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Colorimetry of Displays
NIST maintains a calibration facility for display color measurement and provides calibration services for colorimeters and spectroradiometers measuring display colors. A key component of the facility is a reference spectroradiometer developed at NIST for most accurate display color measurement. The spectroradiometer is a double-grating type and equipped with an input imaging optics with a depolarizer. The uncertainties in chromaticity when measuring cathode ray tube (CRT) or liquid crystal (LCD) displays were analyzed by a series of computer simulation for each source of error. Corrections are applied for wavelength scale and bandpass of the instrument. The uncertainties in color measurement with the NIST reference spectroradiometer measuring CRT or LCD were determined to be ~0.001 (k = 2) or less in chromaticity (x, y) and 1 % in luminance (Y). In addition, a correction matrix for the Four-Color Method (developed by NIST) can be supplied to customer to allow for convenient correction of measured chromaticity of any color of the tested display.     Figure 1

Figure 1. Photo of a flat panel display set for measurement with a spectroradiometer.

Introduction

Colorimeters and spectroradiometers are commonly used to measure the chromaticity and luminance of displays, and useful protocols for color measurement using these instruments are available [1,2]. However, the instruments are normally calibrated against an incandescent standard lamp having a broad, smoothly varying spectral power distribution, while display colors have very different spectral distributions, often incorporating narrow spectral features. As a result, chromaticity errors tend to be much larger than anticipated when these instruments are used to measure displays. For example, commercial tristimulus colorimeters and diode-array spectroradiometers can be calibrated against Illuminant A with uncertainties (k=2) on the order of 0.001 in x,y and 1 % in Y. However, inter-instrument variations for chromaticity measurements of various colors of a display are often found to be as large as 0.01 in x,y and 10 % in Y (corresponding to approximately 10 DE*ab). Such variations are much larger than the accuracy required for many applications. For example, measurement uncertainties within 0.005 in chromaticity are recommended for CRT and LCD color measurements in international standards [3]. To address the higher-accuracy measurement needs, a calibration facility for display measurements is established at NIST, and calibration services for color-measuring instruments are made available [4]. In the NIST facility, several different display colors are measured with a test instrument and the NIST reference spectroradiometer and the differences in measured chromaticity for the particular display are reported. Such results can be used to verify the uncertainty of the test instrument or to apply corrections to the test instrument results to improve its uncertainty.

Figure 2   Figure 3

Figures 2 and 3. NIST calibration facility for display color measurement (left) and the reference spectroradiometer with some test instruments mounted (right).

The NIST reference spectroradiometer consists of imaging optics; a double-grating, scanning monochromator for wavelength selection; and a photomultiplier tube for detection. The instrument has been characterized for stray light, wavelength error, variable bandpass, linearity, and random measurement uncertainties. These results were then incorporated into detailed simulations to estimate the uncertainty in chromaticity measurements of a CRT and an LCD display. The uncertainties are determined to be ~0.001 (k = 2) or less in chromaticity (x,y), depending on color, for a CRT or an LCD [5].

Figure 3 Figure 4

Figures 4 and 5. Construction of the NIST reference spectroradiometer for display measurement (left) and typical display spectra (right).

Four Color Method

A correction method has been developed to improve the accuracy of color measurement of displays using a colorimeter (or a spectroradiometer). Matrix methods such as the one that used to be recommended by ASTM [2] are known for this purpose, but they often did not reduce errors satisfactorily due to experimental noise and signal fluctuations from the display. As these matrix methods are based on tristimulus values, the accuracy of the luminance measurement (Y) affects the accuracy of the corrected chromaticity. A new method (Four-Color method), developed by NIST, utilizes x,y values only, and is independent of Y values. Thus, in principle, it eliminates errors due to luminance measurement variations (due to flicker of display, etc.). Four colors of a display -- red, green, blue, and white -- are measured with a test instrument and a reference instrument, and from the results of (x,y), a 3 ×  correction matrix is obtained. The correction brings the test instrument a similar accuracy as the reference instrument for any color of the particular display. An example of the correction of a tristimulus colorimeter measuring an LCD is shown below. Details are found in references [6,7]. This method has been adopted as a new ASTM standard [8].

Figure 6

Figure 6. Two figures, above showing original errors, below showing errors after correction for chromaticity (x,y) of a flat panel display.

References

  1. ASTM E 1336-91, Standard Test Method for Obtaining Colorimetric Data From a Visual Display Unit by Spectroradiometry (1991).
  2. ASTM E 1455-96, Standard Practice for Obtaining Colorimetric Data From a Visual Display Unit Using Tristimulus Colorimeters (1996).
  3. IEC 61966-3 (2000-03) Multimedia systems and equipment - Colour measurement and management - Part 3: Equipment using cathode ray tubes.
  4. Y. Ohno and S. Brown, Four-Color Matrix Method for Correction of Tristimulus Colorimeters - Part 2 (54 kB) PDF, Proc. IS&T Sixth Color Imaging Conference, 65-68 (1998).
  5. F. Manoocheri, S.W. Brown, and Y. Ohno, NIST Colorimetric Calibration Facility for Displays -- Part 2 (293 kB) PDF, Society for Information Displays '01 DIGEST, 330-333 (2001).
  6. Y. Ohno and J. Hardis, Four-Color Matrix Method for Correction of Tristimulus Colorimeters (39 kB) PDF, Proc. IS&T Fifth Color Imaging Conference, 301-305 (1997).
  7. Y. Ohno and S. Brown, Four-Color Matrix Method for Correction of Tristimulus Colorimeters - Part 2 (54 kB) PDF, Proc. IS&T Sixth Color Imaging Conference, 65-68 (1998).
  8. ASTM E1455-03, Standard Practice for Obtaining Colorimetric Data From a Visual Display Unit Using Tristimulus Colorimeters (2003).
Optical Sensor Group
For technical information or questions, contact:
Yuqin Zong
Phone: (301) 975-2332
Email: yzong@nist.gov		 Steve Brown
Phone: (301) 975-5167
Email: swbrown@nist.gov		 Yoshi Ohno
Phone: (301)-975-2321
Email: ohno@nist.gov

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Online: February 2000   -   Last updated: February 2008