Header for SPIE use NIST Calibration Facility for Display Colorimeters Steven W. Brown and Yoshi Ohno National Institute of Standards and Technology, Gaithersburg, MD 20899 ABSTRACT A calibration facility has been developed at the National Institute of Standards and Technology (NIST) to address the need for high accuracy color measurements of displays. Calibration services tailored to display measurements are planned for colorimeters and spectroradiometers. A key component of the facility, a reference spectroradiometer, has been developed and its uncertainty for display measurements estimated using a series of computer simulations. The simulations predict that the reference spectroradiometer — corrected for wavelength error and variable bandpass — can measure any color of a cathode ray tube (CRT) or liquid crystal (LCD) display with a combined standard uncertainty of approximately 0.001 in chromaticity ( x, y ) and 1 % in luminance (Y). In addition, a new matrix correction technique (the Four-Color Method) has been developed as a means to transfer the calibration from the reference instrument to a test instrument. Using the Four- Color Method, the residual errors with the calibrated instrument for one type of display are reduced to within 0.001 in x,y (or ~1 ∆E * ab) with respect to the reference instrument. To evaluate the overall performance of the system, commercial spectroradiometers and tristimulus colorimeters were calibrated against the reference instrument, measuring both a CRT and an LCD display. The results show that calibrated target instruments can measure various colors of a particular display with a combined standard uncertainty of approximately 0.002 in x,y and 2 % in Y (~2 ∆E * ab). Keywords: calibration, standards, display, colorimeter, spectroradiometer, colorimetry 1. 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 on the order of 0.001 in x,y and 1% in Y [3]. However, inter-instrument variations for chromaticity measurements of various colors of a display are often as large as 0.01 in x, y and 10 % in Y (corresponding to approximately 10 ∆E * 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 [4]. To address the need for higher-accuracy measurements of displays, a program has been established at NIST to develop standards and calibration services for color-measuring instruments. A central component of this program, a calibration facility, has been developed to calibrate instruments for measuring display colors [5]. In this facility, a test instrument and a reference spectroradiometer measure various colors of a particular display (e.g. a CRT or an LCD display). The chromaticity and luminance values are then compared, and the test instrument values are corrected using the Four-Color Method [6,7] to more closely approximate the reference values. In this paper, we describe details of the reference spectroradiometer, including a comprehensive uncertainty analysis, together with recent results on the Four-Color Method. 2. CHARACTERIZATION OF THE REFERENCE SPECTRORADIOMETER The reference spectroradiometer, shown in Fig. 1, 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,