving video content, or other content to be presented on display  14 . 
     Display control circuitry  28  may also include display driver circuitry. Display driver circuitry in circuitry  28  may be implemented using one or more integrated circuits (ICs) and may sometimes be referred to as a driver IC, display driver integrated circuit, or display driver. Display driver circuitry may include, for example, timing controller (TCON) circuitry such as a TCON integrated circuit. If desired, display driver circuitry may be mounted on an edge of a thin-film-transistor substrate layer in display  14  (as an example). Display control circuitry  28  may be coupled to additional circuitry in device  10  such as storage and processing circuitry  34 . 
     Device  10  may include a thermal sensor such as thermal sensor  60 . Thermal sensor  60  may be an internal sensor in device  10  configured to gather temperature data from device  10 . Thermal sensor  60  may be used to measure display temperature, display cover glass temperature (e.g., the temperature associated with a cover glass layer that covers display  14 ), backlight temperature (e.g., a the temperature associated with light-emitting diodes that provide backlight for display  14 ), internal component temperature (e.g., the temperature associated with an internal component in device  10  such as a central processing unit (CPU) or other component), etc. Temperature information gathered by sensor  60  may sometimes be referred to herein as “display temperature” or “device temperature.” 
     Control circuitry such as storage and processing circuitry  34  in device  10  may include microprocessors, microcontrollers, digital signal processor integrated circuits, application-specific integrated circuits, and other processing circuitry. Volatile and non-volatile memory circuits such as random-access memory, read-only memory, hard disk drive storage, solid state drives, and other storage circuitry may also be included in circuitry  34 . Display calibration information may be stored using circuitry  34  or may be stored using display control circuitry  28  or other circuitry associated with display  14 . 
     Circuitry  34  may use wireless communications circuitry  36  and/or input-output devices  50  to obtain user input and to provide output to a user. Input-output devices  50  may include speakers, microphones, sensors, buttons, keyboards, displays, touch sensors, and other components for receiving input and supplying output. Wireless communications circuitry  36  may include wireless local area network transceiver circuitry, cellular telephone network transceiver circuitry, and other components for wireless communication. 
     Display calibration information such as color-specific and temperature-specific adjustment values may be loaded onto device  10  during manufacturing. The stored adjustment values may be used to adjust display colors in order to compensate for changes in display temperature. Adjustment values may be stored in any suitable location in electronic device  10 . For example, adjustment values may be stored in storage and processing circuitry  34  or in display control circuitry  28 . 
     In one suitable embodiment, a display TCON integrated circuit in circuitry  28  may receive input RGB values from storage and processing circuitry  34  and may receive display temperature information from thermal sensor  60 . Based on the input RGB values and the temperature information, the TCON integrated circuit may determine a color-specific and temperature-specific adjustment value for each input RGB value. The TCON integrated circuit may apply the adjustment values to either the input RGB values or to the gain control of the RGB channels. The adjustment values may change the display colors such that the display colors appear as the target color. 
     A portion of an illustrative array of display pixels that may be used in display  14  is shown in  FIG. 6 . As shown in  FIG. 6 , display  14  may have a pixel array with rows and columns of pixels such as display pixels  52 . There may be tens, hundreds, or thousands of rows and columns of display pixels  52 . Each pixel  52  may, if desired, be a color pixel such as a red (R) pixel, a green (G) pixel, a blue (B) pixel or a pixel of another color. Red pixels R, for example, may include a red color filter element over a light generating element (e.g., a liquid crystal pixel element or an OLED pixel element) that absorbs and/or reflects non-red light while passing red light. This is, however, merely illustrative. Pixels  52  may include any suitable structures for generating light of a given color. 
     Pixels  52  may include pixels of any suitable color. For example, pixels  52  may include a pattern of cyan, magenta, and yellow pixels, or may include any other suitable pattern of colors. Arrangements in which pixels  52  include a pattern of red, green, and blue pixels is sometimes described herein as an example. 
     Display control circuitry  28  ( FIG. 5 ) such as a display driver integrated circuit and, if desired, associated thin-film transistor circuitry formed on a display substrate layer may be used to produce signals such as data signals and gate line signals (e.g., on data lines and gate lines respectively in display  14 ) for operating pixels  52  (e.g., turning pixels  52  on and/or off and/or adjusting the intensity of pixels  52 ). During operation, display control circuitry  28  may control the values of the data signals and gate signals to control the light intensity associated with each of the display pixels and to thereby display images on display  14 . 
     Display control circuitry  28  may be used to convert input RGB values (sometimes referred to as digital display control values) for each display pixel  52  into analog display signals for controlling the brightness of each pixel. Control circuitry such as storage and processing circuitry  34  may provide input RGB values (commonly integers with values ranging from 0 to 255) corresponding to the desired pixel intensity of each pixel to display control circuitry  28 . For example, a digital display control value of 0 may result in an “off” pixel, whereas a digital display control value of 255 may result in a pixel operating at a maximum available power. 
     It should be appreciated that these are examples of 24-bit color in which each color channel has eight bits dedicated to it. Alternative embodiments may employ greater or fewer bits per color channel. For example, display  14  may support 18-bit color in which each color has six bits dedicated to it. With this type of configuration, input RGB values may be a set of integers ranging from 0 to 64. Arrangements in which display  14  supports 24-bit color are sometimes described herein as an example. 
     Display control circuitry  28  may be used to concurrently operate pixels  52  of different colors in order to generate light having a color that is a mixture of, for example, primary colors red, green, and blue. As examples, operating red pixels R and blue pixels B at equal intensities may produce light that appears violet, operating red pixels R and green pixels G at equal intensities may generate light that appears yellow, operating red pixels R and green pixels G at maximum intensity while operating blue pixels B at half of maximum intensity may generate light that appears “yellowish,” operating red pixels R, green pixels G, and blue pixels B simultaneously at maximum intensity may generate light that appears white, etc. 
     However, due to variations in display temperature, some internal parameters of the display may change, which may in turn affect the luminance and the chromaticity of the displayed color, even if the RGB input signal is not changed. For example, displayed colors may vary with temperature. 
     A chromaticity diagram illustrating this type of temperature induced color shift is shown in  FIG. 7 . The chromaticity diagram of  FIG. 7  illustrates a two-dimensional projection of a three-dimensional color space. The color generated by a display such as display  14  may be represented by chromaticity values x and y. The chromaticity values may be computed by transforming, for example, three color intensities (e.g., intensities of colored light emitted by a display) such as intensities of red, green, and blue light into three tristimulus values X, Y, and Z and normalizing the first two tristimulus values X and Y (e.g., by computing x=X/(X+Y+Z) and y=Y/(X+Y+Z) to obtain normalized x and y values). Transforming color intensities into tristimulus values may be performed using transformations defined by the International Commission on Illumination (CIE) or using any other suitable color transformation for computing tristimulus values. 
     Any color generated by a display such as display  14  may therefore be represented by a point (e.g., by chromaticity values x and y) on a chromaticity diagram such as the diagram shown in  FIG. 7 . Bounded region  54  of  FIG. 7  represents the limits of visible light that may be perceived by humans (i.e., the total available color space). The colors that may be generated by a display are contained within a subregion of bounded region  54 . 
     Changing display temperatures may have a noticeable impact on colors being displayed on the display. At an initial power-on state, a display may have an initial white point that lies within bounded region  56 . The initial white point of the display within bounded region  56  may appear on the display as a yellowish color. As time passes, the physical display temperature may increase to a stable value. The increase in display temperature may induce a corresponding change in the display white point. For example, as the display warms up to a stable operating temperature, the display white point may shift from bounded region  56  to bounded region  58 . A white point that lies within bounded region  58  may appear accurately rendered (e.g., may appear as a neutral white). If a display continues to warm up beyond a stable operating temperature, the display white point may appear slightly blue. It should be noted that the actual objective display white point may shift from region  56  to region  58  even when the RGB input values do not change. Other colors may experience a similar shift within bounded region  54  as a result of changing display temperature. 
     Displays are sometimes calibrated to compensate for temperature induced white point shifts. Conventional methods involve applying adjustment values (adjustment values that are optimized for white point correction) to RGB input values based on the device temperature. When these white-point-optimized adjustment values are applied to other colors, the colors often experience undesirable color shifts as the display temperature varies. For example, when white-point-optimized adjustment values are applied to skin tone colors, the skin tone colors appear bluish at higher display temperatures. 
     To overcome this type of temperature induced color distortion, adjustment values may be derived for specific colors. Rather than applying white-point-optimized adjustment values to all colors, a set of color-specific adjustment values may be derived for any desired color. For example, a set of adjustment values may be derived for yellowish green, a set of adjustment values may be derived for bluish red, a set of adjustment values may be derived for magenta, etc. As another example, sets of adjustment values may be derived for high-importance colors such as skin tone colors, grass tone colors, and sky tone colors. 
     Each set of color-specific adjustment factors may be used to adjust RGB input values to compensate for temperature changes. The color-specific adjustment values may be used to ensure that a given display color remains at the “target color” even as the display temperature changes. A target color may refer to a display color with a desired luminance and chromaticity. 
     A chromaticity diagram showing illustrative colors for which adjustment values may be derived is shown in  FIG. 8 . Saturated colors may be included in a subregion such as subregion  54 S of bounded region  54 . Subregion  54 S may include saturated primary colors (e.g., saturated red, saturated green, and saturated blue) and saturated secondary colors (e.g., saturated cyan, saturated magenta, and saturated yellow). Subregion  54 N may include neutral colors. Neutral colors may include, for example, colors having equal intensities of red, green, and blue. Colors such as white and gray (e.g., different shades of gray) may be included in region  54 N. 
     A third subregion such as subregion  54 M may include mid-tone colors. Mid-tone colors in subregion  54 M may lie between the saturated colors of region  54 S and the neutral colors of region  54 N. The human eye may be more sensitive to color shifts in mid-tone colors in a display than to color shifts in saturated colors. If desired, color-specific adjustment values may be derived for a set of representative colors that lie in regions  50 M and  50 N to compensate for temperature induced color shifts in these regions of the color space. In general, color-specific adjustment values may be derived for any suitable color or set of colors. Choosing a set of colors that lie in region  50 M and/or  50 N is merely illustrative. 
     If desired, color-specific adjustment values may be derived for predetermined subsets of colors that fall within discrete subregions of region  54  such as subregions  51 ,  53 , and  55 . Subregions  51 ,  53 , and  55  may be regions of the color space for which maintaining display color integrity is of particular importance. For example, subregion  51  may include colors corresponding to different skin tones, subregion  53  may include colors corresponding to different grass tones, and subregion  55  may include colors corresponding to different sky tones (as examples). The human eye may be more sensitive to color shifts in these regions than in other regions of the color space. Additionally, color integrity in these regions may be of particular importance to a user, as a display such as display  14  may often be used to display images that include skin tones, grass tones, sky tones, and other vital colors. Colors that fall in these predetermined subregions of the color space such as regions  51 ,  53 , and  55  may sometimes be referred to as “high-importance” or “high-value” colors. 
     High-importance colors are not limited to regions  51 ,  53 , and  55 . In general, any suitable subregion of a color space may be defined as a region of high-importance colors. The examples of skin, grass, and sky tone colors are merely illustrative. 
       FIG. 9  is a diagram of an illustrative calibration system that may be used to perform temperature adaptive display calibration for a display such as display  14  of device  10 . As shown in  FIG. 9 , calibration system  48  may include calibration computing equipment  46  that is coupled to test apparatus such as test chamber  38 . Calibration computing equipment  46  may include one or more computers, one or more databases, one or more displays, one or more technician interface devices (e.g., keyboards, touch-screens, joysticks, buttons, switches, etc.) for technician control of calibration computing equipment  46 , communications components or other suitable calibration computing equipment. 
     Calibration computing equipment  46  may be coupled to test chamber  38  using a wired or wireless communications path such as path  44 . 
     Test chamber  38  may include a light sensor such as light sensor  40 . Light sensor  40  may include one or more light-sensitive components such as light-sensitive components  45  for gathering display light  42  emitted by display  14  during calibration operations. Light-sensitive components  45  may include, for example, colorimetric light-sensitive components and/or spectrophotometric light-sensitive components that are configured to gather colored light from display  14 . 
     Light sensor  40  may, for example, be a colorimeter having one or more light-sensitive components  45  corresponding to each set of colored pixels in display  14 . For example, a display having red, green, and blue display pixels may be calibrated using a light sensor having corresponding red, green, and blue light-sensitive components  45 . This is, however, merely illustrative. A display may include display pixels for emitting colors other than red, green, and blue, and light sensor  40  may include light-sensitive components  45  sensitive to colors other than red, green, and blue, may include white light sensors, or may include spectroscopic sensors. 
     Test chamber  38  may, if desired, be a light-tight chamber that prevents outside light (e.g., ambient light in a testing facility) from reaching light sensor  40  during calibration operations. 
     During calibration operations, calibration computing equipment  46  may gather information from device  10  such as temperature information and display performance information. Calibration computing equipment  46  may use the temperature information and display performance information gathered from device  10  to generate temperature adaptive display calibration parameters for device  10 . 
     Temperature information may be gathered from device  10  using thermal sensor  60 . Thermal sensor  60  may be an internal sensor in device  10  (as shown in  FIG. 9 ) or may be an external sensor such as an infrared thermal gun or other suitable type of temperature sensor. Thermal sensor  60  may be used to measure any suitable temperature associated with device  10  (e.g., display temperature, display cover glass temperature, backlight temperature, internal component temperature, etc.). 
     Temperature information gathered by thermal sensor  60  and display performance information gathered by light sensor  40  may be provided to calibration computing equipment  46  over path  44 . Display performance information may include measured luminance and chromaticity values. For example, luminance (Y) and chromaticity (x,y) of light emitted by display  14  may be measured for a number of different colors (e.g., a number of different input RGB values). These measurements may be repeated for each color at a number of different temperatures. 
     During calibration operations, device  10  may be placed into test chamber  38  (e.g., by a technician or by a robotic member). Calibration computing equipment  46  may be used to operate device  10  and light sensor  40  during calibration operations. For example, calibration computing equipment  46  may issue a command (e.g., by transmitting a signal over path  44 ) to device  10  to operate some or all pixels of display  14 . While device  10  is operating the pixels of display  14 , calibration computing equipment  46  may operate light sensor  40  to gather display performance information from display  14  corresponding to the light  42  emitted by display  14 . When it is desired to read out one or more temperatures associated with device  10 , calibration computing equipment  46  may issue a command to device  10  to supply a temperature reading from thermal sensor  60  to calibration computing equipment  46  over path  44 . In configurations where thermal sensor  60  is separate from device  10 , calibration computing equipment  46  may, if desired, request a temperature reading directly from sensor  60 . 
     Calibration computing equipment  46  may determine a set of color-specific adjustment values for each color in a set of predetermined colors. The adjustment value may include three values: an adjustment value for the red channel, an adjustment value for the green channel, and an adjustment value for the blue channel. For explanatory purposes, although an adjustment value may include three values, it may sometimes be referred to herein as “adjustment values.” Additionally, the terms “RGB channel gain” and “input RGB values” may sometimes be referred to herein as “RGB values.” 
     Each set of adjustment values may be stored in an RGB adjustment value table. The RGB adjustment value tables may be stored in device  10 . During operation of display  14 , the color-specific adjustment values may be applied to the RGB values so that the displayed colors each appear as the associated target color even though the display may be at different temperatures. 
     Adjustment values may be derived based on display performance information gathered during calibration operations. For example, calibration computing equipment  46  of  FIG. 9  may determine which RGB input values produce a given target color at a given temperature. The RGB values that produce a given target color at a number of different temperatures may be stored in an RGB table such as RGB table  62  of  FIG. 10 , RGB table  64  of  FIG. 11 , and RGB table  66  of  FIG. 12 . RGB tables of this type may be used to determine a set of adjustment values for any desired target color. 
     Table  62  of  FIG. 10  may be used to determine adjustment values optimized for neutral colors such as white and different shades of gray. Neutral colors may be defined by an R:G:B ratio of 1:1:1 (i.e., equal intensities of red, green, and blue light). As shown in  FIG. 10 , table  62  includes RGB values RGB 1  through RGBm, where RGB 1  through RGBm are the RGB values that may produce a neutral color corresponding to the target neutral color (e.g., the target white point) at the temperature T 1  through Tm, respectively. The RGB 1  through RGBm values may be used to compute the adjustment values R 1  through Rm for the red component, G 1  through Gm for the green component, and B 1  through Bm for the blue component for the temperature T 1  through Tm, respectively. 
     Table  64  of  FIG. 11  may be used to determine adjustment values optimized for yellowish colors. Yellowish colors may be defined by an R:G:B ratio of 2:2:1 (i.e., red and green light each have twice the intensity of blue light). As shown in  FIG. 11 , table  64  includes RGB values RGB 1 ′ through RGBm′, where RGB 1 ′ through RGBm′ are the RGB values that may produce a yellowish color corresponding to the target yellowish color at the temperature T 1  through Tm, respectively. The RGB 1 ′ through RGBm′ values may be used to compute the adjustment values R 1 ′ through Rm′ for the red component, G 1 ′ through Gm′ for the green component, and B 1 ′ through Bm′ for the blue component for the temperature T 1  through Tm, respectively. 
     Table  66  of  FIG. 12  may be used to determine adjustment values optimized for greenish blue colors. Greenish blue colors may be defined by an R:G:B ratio of 1:2:3 (i.e., green light has twice the intensity of red light, and blue light has three times the intensity of red light). As shown in  FIG. 12 , table  66  includes RGB values RGB 1 ″ through RGBm″, where RGB 1 ″ through RGBm″ are the RGB values that may produce a greenish blue color corresponding to the target greenish blue color at the temperature T 1  through Tm, respectively. The RGB 1 ″ through RGBm″ values may be used to compute the adjustment values R 1 ″ through Rm″ for the red component, G 1 ″ through Gm″ for the green component, and B 1 ″ through Bm″ for the blue component for the temperature T 1  through Tm, respectively. 
     Each adjustment value may correspond to a single temperature and may be indexed in the RGB adjustment value table by the corresponding temperature. The adjustment values may be determined for each RGB channel at a specific temperature. The adjustment value for an arbitrary temperature T may be computed by using the ratio: 
                       R   A     =       R   T       R   C         ⁢     
     ⁢       G   A     =       G   T       G   C         ⁢     
     ⁢       B   A     =       B   T       B   C                 (   1   )               
where R A , G A , and B A  are the respective adjustment values for each RGB channel at the arbitrary temperature T; R T , G T , and B T  are the respective RGB values interpolated from two RGB sets from the RGB table corresponding to the temperatures T 1 , T 2  that define the smallest temperature interval containing the temperature T; and R C , G C , and B C  are the respective RGB values corresponding to the target color at a stable operating display temperature.
 
     The examples described in connection with  FIGS. 10 ,  11 , and  12  are merely illustrative. In general, a table of color-specific adjustment values may be derived for any desired color (i.e., for any desired R:G:B ratio). For example, a table of color-specific adjustment values may be derived for ten different colors (i.e., ten different R:G:B ratios), more than ten different colors, less than ten different colors, etc. The exemplary colors described in connection with  FIGS. 10 ,  11 , and  12  are merely illustrative. 
     If desired, tables of color-specific adjustment values may be derived for display colors that tend to experience more noticeable shifts as display temperature changes. For example, slight changes in skin tone colors may be more noticeable to the human eye than slight changes in other display colors. Tables of adjustment values may be derived for a group of colors (i.e., a group of R:G:B ratios) corresponding to different skin tones. As additional examples, tables of adjustment values may be derived different grass tones, for different sky tones, etc. 
     The RGB tables described above in connection with  FIGS. 10 ,  11 , and  12  may be derived from sets of color gamuts that are constructed during calibration operations. A color gamut may be constructed in a number of ways. The color gamut may represent the range of possible colors that a display may produce for a given temperature. 
     In one suitable arrangement, the color gamut may be constructed by employing a look-up table based model and the color gamut may be an empirical model. With this type of arrangement, a set of input RGB values may be predetermined. The selection of the set of predetermined RGB values may be based on the number of desired values for each color. For example, six values ranging from 0 to 255 may be chosen for the red component, six values ranging from 0 to 255 may be chosen for the green component and six values ranging from 0 to 255 may be chosen for the blue component. For every combination of the six values for each of the three components, a luminance (Y) and a chromaticity (x,y) may be measured. These measurements may be repeated for a number of different temperatures. 
     For constructing a color gamut at a temperature T 1 , for example, measurements corresponding to a color model and at the temperature T 1  may be taken. The measurements at each of the temperatures T 1  through Tm may show the variation of luminance or the variation of a target color in the form of the correlated color temperature value (as an example). 
     Returning to constructing a color gamut, a predetermined set of RGB values may be defined. In this example, at each operating temperature T 1  through Tm, the luminance (Y) and the chromaticity (x,y) may be measured for each of the RGB values in the predetermined set of RGB values. If the matrix color model is used, four color measurements for pure red, pure green, pure blue and pure white, at each temperature T 1 , through Tm, may be used for the display. For example, pure red may be produced by input RGB values of 255, 0, 0, pure green may be produced by input RGB values of 0, 255, 0, pure blue may be produced by input RGB values of 0, 0, 255 and pure white may be produced by input RGB values of 255, 255, 255. 
     If a look-up table model is used with 216 samples (6×6×6=216), the measurements may be taken of luminance (Y) and chromaticity (x,y) for 216 predetermined RGB values. For example, at a temperature T 1 , a luminance and chromaticity measurement may be taken for each of the 216 predetermined RGB values. Similarly, for a temperature T 2 , another luminance and chromaticity measurement may be taken for each of the 216 predetermined RGB values and so on. The 216 RGB values is provided for explanatory purposes only. If desired, the number of samples per each component may be increased (for example, using seven or more values for each of the individual RGB values), thus increasing the accuracy of the empirical model. 
     Each color gamut CG 1  through CGm may be defined at each temperature T 1  through Tm, respectively. The RGB table may be calculated once the target luminance Y and target chromaticity (x,y) values are set. The calculation of the RGB table may be performed line by line. Each line in the table may correspond to a respective temperature T 1  through Tm such that the RGB table has m lines. For each line k in the RGB table, the RGB values may be computed as follows. For temperature Tk, the target luminance and white point values may correspond to a unique color in the color gamut CGk. The unique color may be produced by a certain RGB value, RGBk. Resolving the RGBk color for a given target color and color gamut may depend on the color model that is used for the display. For example, if the matrix model is used, the following equations are used to compute RGB from Yxy of the target: 
               X   =       Y   y     ⁢   x       ,     
     ⁢     Z   =         Y   y     ⁢         (     1   -   x   -   y     )     ⁢     
     [       r   linear     ⁢     g   linear     ⁢     b   linear       ]     T       =         M       -   1     ⁢   i       ⁡     [   XYZ   ]       T                     R   =     rTRC   -     1   ⁡     [     r   linear     ]                     G   =     gTRC   -     1   ⁡     [     g   linear     ]                     B   =     bTRC   -     1   ⁡     [     b   linear     ]                   where             M   =     [           X   r           X   g           X   b               Y   r           Y   g           Y   b               Z   r           Z   g           Z   b           ]           
wherein rTRC corresponds to the red tone reproduction curve, gTRC corresponds to the green tone reproduction curve, and bTRC corresponds to the blue tone reproduction curve.
 
     If the look-up table model is used, the calculation of the RGB with a defined color gamut as a table of (RGB Yxy) sets may be based on tetrahedral decomposition and tetrahedral interpolation, which are known to one of ordinary skill in the art. 
     The exemplary operating temperatures for constructing a color model may be selected at intervals sufficiently close together such that the color may be adjusted at small enough temperature intervals that there may be no perceptible shift in color. A color model including a luminance measurement Y and a chromaticity measurement (x,y) for each of the predetermined RGB values may be constructed for each of the set of operating temperatures. For example, at an operating temperature T, a color model generated or used by the present embodiment may include a luminance measurement Y and a chromaticity measurement (x,y) for each predetermined RGB value. For example, a color model may contain the following information: 
                                                 TABLE 1                          T1   R1   G1   B1   Y1   (x, y)1           T1   R2   G2   B2   Y2   (x, y)2                 . . .                                             T1   Rn   Gn   Bn   Yn   (x, y)n                        
where the measurements (Yxy) 1  through (Yxy)n correspond to the temperature T 1 . Accordingly, multiple luminance and chromaticity values (Y and (x,y), respectively) may be measured for a variety of predetermined RGB values R 1 ,G 1 ,B 1  to Rn,Gn,Bn at a single operating temperature T 1 . Also, n is the number of luminance and chromaticity measurements taken at each operating temperature.
 
     For every selected operating temperature T 1  through Tm, color gamuts CG 1  through CGm may be constructed for each corresponding temperature. The construction of the color gamuts may be based on the color model that employs the measurements at each temperature T 1  through Tm. The measurements taken at each of the temperatures T 1  through Tm may be selected to cover the range from approximately the cold start-up temperature of the display to the stable operating temperature of the display. In one example, the last or stable operating temperature may be the display temperature after the display has been on for a predetermined period of time. Generally, the color table for the last temperature may be represented as: 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
             
            
               
                   
                 Tm 
                 R1 
                 G1 
                 B1 
                 Y1 
                 (x, y)1 
               
               
                   
                 Tm 
                 R2 
                 G2 
                 B2 
                 Y2 
                 (x, y)2 
               
            
           
           
               
            
               
                 . . . 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Tm 
                 Rn 
                 Gn 
                 Bn 
                 Yn 
                 (x, y)n 
               
               
                   
                   
               
            
           
         
       
     
     Thus, m color gamuts CG 1  through CGm may be constructed using the temperatures, predetermined RGB values, luminance measurements and chromaticity measurements and the color model at each temperature T 1  through Tm. The m color models may, for example, take the form of tables 1 and 2, where table 1 corresponds to color model 1 and where table 2 corresponds to color model m. 
     In another suitable embodiment, a color model may be constructed using a matrix model. The matrix model may, for example, employ measurements of the following colors: the display red, green, blue and white colors, and a set of intermediate gray colors between black and white for tone reproduction curve estimation. For this embodiment, 6 intermediate gray colors may be used. The luminance measurements Y and the chromaticity measurements (x,y) may be taken for a predetermined set of RGB values specified by the following n=4+6 combinations, and the (Yxy)j,k may represent the measurements for the color model k at temperature Tk, k=1 through m and for the combination j, where j may be a natural number from n=1 through n=10. 
     Color Model 1 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
             
            
               
                   
                 T1 
                 255 
                 0 
                 0 
                 Y1, 1 
                 (x, y)1, 1 
               
               
                   
                 T1 
                 0 
                 255 
                 0 
                 Y2, 1 
                 (x, y)2, 1 
               
               
                   
                 T1 
                 0 
                 0 
                 255 
                 Y3, 1 
                 (x, y)3, 1 
               
               
                   
                 T1 
                 255 
                 255 
                 255 
                 Y4, 1 
                 (x, y)4, 1 
               
               
                   
                 T1 
                 204 
                 204 
                 204 
                 Y5, 1 
                 (x, y)5, 1 
               
               
                   
                 T1 
                 153 
                 153 
                 153 
                 Y6, 1 
                 (x, y)6, 1 
               
            
           
           
               
            
               
                 . . . 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 T1 
                 0 
                 0 
                 0 
                 Y10, 1 
                 (x, y)10, 1 
               
               
                   
                   
               
            
           
         
       
     
     Color Model m 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
             
            
               
                   
                 Tm 
                 255 
                 0 
                 0 
                 Y1, m 
                 (x, y)1, m 
               
               
                   
                 Tm 
                 0 
                 255 
                 0 
                 Y2, m 
                 (x, y)2, m 
               
               
                   
                 Tm 
                 0 
                 0 
                 255 
                 Y3, m 
                 (x, y)3, m 
               
               
                   
                 Tm 
                 255 
                 255 
                 255 
                 Y4, m 
                 (x, y)4, m 
               
               
                   
                 Tm 
                 204 
                 204 
                 204 
                 Y5, m 
                 (x, y)5, m 
               
               
                   
                 Tm 
                 153 
                 153 
                 153 
                 Y6, m 
                 (x, y)6, m 
               
            
           
           
               
            
               
                 . . . 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Tm 
                 0 
                 0 
                 0 
                 Y10, m 
                 (x, y)10, m 
               
               
                   
                   
               
            
           
         
       
     
     The tone reproduction curve in the matrix model may be determined at each temperature T 1  through Tm from the measurements Y 5 , k through Y 10 , k using an interpolation method familiar to one of ordinary skill in the art. In this embodiment, linear interpolation may be employed (if desired). 
     In another embodiment, a color model may be constructed using a matrix model where the tone reproduction curves may be independent of the temperature and estimated before the color measurements are taken at the temperature T 1  through Tm. The measurement of the intermediate gray colors may be done at the initial cold or warmed up stable display temperature. The curves may be derived through interpolation one time and may be used for each color model at temperature T 1  through Tm. For this embodiment, the matrix model may employ the measurements of the following colors: the device red, green, blue and white colors. The luminance measurements Y and the chromaticity measurements (x,y) may be taken for a predetermined set of RGB values specified by the following n=4 combinations. Additionally, the (Yxy)j,k values may represent the measurement for the color model k at temperature Tk, k=1 through m and for the combination j, where j may be a natural number from 1 through n=10. 
     Color Model 1 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 5 
               
               
                   
                   
               
             
            
               
                   
                 T1 
                 255 
                 0 
                 0 
                 Y1, 1 
                 (x, y)1, 1 
               
               
                   
                 T1 
                 0 
                 255 
                 0 
                 Y2, 1 
                 (x, y)2, 1 
               
               
                   
                 T1 
                 0 
                 0 
                 255 
                 Y3, 1 
                 (x, y)3, 1 
               
               
                   
                 T1 
                 255 
                 255 
                 255 
                 Y4, 1 
                 (x, y)4, 1 
               
               
                   
                   
               
            
           
         
       
     
     Color Model m 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 6 
               
               
                   
                   
               
             
            
               
                   
                 Tm 
                 255 
                 0 
                 0 
                 Y1, m 
                 (x, y)1, m 
               
               
                   
                 Tm 
                 0 
                 255 
                 0 
                 Y2, m 
                 (x, y)2, m 
               
               
                   
                 Tm 
                 0 
                 0 
                 255 
                 Y3, m 
                 (x, y)3, m 
               
               
                   
                 Tm 
                 255 
                 255 
                 255 
                 Y4, m 
                 (x, y)4, m 
               
               
                   
                   
               
            
           
         
       
     
     In another embodiment, a color model may be constructed using a look-up table model. The luminance measurements Y and the chromaticity measurements (x,y) may be taken for a predetermined set of RGB values specified by the following n=6×6×6 combinations. Six intermediate values may be set for each R,G,B component, and the (Yxy)j,k may represent the measurement for the color model k at temperature Tk, k=1 through m and for the combination j, where j may be a natural number from 1 through n=216. 
     Color Model 1 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 7 
               
               
                   
                   
               
             
            
               
                   
                 T1 
                 255 
                 255 
                 255 
                 Y1, 1 
                 (x, y)1, 1 
               
               
                   
                 T1 
                 255 
                 255 
                 204 
                 Y2, 1 
                 (x, y)2, 1 
               
               
                   
                 T1 
                 255 
                 255 
                 153 
                 Y3, 1 
                 (x, y)3, 1 
               
               
                   
                 T1 
                 255 
                 255 
                 102 
                 Y4, 1 
                 (x, y)4, 1 
               
            
           
           
               
            
               
                 . . . 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 T1 
                 0 
                 0 
                 0 
                 Y216, 1 
                 (x, y)216, 1 
               
               
                   
                   
               
            
           
         
       
     
     Color Model m 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 8 
               
               
                   
                   
               
             
            
               
                   
                 Tm 
                 255 
                 255 
                 255 
                 Y1, m 
                 (x, y)1, m 
               
               
                   
                 Tm 
                 255 
                 255 
                 204 
                 Y2, m 
                 (x, y)2, m 
               
               
                   
                 Tm 
                 255 
                 255 
                 153 
                 Y3, m 
                 (x, y)3, m 
               
               
                   
                 Tm 
                 255 
                 255 
                 102 
                 Y4, m 
                 (x, y)4, m 
               
            
           
           
               
            
               
                 . . . 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Tm 
                 0 
                 0 
                 0 
                 Y216, m 
                 (x, y)216, m 
               
               
                   
                   
               
            
           
         
       
     
     If desired, a color-specific color model may be constructed for each target color. For example, color models for yellowish (R:G:B=2:2:1) may be constructed by taking luminance measurements Y and chromaticity measurements (x,y) for a predetermined set of RGB values having R:G:B ratios of 2:2:1. The m color models for yellowish colors may, for example, take the following form: 
     Color Model 1 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 9 
               
               
                   
                   
               
             
            
               
                   
                 T1 
                 255 
                 255 
                 127 
                 Y1, 1 
                 (x, y)1, 1 
               
               
                   
                 T1 
                 221 
                 221 
                 110 
                 Y2, 1 
                 (x, y)2, 1 
               
               
                   
                 T1 
                 187 
                 187 
                 93 
                 Y3, 1 
                 (x, y)3, 1 
               
            
           
           
               
            
               
                 . . . 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 T1 
                 17 
                 17 
                 6 
                 Y8, 1 
                 (x, y)8, 1 
               
               
                   
                   
               
            
           
         
       
     
     Color Model m 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 10 
               
               
                   
                   
               
             
            
               
                   
                 Tm 
                 255 
                 255 
                 127 
                 Y1, m 
                 (x, y)1, m 
               
               
                   
                 Tm 
                 221 
                 221 
                 110 
                 Y2, m 
                 (x, y)2, m 
               
               
                   
                 Tm 
                 187 
                 187 
                 93 
                 Y3, m 
                 (x, y)3, m 
               
            
           
           
               
            
               
                 . . . 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Tm 
                 17 
                 17 
                 8 
                 Y8, m 
                 (x, y)8, m 
               
               
                   
                   
               
            
           
         
       
     
     Color models of the type shown in tables 9 and 10 may be constructed for any desired color (i.e., any suitable R:G:B ratio). In general, any suitable color model may be used to determine adjustment values for a given color. If desired, the color model that is used to determine adjustment values for a given color may be chosen based on which color model offers the most accurate compensation for changes in display temperature. 
     Moreover, the color models may be a function of multiple input parameters, as opposed to a function of temperature alone. The RGB values, luminance values and chromaticity values may be recorded for multiple input parameters. For example, RGB values may be recorded for combinations of input parameters such as brightness and temperature. Further, the RGB values, luminance values and chromaticity values may be recorded at multiple temperatures at a first brightness level, a second brightness level and so on. Similar to previously discussed methods, the RGB values may be used to determine adjustment values such as attenuation factors. Additionally, interpolation may be used to determine adjustment values for any combination of input parameters and by employing the previously recorded RGB values, luminance values, chromaticity values for the various combinations of input parameters. 
     As the RGB adjustment value table includes a finite number of entries, it may occur that the actual operating temperature of a display falls between temperatures for which entries exist in the RGB adjustment value table. Certain embodiments may use the existing entries of the RGB adjustment value table to interpolate adjustment values for such interim temperatures. The adjustment values corresponding to the interim temperature may be interpolated based on the adjustment values of the entries in the table bounding the interim temperature (e.g., the adjustment values for the nearest temperature above the current operating temperature and the nearest temperature below the current operating temperature). 
     Certain embodiments use linear interpolation to calculate the adjustment value for the interim temperature, while others may use a different form of interpolation. Any known form of interpolation may be employed by various embodiments. Accordingly, RGB values may be determined for display temperatures that are not included in the existing RGB table. Moreover, it may be possible to increase the granularity of the temperatures and corresponding RGB values by interpolating between the existing RGB values and determining additional RGB values for temperatures not originally included in the RGB table. In another embodiment, previous adjustment values may be used to determine a trend and/or a slope of change in adjustment values to more accurately interpolate the next value. 
     Although the RGB values, luminance measurements and chromaticity measurements have been discussed herein as a function of temperature, alternative embodiments may adjust the color output of a display based on other parameters. For example, the RGB values, luminance and chromaticity may be sampled as a function of other parameters including, but not limited to, time, brightness settings, the age of the display or any combination thereof. Accordingly, the RGB table and adjustment constants generated or employed by an embodiment would account for such parameters. 
       FIG. 13  is a flow chart of illustrative steps involved in calibrating a display such as display  14 . 
     At step  102 , a calibration system such as calibration system  48  of  FIG. 9  may be used to gather display performance data from display  14 . For example, light sensor  40  may be used to gather one or more images of display  14  while display  14  is operated in a series of calibration sequences. This may include, for example, measuring luminance values Y and chromaticity values (x,y) while pixels  52  are operated at different intensity levels (e.g., while different RGB input values are provided to display pixel  52 ). The luminance values Y and chromaticity values (x,y) may be recorded as a function of at least one parameter or a combination of parameters. The parameters may be temperature, time, brightness, ambient light, the aging of the display, or any combination thereof. Temperature information such as display temperature may be provided to calibration computing equipment  46  using a thermal sensor in device  10  such as temperature sensor  60 . 
     Additionally, other data values may be recorded (and thus adjusted) such as contrast, tone reproduction curves, or any other visual parameter of the display. The luminance and chromaticity values may be recorded over a time period such as the warming up time of a display. The intervals that the luminance and chromaticity values are recorded may vary. Generally, the intervals may be selected such that when the color of the display is adjusted, it may not be perceptible to a user. 
     At step  104 , a target behavior of the display may be defined. This may include, for example, setting a target color (e.g., target luminance and chromaticity values) for each color in a predetermined set of colors. For example, target colors may be may be set respectively for white, for yellowish, for greenish blue, for redish blue, for greenish red, etc. Target colors may, if desired, be set for high-importance colors such as skin tone colors, grass tone colors, and sky tone colors. 
     At step  106 , calibration computing equipment  46  may use the gathered display performance data such as the measured luminance and chromaticity values to calculate adjustment values as a function of the at least one input parameter. The adjustment values may be organized into RGB adjustment value tables. A table of adjustment values may be derived for each color in the predetermined set of colors. As previously discussed, the adjustment values may be attenuation factors for the RGB channels in display  14 . 
     At step  108 , additional adjustment values may be determined by interpolating from the temperatures and adjustment values in the RGB adjustment value tables. By employing interpolation to determine these additional adjustment values, it may be possible to determine adjustment values for any temperature. The additional adjustment values may be stored in the RGB adjustment value table. 
     At step  110 , the RGB adjustment value tables may be stored in device  10 . If desired, the RGB adjustment value tables may be stored in display control circuitry  28  in device  10  or may be stored in any other suitable location in device  10  such as storage and processing circuitry  34 . 
       FIG. 14  is a flow chart of illustrative steps involved in adjusting display colors during operation of display  14  to minimize undesirable color shifts as the display temperature changes. 
     At step  112 , display control circuitry  28  may receive temperature information such as display temperature from a thermal sensor in device  10  such as temperature sensor  60 . 
     At step  114 , display control circuitry  28  may receive incoming RGB subpixel values (which may sometimes be referred to as input RGB values, data, display data, digital display control values, or display control signals) and may determine the color associated with the incoming RGB values. If desired, display control circuitry  28  may optionally linearize the incoming subpixel values to remove display gamma non-linearity (e.g., if the display gamma is not equal to one). If the display gamma is equal to one, the step of linearizing the incoming RGB values may be omitted. 
     Step  114  may include, for example, determining the R:G:B ratio associated with the incoming RGB values. For example, display control circuitry  28  may determine that the R:G:B ratio of incoming RGB values is 2:2:1 and may therefore determine that the color associated with the incoming RGB values is yellowish. As another example, step  114  may include determining that the color associated with incoming RGB values is a skin tone color. 
     At step  116 , display control circuitry  28  may determine which RGB adjustment value table most closely corresponds to the color associated with the incoming RGB values. If the color associated with the incoming RGB values does not exactly match one of the colors for which adjustment values have been stored, interpolation techniques may be used to determine an appropriate set of adjustment values based on the color associated with the incoming RGB values. For example, a combination of Inverse Distance Weighting and Delaunay Triangulation may be used to interpolate RGB adjustment values for the incoming RGB values. 
     At step  118 , display control circuitry  28  may determine an adjustment value for the incoming RGB values based on the sampled display temperature and the color associated with the incoming RGB values. If the display operating temperature falls between temperatures for which entries exist in the RGB adjustment value table, the existing entries in the RGB adjustment value table may be used to interpolate adjustment values for the actual display temperature. 
     At step  120 , display control circuitry  28  may apply the appropriate adjustment value to each incoming RGB subpixel value to obtain adapted RGB subpixel values. This may include, for example, multiplying the display input value for red by a red correction coefficient, multiplying the display input value for green by a green correction coefficient, and multiplying the display input value for blue by a blue correction coefficient. 
     The resulting adapted linearized subpixel values may then optionally be de-linearized (e.g., to restore the non-linear display gamma) to obtain adapted subpixel vales R′, G′, and B′. The adapted RGB values may then be supplied to pixels  52  in display  14 . 
       FIG. 15  is a flow chart of illustrative steps involved in adjusting display colors during operation of display  14  based on which colors exhibit perceivable changes as the display temperature changes. 
     At step  124 , display control circuitry  28  may sample the temperature of display  14  using a thermal sensor such as temperature sensor  60 . Display control circuitry  28  may also determine the color associated with incoming RGB values. This may include, for example, determining the R:G:B ratio associated with the incoming RGB values. 
     If it is determined that the color associated with incoming RGB values is not a high-importance color, processing may proceed to step  126 , as indicated by line  128 . As described in connection with  FIG. 8 , a high-importance color may be a color for which maintaining display color integrity is of particular importance. High-importance colors may include skin tone colors, grass tone colors, sky tone colors, and other high-importance colors. Display control circuitry  28  may determine whether the color associated with incoming RGB values is a high-importance color by determining whether the color falls within a predetermined region of high-importance colors in a color space (e.g., regions such as regions  51 ,  53 , and  55  of a color space as shown in  FIG. 8 ). 
     At step  126 , display control circuitry  28  may apply adjustment values of a first type to the incoming RGB values to obtain adapted RGB values. The adjustment values applied to the incoming RGB values may be determined based on the display temperature sampled during step  124 . The adjustment values of the first type may, for example, be optimized for neutral colors such as white and gray. 
     If it is determined that the color associated with the incoming RGB values is a high-importance color (e.g., a skin tone color, a grass tone color, a sky tone color, or other high-value color), processing may proceed to step  130 , as indicated by line  132 . Display control circuitry  28  may determine that the color associated with incoming RGB values is a high-importance color by determining that the color falls within a predetermined region of high-importance colors in a color space (e.g., regions such as regions  51 ,  53 , and  55  of a color space as shown in  FIG. 8 ). 
     At step  130 , display control circuitry  28  may apply adjustment values of a second type to the incoming RGB values to obtain adapted RGB values. The adjustment values applied to the incoming RGB values may be determined based on the display temperature sampled during step  124 . The adjustment values of the second type may be optimized for different high-importance colors. For example, one or more tables of adjustment values of the second type may be optimized for skin tone colors, one or more tables of adjustment values of the second type may be optimized for grass tone colors, and one or more tables of adjustment values of the second type may be optimized for sky tone colors. These examples are merely illustrative. In general, any subregion of a color space may be defined as a region of high-importance colors. 
     Display control circuitry  28  may determine which table of adjustment values to use based on the color associated with the incoming RGB values. For example, if the color falls within region  51  of the color space of  FIG. 8 , control circuitry  28  may use adjustment values that are optimized for skin tone colors. If the color falls within region  53  of the color space of  FIG. 8 , control circuitry  28  may use adjustment values that are optimized for grass tone colors. If the color falls within region  55  of the color space of  FIG. 8 , control circuitry  28  may use adjustment values that are optimized for sky tone colors (as examples). 
     The foregoing is merely illustrative of the principles of this invention and various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention. The foregoing embodiments may be implemented individually or in any combination.