PATENT DOCUMENT

Publication Number: US-8988682-B2
Application Number: US-201314058055-A
Country: US
Kind Code: B2

Title: High accuracy imaging colorimeter by special designed pattern closed-loop calibration assisted by spectrograph

Abstract:
This specification describes various embodiments that relate to methods for providing a wideband colorimeter that can include more accurate outputs. In one embodiment, a narrowband instrument, such as a spectrometer or spectrograph, can be used for calibration of a wideband colorimeter, so that more accurate outputs can be provided. In one embodiment, an optical test equipment, which consists of both a wideband colorimeter and a narrowband spectrograph, can be used for providing a more accurately calibrated wideband colorimeter. As an example, a spectra-camera, which is a hybrid system consisting of both a wideband colorimeter and a narrowband spectrograph, can be used for simultaneous testing by both the wideband colorimeter and the narrowband spectrograph. By doing simultaneous testing, accurate calibration of the wideband colorimeter can be achieved. This specification further describes a mathematical model to characterize a wideband three channel colorimeter with a narrowband multiple channel spectrometer.

Claims:
What is claimed is: 
     
       1. A system configured to use a narrowband color measurement device to calibrate an output of a wideband color measurement device, the system comprising:
 a splitter configured to split an image of a test pattern into a first image and a second image; 
 a first image pipeline to direct the first image to a narrowband device configured to capture a first data from the first image; 
 a second image pipeline to direct the second image to a wideband device configured to capture a second data from the second image, wherein the captured first data and the captured second data are used to determine a best fit correction matrix relating color data of the captured first data and the captured second data, and the best fit correction matrix is evaluated for being acceptable as a calibration parameter for a color measurement of the wideband device. 
 
     
     
       2. The system of  claim 1 , wherein the best fit correction matrix is used for calibration of the wideband device. 
     
     
       3. The system of  claim 2 , wherein the wideband device is a colorimeter. 
     
     
       4. The system of  claim 3 , wherein the colorimeter includes a Bayer filter. 
     
     
       5. The system of  claim 3 , wherein the colorimeter includes a color wheel. 
     
     
       6. The system of  claim 2 , wherein the narrowband device is a spectrometer. 
     
     
       7. A method for using a narrowband color measurement device to calibrate an output of a wideband color measurement device, the method comprising:
 splitting an image of a test pattern into a first image and a second image; 
 directing the first image to a narrowband device using a first image pipeline; 
 capturing a first data from the first image using the narrowband device; 
 directing the second image to a wideband device using a second image pipeline; 
 capturing a second data from the second image using the wideband device; 
 determining, using the captured first data and the captured second data, a best fit correction matrix relating color data of the captured first data and the captured second data; and 
 evaluating the best fit correction matrix for being acceptable as a calibration parameter for a color measurement of the wideband device. 
 
     
     
       8. The method of  claim 7 , wherein the test pattern belongs to a set of predetermined test patterns comprising 61 unique digital color stimulus patterns. 
     
     
       9. The method of  claim 7 , wherein the wideband device is a colorimeter. 
     
     
       10. The method of  claim 9 , wherein the colorimeter includes a Bayer filter. 
     
     
       11. The method of  claim 9 , wherein the colorimeter includes a color wheel. 
     
     
       12. The method of  claim 7 , wherein the narrowband device is a spectrometer. 
     
     
       13. The method of  claim 7 , wherein the best fit correction matrix is a 3×3 matrix. 
     
     
       14. The method of  claim 7 , wherein the best fit correction matrix is a 3×4 matrix. 
     
     
       15. A non-transitory computer-readable medium storing instructions that when executed by a processor of a computing device cause the computing device to perform the steps of:
 capturing a first data corresponding to a first image from a narrowband device; 
 capturing a second data corresponding to a second image from a wideband device, wherein the first image and the second image are derived from splitting an image of a test pattern; 
 determining, based on the captured first data and the captured second data, a best fit correction matrix relating color data of the captured first data and the captured second data; and 
 evaluating the best fit correction matrix for being acceptable as a calibration parameter for a color measurement of the wideband device. 
 
     
     
       16. The non-transitory computer-readable medium as recited in  claim 15 , further comprising the steps of:
 modifying an output of the wideband device using the best fit correction matrix. 
 
     
     
       17. The non-transitory computer-readable medium as recited in  claim 15 , wherein the best fit correction matrix is a 3×3 matrix. 
     
     
       18. The non-transitory computer-readable medium as recited in  claim 15 , wherein the best fit correction matrix is a 3×4 matrix. 
     
     
       19. The non-transitory computer-readable medium as recited in  claim 15 , further comprising the steps of:
 when the best fit correction matrix is determined to be unacceptable as the calibration parameter for the color measurement of the wideband device, capturing a new first image and a new second image. 
 
     
     
       20. The non-transitory computer-readable medium as recited in  claim 15 , wherein the wideband device is a colorimeter and the narrowband device is a spectrometer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Application Ser. No. 61/717,523 filed Oct. 23, 2012, which is incorporated herein in its entirety by reference. 
    
    
     FIELD OF THE DESCRIBED EMBODIMENTS 
     The described embodiments relate generally to colorimetry and more particularly to calibrating a low cost colorimeter. 
     BACKGROUND 
     Color measurement instruments fall into two general categories: wideband (or broadband) and narrowband. A wideband measurement instrument reports up to 3 color signals obtained by optically processing the input light through wideband filters. Photometers are the simplest example, providing a measurement only of the luminance of a stimulus. Their primary use is in determining the nonlinear calibration function of displays. Densitometers are an example of wideband instruments that measure optical density of light filtered through red, green and blue filters. Colorimeters are another example of wideband instruments that directly report tristimulus (XYZ) values, and their derivatives such as CIELAB. A colorimeter, sometimes also called an imaging photometer, is an imaging device which behaves like a camera. The imaging colorimeter can be a time-sequential type or Bayer-filter type. Under the narrowband category fall instruments that report spectral data of dimensionality significantly larger than three. 
     Spectrophotometers and spectroradiometers are examples of narrowband instruments. These instruments typically record spectral reflectance and radiance respectively within the visible spectrum in increments ranging from 1 to 10 nm, resulting in 30-200 channels. They also have the ability to internally calculate and report tristimulus coordinates from the narrowband 15 spectral data. Spectroradiometers can measure both emissive and reflective stimuli, while spectrophotometers can measure only reflective stimuli. A spectrometer or spectrograph is a narrowband device which can quantify and measure the spectrum. 
     The main advantage of wideband instruments such as densitometers and colorimeters is that they are inexpensive and can read out data at very fast rates. However, the resulting measurement is only an approximation of the true tristimulus signal, and the quality of this approximation varies widely depending on the nature of the stimulus being measured. Accurate colorimetric measurement of arbitrary stimuli under arbitrary illumination and viewing conditions requires spectral measurements afforded by the more expensive narrowband instruments. Compared with measuring instruments without spatial resolutions, such as spectrometers, this technology offers the following advantages: (a) Substantial time-savings with simultaneous capture of a large number of measurements in a single image and (b) Image-processing functions integrated in the software permit automated methods of analysis, e.g. calculation of homogeneity or contrast. 
     However, the absolute measuring precision of imaging photometers and colorimeters is not as high as spectrometers. This is because of the operational principle using a CCD Sensor in combination with optical filters, which can only be adapted to the sensitivity of the human eye with limited precision. Therefore, the imaging colorimeters are the instruments of choice for measurement of luminance and color distribution of panel graphics and control elements in the display test industry, including but not limited to homogeneity, contrast, mura and modulation transfer function (MTF). 
     Therefore, what is desired is an alternative to wideband colorimeters that can include more accurate outputs. 
     SUMMARY OF THE DESCRIBED EMBODIMENTS 
     This specification describes various embodiments that relate to methods for providing a wideband colorimeter that can include more accurate outputs. In one embodiment, a narrowband instrument, such as a spectrometer or spectrograph, can be used for calibration of a wideband colorimeter, so that more accurate outputs can be provided. In one embodiment, an optical test equipment, which consists of both a wideband colorimeter and a narrowband spectrograph, can be used for providing a more accurately calibrated wideband colorimeter. As an example, a spectra-camera, which is a hybrid system consisting of both a wideband colorimeter and a narrowband spectrograph, can be used for simultaneous testing by both the wideband colorimeter and the narrowband spectrograph. By doing simultaneous testing, accurate calibration of the wideband colorimeter can be achieved. This specification further describes a mathematical model to characterize a wideband three channel colorimeter with a narrowband multiple channel spectrometer. 
     In one embodiment, a method for correcting an output of a wideband color measurement device through use of a narrowband color measurement device is disclosed. The method includes configuring the wideband color measurement device and the narrowband color measurement device to measure color, stimulating the wideband color measurement device and the narrowband color measurement device with predetermined test patterns, capturing color measurement data from the wideband color measurement device and the narrowband color measurement device, determining a correction matrix relating the captured color measurement data, and correcting the color measurement output of the wideband color measurement device with the correction matrix. In one embodiment, the predetermined test patterns include 61 unique digital color stimulus patterns. In one embodiment, the wideband color measurement device is a colorimeter. In one embodiment, the narrowband color measurement device is a spectrometer. 
     In one embodiment, a method for using a narrowband color measurement device to calibrate a wideband color measurement device is disclosed. The method includes presenting predetermined test patterns, configuring the narrowband device and the wideband device to concurrently measure color data from the predetermined test patterns, capturing the color data from the wideband device and the narrowband device, determining a best fit correction matrix relating the captured color data, evaluating if the best fit correction matrix is acceptable as a calibration parameter for color measurements from the wideband device, and returning to the step of presenting predetermined test patterns when the best fit correction matrix is not acceptable as the calibration parameter. In one embodiment, the method further includes using the best fit correction matrix as the calibration parameter for the color measurements from the wideband device when the best fit correction matrix is acceptable as the calibration parameter. In one embodiment, the best fit correction matrix is a 3×3 matrix. In one embodiment, the best fit correction matrix is a 3×4 matrix. In one embodiment, the predetermined test patterns include 61 unique digital color stimulus patterns. In one embodiment, the wideband device is a colorimeter. In one embodiment, the narrowband device is a spectrometer. 
     In one embodiment, a system configured to using a narrowband color measurement device to calibrate an output of a wideband color measurement device is disclosed. The system includes a splitter configured to split an image of a test pattern into a first image and a second image, a first image pipeline configured to direct the first image to a narrowband device, a narrowband device configured to capture a first data from the first image, a second image pipeline configured to direct the second image to the wideband device, and a wideband device configured to capture a second data from the second image. The captured first data and the captured second data are used to determine a correction matrix relating the captured first and second data. In one embodiment, the correction matrix is used for calibration of the wideband device. In one embodiment, the wideband device is a colorimeter. In one embodiment, the narrowband device is a spectrometer. In one embodiment, the test pattern belongs to a set of predetermined test patterns that includes 61 unique digital color stimulus patterns. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The described embodiments and the advantages thereof may best be understood by reference to the following description taken in conjunction with the accompanying drawings. These drawings in no way limit any changes in form and detail that may be made to the described embodiments by one skilled in the art without departing from the spirit and scope of the described embodiments. 
         FIGS. 1A-1C  illustrate three widely used types of spectrometer configurations: ( 1 A) Crossed Czerny-Turner, ( 1 B) Lens-Grating-Lens, and ( 1 C) Mirror-Grating-Mirror. 
         FIGS. 2A-2B  illustrate two color separating filter methods for a colorimeter: ( 2 A) time-sequential filter and ( 2 B) Bayer filter. 
         FIG. 3  illustrates an embodiment of a spectra-camera, which can be used for calibration of a wideband colorimeter with a narrowband spectrometer, in accordance with one embodiment described in the specification. 
         FIG. 4  illustrates a flow chart showing method steps for performing concurrent wideband colorimeter and narrowband spectrometer testing in a spectra-camera, in accordance with one embodiment described in the specification. 
         FIG. 5  shows error ranges for 14 patterns before correction is applied. 
         FIG. 6  shows error ranges for 14 patterns after correction is applied. 
         FIG. 7  illustrates a flow chart of method steps for correcting the output of a wideband colorimeter, in accordance with one embodiment described in the specification. 
         FIG. 8  illustrates a flow chart showing method steps for performing calibration of a wideband color measurement device with a narrowband color measurement device, so that more accurate outputs of the wideband color measurement device can be provided, in accordance with one embodiment described in the specification. 
         FIG. 9  is a block diagram of an electronic device suitable for implementing some of the described embodiments. 
     
    
    
     DETAILED DESCRIPTION OF SELECTED EMBODIMENTS 
     Representative applications of methods and apparatus according to the present application are described in this section. These examples are being provided solely to add context and aid in the understanding of the described embodiments. It will thus be apparent to one skilled in the art that the described embodiments may be practiced without some or all of these specific details. In other instances, well known process steps have not been described in detail in order to avoid unnecessarily obscuring the described embodiments. Other applications are possible, such that the following examples should not be taken as limiting. 
     In the following detailed description, references are made to the accompanying drawings, which form a part of the description and in which are shown, by way of illustration, specific embodiments in accordance with the described embodiments. Although these embodiments are described in sufficient detail to enable one skilled in the art to practice the described embodiments, it is understood that these examples are not limiting; such that other embodiments may be used, and changes may be made without departing from the spirit and scope of the described embodiments. 
     Spectrometer is an example of a narrowband color measurement device which can quantify and measure the spectrum.  FIGS. 1A-1C  illustrate the three basic types of spectrometer configurations that are widely used and that can be used for parallel testing configuration with a spectra-camera. They are the Crossed Czerny-Turner ( FIG. 1A ), Lens-Grating-Lens ( FIG. 1B ), and Mirror-Grating-Mirror ( FIG. 1C ) configurations. 
     As  FIGS. 1A-1C  demonstrate, all spectrometers have these four key elements: (1) collimator  110 , (2) diffractive grating  120 , (3) focusing element  130 , and (4) detector array  140 . The beam will be first collimated at a curved mirror or lens. As the name indicates, the function of a collimator  110  is to collimate the beams in a controlled manner. The collimated beam is then diffracted by a grating  120 . The diffraction grating  120  causes the collimated beam to diverge in angle space with different wavelength outputs. After the grating  120  diffracts the beam, different wavelength output beams will propagate in different direction. To make sure that these outputs beams hit the right detectors, there is usually a focusing mirror or lens positioned to ensure that all the output beams passing through the focusing mirror or lens will focus on the right detector. The output beams with different wavelength will then be detected and absorbed by a detector array  140 . 
     For transmission grating based spectrometer, the Crossed Czerny-Turner spectrometer uses all curved mirrors to realize the collimation and focusing function. The Lens-Grating-Lens (LGL) spectrometer uses two lenses and a grating, while the Mirror-Grating-Mirror (MGM) spectrometer also uses two curved mirrors and a grating.  FIG. 1A  shows Crossed Czerny-Turner spectrometer  100 , which uses curved mirrors for collimator  110  and focusing element  130 .  FIG. 1B  shows Lens-Grating-Lens (LGL) spectrometer  102 , which uses lenses for collimator  110  and focusing element  130 , while  FIG. 1C  shows Mirror-Grating-Mirror (MGM) spectrometer  104 , which also uses curved mirrors for collimator  110  and focusing element  130 . 
     Spectrometers can be used for basic display parametric testing. As the name indicates, the display basic parameters usually refer to test items which do not involve the use of imaging algorithms, such as display white luminance, contrast, uniformity, gamma, color gamut, etc. The definition of these test items can be found in these four popular standards:
         (1) VESA FPDM (‘Video Electronics Standards Association’ Flat Panel Display Measurements&#39; standard),   (2) ISO 13406-2 (International Organization for Standardization 13406-2: “Ergonomic requirements for work with visual displays based on flat panels—Part 2: Ergonomic requirements for flat panel displays”),   (3) TCO &#39;05 (Tjänstemännens Centralorganisation &#39;05), and   (4) SPWG 3.5 (Standard Panels Working Group standard 3.5).
 
These standards are slightly different from each other due to historical reasons and because they target different panel sizes.
       

     Imaging colorimeter is an example of a wideband color measurement device. Imaging colorimeter, sometimes also called as imaging photometer, is an imaging device which behaves like a camera. The imaging colorimeter can be a time-sequential type or Bayer filter type. The time-sequential type colorimeter separates the measurement objective color in a time sequential manner by using a spinning color wheel, which is shown in  FIG. 2A . At any particular moment, the measurement objective photons with only a certain color will be transmitting through the filter and hitting the embedded CCD or CMOS imager inside the colorimeter. The overall display color information and imaging can be reconstructed after at least one cycle of the color wheel spinning A second type of imaging colorimeter separates the color channels by Bayer filters, which are shown in  FIG. 2B . A Bayer filter is a color filter array which is composed of periodically aligned 2×2 filter element. The 2×2 filter element is composed of two green filter elements, one red filter and one blue filter element. The Bayer filter sits on top of a square grid of photo sensors. 
     There are advantages and disadvantages to both types of colorimeters. The time-sequential colorimeter will be more precise, but also more time consuming. The Bayer filter colorimeter, on the other hand, has the one-shot capability to extract the color information with resolution loss. Additionally, there is also a third type of spatial Foveon filter which can separate the color by vertical stack photodiode layer. In the Foveon filter, the red, green, and blue (RGB) color sensitive pixels lie stacked on top of each other, in layers, instead of spread on a single layer as is found in the Bayer filter. Foveon filter can have the advantage that color artifacts normally associated with the Bayer filter are eliminated and light sensitivity is increased, but there are very few applications of the Bayer filter in cameras and none in off-the-shelf colorimeters. 
     Imaging colorimeter can be used for artifact testing, which is more complicated than basic display parametric testing and without clear boundary. The artifacts refer to the human perception of the display visual artifacts. The detected artifacts can be classified into two categories: (a) Static artifacts, where artifacts do not change over time; and (b) Dynamic artifact, where the artifacts are more visible during a certain time frame. Dynamic artifacts include flickering (i.e., luminance exhibits a frequency pattern which can cause human eye fatigue), ripper, and dynamic cross-talk. For both static and dynamic artifacts, depending on the viewing condition, they can be further classified as on-axis artifacts which are visible at normal view and off-axis artifacts which are visible at tilt angle view. 
     The use of imaging colorimeters for fast capture of photometric and colorimetric quantities with spatial resolution is very attractive. Compared with measuring instruments without spatial resolutions, such as spectrometers, imaging colorimeters offer the advantage of substantial time savings since a single image can simultaneously capture a large number of measurements. Imaging colorimeters also offer the advantage of being able to perform automated methods of analysis, such as calculation of homogeneity or contrast, because of image processing functions integrated in the imaging software. Additionally, imaging colorimeters are relatively inexpensive as compared to spectrometers. 
     However, the imaging colorimeters and photometers have a lower measuring precision than spectrometers. This is because imaging colorimeters operate using a CCD (charge-coupled device) sensor in combination with optical filters. For example, in the case of the Bayer filter, there are only three types of color filters (i.e., red, green, and blue), so there is lacking the precision found in spectrometers, where the visible spectrum can be partitioned by increments ranging from 1 to 10 nm, resulting in 30-200 channels. Therefore, in a spectra-camera used for display testing, the imaging colorimeters are best utilized for measurement of luminance and color distribution of panel graphics and control elements, including but not limited to homogeneity, contrast, mura (i.e., luminance non-uniformity of a display device) and MTF (Modulation Transfer Function). 
       FIG. 3  illustrates a display test equipment (e.g., Spectra-camera), which can simultaneously incorporate the testing objectives of both a wideband and a narrowband device. The narrowband device is a high accuracy device measuring a single spot on a display, so there is little or no spatial resolution. Examples of a narrowband device include a spectrometer together with or without a filter based probe. A narrowband device can perform display parametric testing, which includes testing basic display attributes, such as brightness, contrast, color, gamut, gamma, etc. The narrowband device can have the following features: (1) expensive, (2) variable measurement spot, and (3) limited capability for dynamic artifacts, but only flickering can be detected. The wideband device, on the other hand, is a low accuracy device measuring a large area on a display, so there is high spatial resolution. Examples of a wideband device include a time-sequential type or Bayer filter type imaging colorimeter. A wideband device can perform display artifact testing, which includes testing visual artifacts, light leakage, yellow mura, LED (light-emitting diode) hotspot, backlight damage, etc. The wideband device can have the following features: (1) long image pipeline (complicated), (2) less accurate than the narrowband device, and (3) incapable for dynamic artifacts. 
     Since both the wideband and narrowband devices have their advantages, what is desired is a display test equipment that can simultaneously incorporate the testing objectives of both the narrowband device and the wideband device. Such a display test equipment can be called a “Spectra-camera”. A spectra-camera is a hybrid test system, which can simultaneously perform the testing functions of both the narrowband device and the wideband device. In one embodiment, a spectra-camera can be a hybrid test system consisting of both a narrowband device and a wideband device. By splitting an image of a display to be tested into two parts, a spectra-camera can send the two parts to both devices at the same time for testing. For example, the part sent to the narrowband device can be an image of a spot on the display, while the part sent to the wideband device can be an image of the entire display minus the spot. 
       FIG. 3  illustrates an embodiment of a spectra-camera  300 . Spectra-camera  300  can read the spectrum and form the image. Incoming light  310  from a display  320  to be tested enters a slit and hit an aperture mirror  330 . A hole  335  in the aperture mirror  330  can enable some of the light to be sampled into a fiber connector  340 . It is not shown, but in another embodiment a beam splitter, instead of an aperture mirror, can enable some of the light to be sampled into a fiber connector  340 . The sample light can go through a lens  350  and a slit  360  to avoid stray light. The sample light beam eventually goes through a Crossed Czerny-Turner spectrometer  370  (with collimating mirror  372 , grating  374 , focusing mirror  376 ) and the diffracted output is collected by a detector array  378 . In the mean time, aperture mirror can also reflect the rest of the light from the test display into a user defined camera  380 . It is not shown, but in another embodiment a beam splitter, instead of an aperture mirror, can also reflect the rest of the light from the test display into a user defined camera  380 . Camera  380  can form an image to be used for imaging analysis. In the embodiment shown in  FIG. 3 , mirror  385  can be used to direct the display image into camera  380 . In another embodiment that is not shown, the display image can be sent directly into camera  380  without the use of mirror  385 , if camera  380  is positioned differently to receive the display image. Spectra-camera  300  has the advantage of a wide test coverage, since both display parametric testing and display artifact testing can be performed at the same time. Doing both tests at the same time reduces test time and boost display testing throughput. 
     In one embodiment, spectrometer  370  can be configured to perform display parametric testing. In one embodiment, spectrometer  370  can be a Crossed Czerny-Turner spectrometer. In another embodiment, spectrometer  370  can be a Lens-Grating-Lens (LGL) spectrometer or a Mirror-Grating-Mirror (MGM) spectrometer. In one embodiment, spectrometer  370  can be configured to be detachable from spectra-camera  300 . 
     In one embodiment, user defined camera  380  can be configured to perform display artifact testing. In one embodiment, user defined camera  380  can be an imaging colorimeter. In an embodiment, the imaging colorimeter can be a time-sequential type or Bayer filter type. In another embodiment, the imaging colorimeter can be spatial Foveon filter type. In one embodiment, user defined camera  380  can be configured to be detachable from spectra-camera  300 . 
     In one embodiment, a spectra-camera can perform parallel testing, where display artifact testing can occur with the spectrometer on. The display artifact testing can detect various defective display symptoms, such as LED hotspot, dot defect, yellow mura, and line defect. Concurrently, the spectrometer can be used to perform display parametric testing, so that basic attributes, such as brightness, contrast, color gamut, gamma, etc., are measured. 
       FIG. 4  illustrates a flow chart showing method steps for performing concurrent wideband colorimeter and narrowband spectrometer testing. In one embodiment, a method for performing concurrent wideband colorimeter and narrowband spectrometer testing starts with step  410 , which splits an image of a display to be tested into a first image and a second image. The method continues in step  420  by sending the first image for wideband colorimeter testing. The method sends, concurrently with the first image, the second image for narrowband spectrometer testing in step  430 . Then the method performs wideband colorimeter testing on the first image (step  440 ) concurrently with the narrowband spectrometer testing on the second image (step  450 ). Concurrent testing can allow for calibration of the wideband colorimeter using the narrowband spectrometer, which is more accurate for color testing. 
     In one embodiment, a spectra-camera can perform self-calibration, where the high precision spectrometer can be used to calibrate the low precision imaging colorimeter. This can be accomplished by linking the high precision spectrometer to the low precision imaging colorimeter and performing the equipment self calibration triggered by the same standard illuminant. In one embodiment, the same standard illuminant is used to simultaneously calibrate the high precision spectrometer and the low precision imaging colorimeter in parallel. For the high precision spectrometer, the calibration process flow can include the following process steps: 
     (1) Luminance Reference Normalization, 
     (2) Spectral Radiance Normalization, 
     (3) Photodiode Position Check, 
     (4) Spectrum, and 
     (5) Tristimulus values XYZ. 
     For the low precision imaging colorimeter, the parallel calibration process flow can include the following process steps: 
     (1) ADC (analog-to-digital converter), 
     (2) Bad Pixel Correction, 
     (3) Gain/Offset Correction, 
     (4) Flat Field Correction, 
     (5) Luminance Correction, 
     (6) Linearity Correction, 
     (7) Focus Correction, 
     (8) Chromaticity Correction, 
     (9) Spatial Correction, 
     (10) Instrument Correlation Correction, and 
     (11) Tristimulus values XYZ. 
     The tristimulus values XYZ of the low precision imaging colorimeter is compared against the tristimulus values XYZ of the high precision spectrometer. If they are within a tolerance specification such as 0.0015 for XYZ, then the tolerance specification is met and the low precision imaging colorimeter is deemed to be calibrated. If the tolerance specification is not met, then the low precision imaging colorimeter needs to recalibrated by repeating the colorimeter calibration process from the Luminance Correction process step (i.e., step (5)). At the end of the colorimeter recalibration process, the tristimulus values XYZ of the colorimeter is again compared against the tristimulus values XYZ of the spectrometer. Calibration of the colorimeter is complete if the tolerance specification is met. 
     In one embodiment, a mathematical model can be used to characterize a wideband three channel colorimeter with a narrowband multiple channel spectrometer. 
     In the device characterization field, a critical component is multidimensional data fitting and interpolation. Generally, the data samples generated by the characterization process in both device-dependent and device-independent spaces will constitute only a small subset of all possible digital values that could be encountered in either space. One reason for this is that the total number of possible samples in a color space is usually prohibitively large for direct measurement of the characterization function. As an example, R, G, B signals can be represented with 8 bit precision. Thus the total number of possible colors is 2 24 =16,777,216. Clearly this is an unreasonable amount of data to be acquired manually. However, since the final characterization function will be used for transforming arbitrary image data, the characterization should be defined for all possible inputs within some expected domain. To accomplish this, some form of data fitting or interpolation can be performed on the characterization samples. In model based characterization, the underlying physical model serves to perform the fitting or interpolation for the forward characterization function. 
     In one embodiment, the 4-color correction matrix concept can be extended to 61 test points. In one embodiment, this can be a set of predetermined test patterns that includes 61 unique digital color stimulus patterns. Then another 14 random colors will be used to verify that the wideband colorimeter data closely matches the narrowband spectrometer data. The X, Y and Z values are converted to x and y before the error calculation. Table 1 shows an example of 61 unique digital color stimulus patterns that can be used as a set of predetermined test patterns. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Patterns that can be used for calibration 
               
            
           
           
               
               
               
               
            
               
                   
                 Digital 
                 Digital 
                 Digital 
               
               
                   
                 Count R 
                 Count G 
                 Count B 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Pattern 1 
                 0 
                 0 
                 0 
               
               
                   
                 Pattern 2 
                 17 
                 0 
                 0 
               
               
                   
                 Pattern 3 
                 34 
                 0 
                 0 
               
               
                   
                 Pattern 4 
                 51 
                 0 
                 0 
               
               
                   
                 Pattern 5 
                 68 
                 0 
                 0 
               
               
                   
                 Pattern 6 
                 85 
                 0 
                 0 
               
               
                   
                 Pattern 7 
                 102 
                 0 
                 0 
               
               
                   
                 Pattern 8 
                 119 
                 0 
                 0 
               
               
                   
                 Pattern 9 
                 136 
                 0 
                 0 
               
               
                   
                 Pattern 10 
                 153 
                 0 
                 0 
               
               
                   
                 Pattern 11 
                 170 
                 0 
                 0 
               
               
                   
                 Pattern 12 
                 187 
                 0 
                 0 
               
               
                   
                 Pattern 13 
                 204 
                 0 
                 0 
               
               
                   
                 Pattern 14 
                 221 
                 0 
                 0 
               
               
                   
                 Pattern 15 
                 238 
                 0 
                 0 
               
               
                   
                 Pattern 16 
                 255 
                 0 
                 0 
               
               
                   
                 Pattern 17 
                 0 
                 17 
                 0 
               
               
                   
                 Pattern 18 
                 0 
                 34 
                 0 
               
               
                   
                 Pattern 19 
                 0 
                 51 
                 0 
               
               
                   
                 Pattern 20 
                 0 
                 68 
                 0 
               
               
                   
                 Pattern 21 
                 0 
                 85 
                 0 
               
               
                   
                 Pattern 23 
                 0 
                 102 
                 0 
               
               
                   
                 Pattern 23 
                 0 
                 119 
                 0 
               
               
                   
                 Pattern 24 
                 0 
                 136 
                 0 
               
               
                   
                 Pattern 25 
                 0 
                 153 
                 0 
               
               
                   
                 Pattern 26 
                 0 
                 170 
                 0 
               
               
                   
                 Pattern 27 
                 0 
                 187 
                 0 
               
               
                   
                 Pattern 28 
                 0 
                 204 
                 0 
               
               
                   
                 Pattern 29 
                 0 
                 221 
                 0 
               
               
                   
                 Pattern 30 
                 0 
                 238 
                 0 
               
               
                   
                 Pattern 31 
                 0 
                 255 
                 0 
               
               
                   
                 Pattern 32 
                 0 
                 0 
                 17 
               
               
                   
                 Pattern 33 
                 0 
                 0 
                 34 
               
               
                   
                 Pattern 34 
                 0 
                 0 
                 51 
               
               
                   
                 Pattern 35 
                 0 
                 0 
                 68 
               
               
                   
                 Pattern 36 
                 0 
                 0 
                 85 
               
               
                   
                 Pattern 37 
                 0 
                 0 
                 102 
               
               
                   
                 Pattern 38 
                 0 
                 0 
                 119 
               
               
                   
                 Pattern 39 
                 0 
                 0 
                 136 
               
               
                   
                 Pattern 40 
                 0 
                 0 
                 153 
               
               
                   
                 Pattern 41 
                 0 
                 0 
                 170 
               
               
                   
                 Pattern 42 
                 0 
                 0 
                 187 
               
               
                   
                 Pattern 43 
                 0 
                 0 
                 204 
               
               
                   
                 Pattern 44 
                 0 
                 0 
                 221 
               
               
                   
                 Pattern 45 
                 0 
                 0 
                 238 
               
               
                   
                 Pattern 46 
                 0 
                 0 
                 255 
               
               
                   
                 Pattern 47 
                 17 
                 17 
                 17 
               
               
                   
                 Pattern 48 
                 34 
                 34 
                 34 
               
               
                   
                 Pattern 49 
                 51 
                 51 
                 51 
               
               
                   
                 Pattern 50 
                 68 
                 68 
                 68 
               
               
                   
                 Pattern 51 
                 85 
                 85 
                 85 
               
               
                   
                 Pattern 52 
                 102 
                 102 
                 102 
               
               
                   
                 Pattern 53 
                 119 
                 119 
                 119 
               
               
                   
                 Pattern 54 
                 136 
                 136 
                 136 
               
               
                   
                 Pattern 55 
                 153 
                 153 
                 153 
               
               
                   
                 Pattern 56 
                 170 
                 170 
                 170 
               
               
                   
                 Pattern 57 
                 187 
                 187 
                 187 
               
               
                   
                 Pattern 58 
                 204 
                 204 
                 204 
               
               
                   
                 Pattern 59 
                 221 
                 221 
                 221 
               
               
                   
                 Pattern 60 
                 238 
                 238 
                 238 
               
               
                   
                 Pattern 61 
                 255 
                 255 
                 255 
               
               
                   
                   
               
            
           
         
       
     
     As an example, after the measurement using the narrowband spectrometer and the wideband colorimeter, the color correction matrix can be obtained in the below format: 
     
       
         
           
               
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 aX[1] = −0.014609 
                 aY[1] = −0.017631 
                 aZ[1] = 0.024884 
               
               
                   
                 aX[2] = 0.931186 
                 aY[2] = 0.068468 
                 aZ[2] = −0.003951 
               
               
                   
                 aX[3] = −0.045284 
                 aY[3] = 0.817216 
                 aZ[3] = 0.004081 
               
               
                   
                 aX[3] = −0.004684 
                 aY[4] = −0.011521 
                 aZ[4] = 0.850434 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 lists the raw data during the measurement, before and after using the correction matrix 
               
            
           
           
               
               
               
               
               
            
               
                   
                 ORIGINAL 
                 ORIGINAL 
                 CORRECTED 
                 CORRECTED 
               
               
                 COLOR 
                 ERROR x 
                 ERROR y 
                 ERROR x 
                 ERROR y 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 White 
                 0.005803118 
                 0.003390245 
                 0.001357612 
                 0.001791935 
               
               
                 Fuchsia 
                 0.010929051 
                 0.010255611 
                 0.002357682 
                 0.001699973 
               
               
                 Red 
                 −0.010552573 
                 0.014348118 
                 0.00096754 
                 −0.000131641 
               
               
                 Silver 
                 0.004753463 
                 0.002249964 
                 0.000338233 
                 0.000696155 
               
               
                 Gray 
                 0.004292981 
                 0.000438794 
                 1.94125E−05 
                 −0.000893157 
               
               
                 Olive 
                 0.000992562 
                 −0.00065155 
                 −0.000431134 
                 −0.000251556 
               
               
                 Purple 
                 0.008693544 
                 0.008715242 
                 0.000495887 
                 0.000663561 
               
               
                 Maroon 
                 −0.008015031 
                 0.011809138 
                 −0.000679522 
                 0.001593174 
               
               
                 Aqua 
                 −0.000848702 
                 −0.00094957 
                 −0.000900931 
                 0.00064071 
               
               
                 Lime 
                 −0.005728015 
                 0.000750034 
                 −0.001553989 
                 −0.000941518 
               
               
                 Teal 
                 −0.000344654 
                 −0.00347737 
                 −0.000159383 
                 −0.001441836 
               
               
                 Green 
                 −0.004834236 
                 0.001791446 
                 −0.000329158 
                 0.001183251 
               
               
                 Blue 
                 0.00401903 
                 −0.003069638 
                 −0.000796601 
                 −0.000594766 
               
               
                 Navy 
                 0.005024664 
                 −0.002098077 
                 0.000747138 
                 0.000974822 
               
               
                   
               
            
           
         
       
     
     Table 2 shows the errors in x and y for 14 color patterns as determined for the case before the correction matrix was used (i.e., original error) and for the case after the correction matrix was used (i.e., corrected error). It can be seen that, after using the correction, the wideband colorimeter accuracy can be improved by one order of magnitude. The data in Table 2 is plotted in  FIGS. 5 and 6 , and summarized in Table 3 (summary table).  FIG. 5  shows a plot of the original errors in x and y (i.e., before using the correction matrix) for 14 color patterns, while  FIG. 6  shows a plot of the corrected errors in x and y (i.e., after using the correction matrix) for the same 14 color patterns. Table 3 below summarizes the averages and standard deviations of the errors both for before using correction matrix (i.e., original) and for after using the correction matrix (i.e., corrected). 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Summary table of the errors in x and y 
               
            
           
           
               
               
               
               
               
            
               
                   
                 x AVERAGE 
                 y AVERAGE 
                 x STD DEV 
                 y STD DEV 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 Before 
                 0.001013229 
                 0.003107314 
                 0.006387198 
                 0.005811428 
               
               
                 Correction 
               
               
                 After 
                 0.000102342 
                 0.000356365 
                 0.001028542 
                 0.001064983 
               
               
                 Correction 
               
               
                   
               
            
           
         
       
     
     From the above existing data, it can be seen that the proper pattern choices can help improve the accuracy and precision level of the wideband colorimeter by up to a factor of 5-10 times. 
       FIG. 7  is a flow chart of a method  700 , including steps for correcting the output of a wideband colorimeter, in accordance with one embodiment described in the specification. As shown in  FIG. 7 , the method  700  begins at step  710 , where the method configures a wideband color measurement device and a narrowband color measurement device to measure color. Then, at step  720 , the method stimulates both the wideband color measurement device and the narrowband color measurement device with predetermined test patterns. In one embodiment, the test patterns can be as described in Table 1 above. In another embodiment, the test patterns include 61 unique digital color stimulus patterns. In other embodiments, more than 61 test patterns can be used. Next, at step  730 , the method captures color measurement data from the wideband color measurement device and the narrowband color measurement device. After step  730 , the method proceeds to step  740 , where the method determines a correction matrix relating the captured data. In one embodiment, the wideband and the narrowband color measurement device outputs can be related by the correction matrix. Then the method proceeds to step  750 , where the method corrects the color measurement output of the wideband color measurement device with the correction matrix. 
       FIG. 8  illustrates a flow chart showing method steps for performing calibration of a wideband color measurement device with a narrowband color measurement device, so that more accurate outputs of the wideband device can be provided, in accordance with one embodiment described in the specification. In one embodiment, the method shown in  FIG. 8  can be performed using a device, such as a spectra camera illustrated in  FIG. 3 . As shown in  FIG. 8 , the method  800  begins at step  810 , where the method presents predetermined test patterns. Then, at steps  820  and  830 , the method configures the narrowband color measurement device and the wideband color measurement device to concurrently measure color data from the predetermined test patterns. In one embodiment, the test patterns can be as described in Table 1 above. In another embodiment, the test patterns include 61 unique digital color stimulus patterns. In other embodiments, more than 61 test patterns can be used. Next, at step  840 , the method captures the color data from the wideband color measurement device and the narrowband color measurement device. After step  840 , the method proceeds to step  850 , where the method determines a best fit correction matrix relating the captured color data. In one embodiment, the best fit correction matrix can be a 3×3 matrix. In another embodiment, the best fit correction matrix can be a 3×4 matrix. In the 3×4 matrix, the last column corresponds to a constant offset, because for very low brightness, the effect will be more important to make it more accurate. Then the method proceeds to step  860 , where there is an evaluation if the best fit correction matrix is acceptable as a calibration parameter for color measurements from the wideband device. If the best fit correction matrix is acceptable as a calibration parameter, then the method proceeds to step  870 , where the method uses the best fit correction matrix as the calibration parameter for the color measurements from the wideband device. If the best fit correction matrix is not acceptable, then the method returns to step  810 , where the method repeats the calibration process again, starting with presentation of predetermined test patterns. 
       FIG. 9  is a block diagram of an electronic device suitable for controlling some of the processes in the described embodiments. Electronic device  900  can illustrate circuitry of a representative computing device. Electronic device  900  can include a processor  902  that pertains to a microprocessor or controller for controlling the overall operation of electronic device  900 . Electronic device  900  can include instruction data pertaining to operating instructions, such as instructions for implementing and controlling a user equipment, in a file system  904  and a cache  906 . File system  904  can be a storage disk or a plurality of disks. In some embodiments, file system  904  can be flash memory, semiconductor (solid state) memory or the like. The file system  904  can typically provide high capacity storage capability for the electronic device  900 . However, since the access time for the file system  904  can be relatively slow (especially if file system  904  includes a mechanical disk drive), the electronic device  900  can also include cache  906 . The cache  906  can include, for example, Random-Access Memory (RAM) provided by semiconductor memory. The relative access time to the cache  906  can be substantially shorter than for the file system  904 . However, cache  906  may not have the large storage capacity of file system  904 . The electronic device  900  can also include a RAM  920  and a Read-Only Memory (ROM)  922 . The ROM  922  can store programs, utilities or processes to be executed in a non-volatile manner. The RAM  920  can provide volatile data storage, such as for cache  906 . 
     Electronic device  900  can also include user input device  908  that allows a user of the electronic device  900  to interact with the electronic device  900 . For example, user input device  908  can take a variety of forms, such as a button, keypad, dial, touch screen, audio input interface, visual/image capture input interface, input in the form of sensor data, etc. Still further, electronic device  900  can include a display  910  (screen display) that can be controlled by processor  902  to display information, such as test results, to the user. Data bus  916  can facilitate data transfer between at least file system  904 , cache  906 , processor  902 , and input/output (I/O) controller  913 . I/O controller  913  can be used to interface with and control different devices such as camera, spectrometer or motors to position mirror/lens through appropriate codecs. For example, control bus  914  can be used to control camera  928 . 
     Electronic device  900  can also include a network/bus interface  911  that couples to data link  912 . Data link  912  can allow electronic device  900  to couple to a host computer or to accessory devices or to other networks such as the internet. The data link  912  can be provided over a wired connection or a wireless connection. In the case of a wireless connection, network/bus interface  911  can include a wireless transceiver. Sensor  926  can take the form of circuitry for detecting any number of stimuli. For example, sensor  926  can include any number of sensors for monitoring a environmental conditions such as for example a light sensor such as a photometer, a temperature sensor and so on. 
     The various aspects, embodiments, implementations or features of the described embodiments can be used separately or in any combination. Various aspects of the described embodiments can be implemented by software, hardware or a combination of hardware and software. The described embodiments can also be embodied as computer readable code on a non-transitory computer readable medium executable by a processor. The computer readable medium is any data storage device that can store data which can thereafter be read by a computer system. Examples of the computer readable medium include read-only memory, random-access memory, CD-ROMs, HDDs, SSDs (solid-state drives), DVDs, magnetic tape, and optical data storage devices. The computer readable medium can also be distributed over network-coupled computer systems so that the computer readable code is stored and executed in a distributed fashion. 
     The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the described embodiments. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the described embodiments. Thus, the foregoing descriptions of specific embodiments are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the described embodiments to the precise forms disclosed. It will be apparent to one of ordinary skill in the art that many modifications and variations are possible in view of the above teachings.

Metadata:
Filing Date: 20131018
Publication Date: 20150324
Grant Date: 20150324
Priority Date: 20121023
Inventors: YIN YE
MARCU GABRIEL G.
WU JIAYING
Assignee: APPLE INC
CPC Classifications: [{"code": "G01J3/51", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/513", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/0297", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/50", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/0218", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/28", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01J3/0243", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/524", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/0291", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/465", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/28", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01J3/502", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/506", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/0294", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/462", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/28", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01J3/506", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/513", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/0294", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/502", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/0243", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/465", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/462", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/0218", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/0291", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/0297", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/51", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/50", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/524", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 50485074