PATENT DOCUMENT

Publication Number: US-9595239-B2
Application Number: US-201514614754-A
Country: US
Kind Code: B2

Title: Color display calibration system

Abstract:
A display calibration system may be used to calibrate a display. The display calibration system may have an image sensor that captures images of the display. An electrically controllable tunable filter such as an electrically tunable liquid crystal birefringence cell filter may be used to produce various wavelength-dependent filter characteristics in accordance with a variety of different filter settings. Control circuitry in the display calibration system may be used to adjust the filter to implement one or more color matching functions or to produce other desired filter characteristics for the display calibration system. The image sensor may measure light emitted from the display while the control circuitry is adjusting the filter. The control circuitry may then process the measured light to produce calibration data for the display. The calibration data may be loaded into the display by the control circuitry to calibrate the display.

Claims:
What is claimed is: 
     
       1. A display calibration system for calibrating a display, comprising:
 control circuitry; 
 an electrically tunable filter with a transmission characteristic that is controlled by the control circuitry, wherein the electrically tunable filter comprises first and second polarizers, a liquid crystal cell that is interposed between the first and second polarizers, and electrodes with which the control circuitry adjusts the liquid crystal cell in accordance with a plurality of filter settings; and 
 an image sensor that receives light from the display through the electrically tunable filter, wherein the control circuitry captures images of the display corresponding to each of the filter settings to produce calibration data to calibrate the display. 
 
     
     
       2. The display calibration system defined in  claim 1  further comprising at least one lens interposed between the display and the image sensor. 
     
     
       3. The display calibration system defined in  claim 2  wherein the control circuitry is configured to process the images of the display to produce the display calibration data. 
     
     
       4. The display calibration system defined in  claim 3  wherein the control circuitry is configured to load the display calibration data into the display to calibrate the display. 
     
     
       5. The display calibration system defined in  claim 1  wherein the control circuitry is configured to adjust the electrically tunable filter to implement at least one color matching function. 
     
     
       6. The display calibration system defined in  claim 5  wherein the control circuitry processes the captured images to produce the display calibration data and stores the display calibration data in the display to calibrate the display. 
     
     
       7. The display calibration system defined in  claim 1  wherein the transmission characteristic varies as a function of wavelength. 
     
     
       8. The display calibration system defined in  claim 1  wherein each electrode comprises an array of individually controllable electrodes. 
     
     
       9. A display calibration system for calibrating a display, comprising:
 control circuitry; 
 an electrically tunable filter with a transmission characteristic that is controlled by the control circuitry, wherein the electrically tunable filter comprises a liquid crystal cell with an electrically controllable birefringence and transparent electrodes to which the control circuitry applies adjustable voltages to adjust the electrically tunable filter; and 
 an image sensor that receives light from the display through the electrically tunable filter, wherein the control circuitry adjusts the electrically tunable filter in accordance with a plurality of filter settings and captures images of the display corresponding to each of the filter settings to produce calibration data to calibrate the display. 
 
     
     
       10. The display calibration system defined in  claim 9  wherein the electrically tunable filter comprises an input polarizer and an analyzer polarizer and wherein the liquid crystal cell is interposed between the input polarizer and the analyzer polarizer. 
     
     
       11. The display calibration system defined in  claim 9  further comprising at least one lens interposed between the display and the image sensor. 
     
     
       12. The display calibration system defined in  claim 11  wherein the control circuitry is configured to process the images of the display to produce the display calibration data and is configured to load the display calibration data into the display to calibrate the display. 
     
     
       13. The display calibration system defined in  claim 9  wherein the liquid crystal cell is interposed between the transparent electrodes. the transmission characteristic in accordance with a plurality of filter settings, wherein the image sensor receives the light from the display while the electrically tunable filter is adjusted in accordance with each of the filter settings. 
     
     
       14. A method of calibrating a display that emits light, comprising:
 with an image sensor, receiving light from the display through an electrically tunable filter having first and second polarizers and transparent electrodes and having a transmission characteristic that varies as a function of wavelength; and 
 applying adjustable voltages to the electrodes to adjust the electrically tunable filter to change the transmission characteristic in accordance with a plurality of filter settings, wherein the image sensor receives the light from the display while the electrically tunable filter is adjusted in accordance with each of the filter settings. 
 
     
     
       15. The method defined in  claim 14  wherein the electrically tunable filter comprises an electrically controllable birefringence cell and wherein applying adjustable voltages to the electrodes electrically adjusts the electrically controllable birefringence cell. 
     
     
       16. The method defined in  claim 15  wherein the electrically controllable birefringence cell comprises a liquid crystal cell and wherein applying adjustable voltages to the electrodes electrically adjusts the liquid crystal cell. 
     
     
       17. The method defined in  claim 16  wherein the liquid crystal cell is interposed between the first and second polarizers. 
     
     
       18. The method defined in  claim 17  wherein at least one setting of the plurality of filter settings is a color matching setting.

Description:
BACKGROUND 
     This relates generally to displays, and, more particularly, to calibrating color displays. 
     Electronic devices often include displays. Color displays include pixels of different colors. For example, a display may include an array of pixels based on red, green, and blue light-emitting diodes. Due to manufacturing variations, light-emitting diodes may exhibit variations in output light intensity and wavelength. Unless care is taken, these variations may cause a display to exhibit undesired color shifts and other visible artifacts. 
     Display calibration equipment has been developed to help ensure that displays perform satisfactorily. During manufacturing, the performance of a display may be measured using the calibration equipment. Calibration settings may then be stored in the display to compensate for any detected variations in display performance. 
     Some calibration equipment is based on spectroradiometers. Spectroradiometers can be used to make accurate color measurements, but can be slow and costly when spatial information on display performance is desired. Filter-based colorimeters are less expensive and work well when making color measurements on broadband light sources such as incandescent light. Filter-based colorimeters may not, however, offer desired levels of accuracy when evaluating narrowband light sources such as certain light-emitting diodes. 
     It would therefore be desirable to be able to provide improved display calibration equipment. 
     SUMMARY 
     A display calibration system may be used to calibrate a display. The display may have an array of pixels such as an array of pixels having respective light-emitting diodes. Calibration operations may involve evaluating the output of the pixels using the display calibration system while the display is using the pixels to generate light. 
     The display calibration system may have an image sensor that captures images of the display. An electrically controllable filter may be used to produce various wavelength-dependent filter characteristics in accordance with a variety of different filter settings. 
     Control circuitry in the display calibration system may be used to adjust the adjustable filter to implement one or more color matching functions or other desired filter characteristics for the display calibration system. The image sensor may measure light emitted from the display during calibration operations while the control circuitry is adjusting the filter. 
     The control circuitry may then process the measured light to produce display calibration data. The calibration data may be loaded into the display by the control circuitry to calibrate the display. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of an illustrative display calibration system in accordance with an embodiment. 
         FIG. 2  is a diagram of an illustrative color matching function that may be used in analyzing display output with the display calibration system in accordance with an embodiment. 
         FIG. 3  is a diagram showing how a desired light transmission function can be implemented by using an adjustable filter at multiple settings in accordance with an embodiment. 
         FIG. 4  is a cross-sectional view of an illustrative filter based on an electrically controllable birefringence cell in accordance with an embodiment. 
         FIG. 5  is a diagram showing illustrative transmittances that may be implemented by adjusting an adjustable filter in accordance with an embodiment. 
         FIG. 6  is a flow chart of illustrative steps involved in calibrating a display using a display calibration system in accordance with an embodiment. 
         FIG. 7  is a diagram of an illustrative adjustable filter based on a set of swappable static filters in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a diagram showing how a display in an electronic device may be calibrated using a display calibration system. Electronic device  10  includes display  14 . Device  10  may be a laptop computer, a stand-alone display, an embedded system such as a kiosk or vehicle with a display, a television, a computer with an integrated display, a tablet computer, a cellular telephone, a wrist-watch device, other miniature handheld or wearable equipment, or other electronic equipment that includes a display. 
     Display  14  includes an array of pixels  12  and control circuitry such as storage  16 . There may be any suitable numbers of rows and columns of pixels  12  in display  14  (e.g., tens, hundreds, thousands, tens of thousands, or more). Pixels  12  may be based on organic light-emitting diodes, crystalline semiconductor diodes (e.g., micro-light-emitting diodes), liquid crystal display structures, electrophoretic display structures, plasma display structures, electrowetting display structures, or other display technologies. 
     Pixels  12  may include pixels of different colors. For example, pixels  12  may include red, green, and blue pixels or other sets of colors. Display  14  may have pixels of three different colors, pixels of four different colors, etc. 
     Due to manufacturing variations, pixels  12  in the array of pixels in display  14  will not all perform identically when initially assembled into display  14 . Variations in the performance of pixels  12  may cause different portions of display  14  to produce outputs levels of varying intensity and color (i.e., display  14  may exhibit spatially varying color shifts and intensity fluctuations). 
     During calibration operations, display calibration system  38  may acquire color and intensity data on the output of display  14 . In particular, display calibration system  38  may use image sensor and filter circuitry  22  to make measurements on light  40  that is emitted from display  14  while display  14  is emitting test patterns of light  40 . This allows display calibration system  38  to evaluate the performance of pixels  12  in display  14  as a function of position in lateral dimensions X and Y. The resolution of system  38  may be sufficient to capture performance data for each individual pixel  12  in display  12  or a lower resolution may be used to enhance calibration throughput. Illustrative configurations for display calibration system  38  in which system  38  gathers data for each pixel  12  in display  14  by analyzing images of display  14  are sometimes described herein as an example. 
     As shown in  FIG. 1 , image sensor and filter circuitry  22  may include photosensitive components such as two-dimensional image sensor  24 . Image sensor  24  may be, for example, a charge-coupled device (CCD) image sensor or other solid state image sensor. The resolution of sensor  24  may, if desired, be sufficiently large to capture data from the array of pixels  12  in display  14 . 
     During display calibration operations, control circuitry  20  may direct display  14  to display test patterns on pixels  12  by sending test patterns and/or control signals to display  14  over path  32 . Light  40  that is emitted by display  14  may be detected and measured by image sensor  24  after passing through filter structures  26 . Measured light data from sensor  24  (i.e., color and intensity data on the light emitted by the pixels of display  14 ) may be provided to control circuitry  20  by path  34 . 
     Filter structures  26  may include an electrically tunable filter such as tunable filter  28  and one or more lenses  30 . Tunable filter  28  may be located between a lens and image sensor  24 , may be located between a pair of front and rear lenses, or tunable filter  28  may be located between a rear lens and display  14  (i.e., a lens  30  may be located between image sensor  24  and tunable filter  28 ). Lenses  30  may be single-element or multi-element lenses and may be used to focus an image of the array of pixels  12  on the front face of display  14  onto image sensor  24  so that the performance of each pixel can be evaluated from the image data produced by image sensor  24 . 
     Control circuitry  20  can adjust tunable filter  28  by supplying control signals to tunable filter  28  over path  36 . During display calibration operations, filter  28  may be adjusted to exhibit different wavelength-dependent filter responses. A desired filter characteristic (i.e., a desired wavelength-dependent transmission curve) can be implemented by processing multiple measurements from image sensor  24  each of which is taken using a different filter configuration for filter (i.e., by using the principal of superposition). 
     Consider, as an example, the illustrative wavelength-dependent filter function f that is represented by curve  42  in the graph of  FIG. 2 . During calibration operations, it may be desirable to use circuitry  22  to implement one or more filter functions such as function f using circuitry  22 . This allows light  40  to be filtered before being measured by image sensor  24 . Functions such as function f may have any suitable transmission versus wavelength characteristic. With one illustrative configuration, tunable filter  28  may be used to implement trichromatic color matching functions (i.e., functions such as function f may be color matching functions that replicate the eye response of a standard observer). The color matching functions implemented by filter  28  may be, for example, color matching functions established by the International Commission on Illumination (CIE) such as the color matching functions associated with the CIE 1931 XYZ color space or other suitable color matching functions. In general, filter functions such as function f may have any suitable shape. The use of filter  28  to implement a wavelength-dependent filter characteristic such as a color matching function is merely illustrative. 
       FIG. 3  is a graph illustrating how a desired filter function can be implemented by taking measurements using different settings for filter  28 . In the example of  FIG. 3 , curve  42  corresponds to a desired filter function (light transmission T versus wavelength λ). During a first measurement with image sensor  24 , control circuitry  20  adjusts filter  28  to produce the filter characteristic represented by line  44 . During a second measurement with image sensor  24 , control circuitry  20  adjusts filter  28  to produce the filter characteristic represented by line  46 . Desired filter function  42  is the result of the superposition of lines  44  and  46 . In the example of  FIG. 3 , the superposition of two filter settings is used in producing a desired filter function. This is merely illustrative. Any suitable number of different filter settings may be used by filter  28  when acquiring data on the output of display  14  with image sensor  24  to implement a desired filter function (e.g., one or more filter settings, two or more, three or more five or more, ten or more, etc.). Desired filter function attributes such as desired slope and curvature attributes for the filter function may be realized by using a sufficient number of filter settings. 
     Filter  28  may be an electrically controllable filter that is based on an electrically controllable birefringence cell. Filter  28  may, as an example, be a filter that exploits the linear electro-optic effect (e.g., a filter based on a material such as lithium niobate that exhibits a linear changes in index of refraction as a function of applied electric field). With another suitable arrangement, which is sometimes described herein as an example, filter  28  may be based on a liquid crystal cell that changes index of refraction as a function of applied electric field (i.e., filter  28  may be an electrically tunable liquid crystal birefringence cell filter). Other types of filter technology may be used in implementing electrically tunable filter  28 , if desired. 
     A cross-sectional side view of filter  28  in an illustrative configuration in which filter  28  has been based on a liquid crystal cell is shown in  FIG. 4 . As shown in  FIG. 4 , filter  28  may have input polarizer  60  to polarize incoming light  40  from display  14 . Filter  28  may also have analyzer polarizer  62 . Polarizers  60  and  62  may be linear polarizers with parallel transmission axes. Light  40  from display  14  is linearly polarized upon passing through input polarizer  60 . Liquid crystal cell  58  has an adjustable index of refraction. Light  40  that has been linearly polarized by polarizer  60  passes through cell  58  to analyzer polarizer  62 . Analyzer polarizer  62  allows output light  40 ′ that is linearly polarized in alignment with the polarization axis of polarizer  62  to exit cell  58 . 
     Liquid crystal cell  58  may be interposed between polarizers  60  and  62 . Liquid crystal cell  58  may have transparent electrodes  54  and  56 , which are connected to respective terminals  50  and  52 . During operation, control circuitry  20  may apply controlled voltages across terminals  50  and  52 , so that electrodes  54  and  56  produce an electric field across liquid crystal cell  58  that has a desired strength. If desired, electrodes  54  and  56  may be segmented (e.g., electrodes  54  and  56  may each include a 32×32 array of individually controllable electrodes or other suitable number of electrodes). The use of an array of individually controllable electrodes may allow control circuitry  20  to adjust cell  58  by different amounts across the surface of cell  58  (e.g., to even out spatial variations in cell performance, etc.). This allows system  38  to compensate for spatial variations that might otherwise affect calibration measurement accuracy. Cell  58  may be calibrated by making measurements using filter  28  while providing reference light  40  using one or more reference light sources in place of display  14 . The reference light sources may be monochromatic sources or other known sources and may be used to calibrate system  38  before system  38  is used to test display  14 . Reference light sources may, if desired, be mounted in the plane of display  14 . 
     Liquid crystal cell  58  may exhibit an adjustable birefringence. The ordinary and extraordinary axes of liquid crystal  58  may be perpendicular to each other and may be oriented at 45° angles with respect to the transmission axis of polarizers  60  and  62 . In this configuration, the transmission T of filter  28  may be given by equation 1, where d is the thickness of cell  58 , Δn is the difference between the extraordinary axis index of refraction n e  and the ordinary axis index of refraction n o  for liquid crystal  58 , and λ is the wavelength of light  40 .
 
 T =cos 2   [πd Δn/λ]   (1)
 
     The value of Δn (i.e., the amount of birefringence of liquid crystal  58 ) is a function of applied electric field (i.e., voltage V across terminals  50  and  52 ). When the voltage V has a first value, the transmission T of filter  28  may have a first characteristic such as the transmission characteristic shown by line  70  of  FIG. 5 . When V has a second value that is different than the first value, the transmission T of filter  28  may have a second characteristic such as the transmission characteristic shown by line  72  of  FIG. 5 . Characteristics  72  and  70  may correspond to lines  44  and  46  of  FIG. 3  (as an example). By taking multiple measurements of the light output from display  14  each corresponding to a different respective setting for filter  28 , a desired transmission function may be implemented (e.g., a color matching function or other suitable filter function). 
       FIG. 5  is a flow chart of illustrative steps involved in using display calibration system  38  to calibrate display  14 . 
     At step  80 , control circuitry  20  of system  38  may direct display  14  to display a desired test pattern (test image) on pixels  12 . Light  40  from the test pattern on display  14  passes to circuitry  22  of test system  38 , as shown in  FIG. 1 . 
     At step  82 , control circuitry  20  may adjust tunable filter  28  using a filter setting that results in a desired transmission characteristic T for filter  28 , as described in connection with  FIGS. 3, 4, and 5 . 
     Image sensor  24  may then be used to capture a two-dimensional image of light  40  from pixels  12  of display  14  (step  84 ). 
     As illustrated by line  90 , additional data may be captured using one or more additional settings for tunable filter  28  (e.g., so that a color matching function or other desired filter function can be implemented through the principal of superposition). 
     After data has been gathered from image sensor  24  for a desired number of different filter settings for filter  28 , control circuitry  20  may produce calibration data for display  14  (step  86 ). The calibration data may include pixel-by-pixel calibration adjustments to the output of each pixel  12  so that color and intensity can be corrected across all pixels  12  in display  14  (i.e., the calibration data may include spatial color calibration data). Global display calibration data (i.e., color and intensity adjustments that apply to some or all of pixels  12  equally) may also be produced, if desired. The measurement and calibration process may be performed at a single luminance level for the pixels, or at multiple luminance levels for the pixels. Measurement and calibration at different luminance levels can provide improved color fidelity when the gamma response curves of the subpixel color channels are subject to variability. Moreover, this technique can be used to perform measurement and calibration for colors that require the operation of two or three subpixels, for cases where the display output is subject to crosstalk between subpixels. The measurement and calibration process may also be performed for different target color spaces. For example, under some circumstances it may be desirable for the display to render in the sRGB color space, but for other circumstances it may be desirable for the display to render in color spaces such as Adobe RGB, DCI-P3, or Rec 2020. For example, the display may draw on these different calibration settings when switched between one color space mode and another by a display color management system. Calibration data may be produced by processing acquired data from image sensor  24  that corresponds to one or more different test patterns being displayed on display  14 , one or more different filter settings for filter  28 , one or more different resulting color matching functions or other transmission functions implemented using filter  28 , and/or other settings for system  38 . 
     After processing the acquired data from sensor  24  to produce display calibration data for display  14 , control circuitry  20  of system  38  may load the display calibration data into storage  16  of display  14  (step  88 ). When display  14  is operated, display  14  can apply the calibration data that has been loaded into storage  16  (e.g., color offsets and other corrections may be applied), so that the images displayed by pixels  12  are accurate. 
     If desired, filter  28  can be implemented using a set of swappable static spectral filters (e.g., filters implemented using dyes, gratings, dielectric stacks, and/or or other illustrative filter structures). As shown in  FIG. 7 , for example, static filters  92  may be mounted in a static filter support structure such as structure  94 . Structure  94  may be a rotatable filter wheel structure, a linear translating filter holder, or other structure that receives and supports multiple filters  92 . Each filter  92  may be a static spectral filter that is configured to make measurements on a particular narrowband light source color for one of the pixel colors of pixels  12  and/or filters  92 , may be a filter corresponding to a color matching function, may be a filter that covers all or part of a spectral range of interest (e.g., in a configuration in which multiple filter measurements are made and processed using control circuitry  20 ), or may be a filter that covers another particular spectral range of interest. Alternatively, blazed or holographic gratings with a linear array detector may be used in system  38  to enable the simultaneous measurement of a large spectral range in a single measurement. 
     Actuator  90  (e.g., a motor for rotating a filter wheel, a linear actuator for translating a linear set of filters  92 , etc.) may be used to switch each desired filter  92  into use. Actuator  90  may be controlled by control circuitry  20  so that suitable filters  92  may be selected while image sensor  24  collects corresponding measurements on light  40  from display  14 . Control circuitry  20  may, for example, switch each of filters F 1  . . . FN into use in series until all filters  92  have been used or may switch a selected subsets of filters  92  into use one after the other. There may be three or more filters  92 , four or more filters  92 ,  5 - 10  filters  92 , fewer than  100  filters  92 , or any other suitable number of static spectral filters  92  that are used to make measurements for filter  28 . 
     To ensure that the measurements made with filters  92  are sufficiently accurate to allow system  38  to make color calibration measurements on display  14 , each filter  92  may have a calibrated response in at least a narrow bandwidth around the narrow linewidths of the light sources of display  14 . Each filter  92  may, for example, have at least a full-width half-maximum bandwidth of about 20-30 nm, 40-50 nm, less than 60 nm, less than 75 nm, less than 200 nm, more than 50 nm, or other suitable value. Filters that exhibit transmission over larger ranges of wavelengths may also be used. For example, filters  92  may transmit light across most or all of the visible light spectrum (which extends from about 380 nm to 750 nm). For accurate color calibration measurements, the responses of the filters are known at least in the vicinities of the light source spectra being measured. 
     System  38  may be calibrated prior to use in making display calibration measurements. For example, a reference light source (or sources) may be used to generate reference light  40  in place of display  14 . During reference light source measurements, image sensor  24  may make measurements using various different filter settings for filter  28  (e.g., different filters  92  may be swapped into use). After a sufficient number of measurements have been made to characterize the performance of image sensor and filter circuitry  22  (i.e., after system  38  has been calibrated against the reference light source(s) using all filters  92 ), system  38  may be used to make calibrating measurements on displays such as display  14 . 
     The foregoing is merely illustrative and various modifications can be made by those skilled in the art without departing from the scope and spirit of the described embodiments. The foregoing embodiments may be implemented individually or in any combination.

Metadata:
Filing Date: 20150205
Publication Date: 20170314
Grant Date: 20170314
Priority Date: 20150205
Inventors: DROLET JEAN-JACQUES
DRZAIC PAUL S.
Assignee: APPLE INC
CPC Classifications: [{"code": "G02F1/137", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G3/006", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G2360/145", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2340/06", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G5/026", "inventive": true, "first": true, "tree": "[]"}, {"code": "G09G2320/0693", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G3/2003", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/29", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G2320/0693", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G3/2003", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/29", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G2360/145", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2340/06", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G3/006", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G5/026", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02F1/137", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 56565272