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CIE fundamentals for color measurements

CIE fundamentals for color measurements - Paper for IS&T...

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- 1 - Paper for IS&T NIP16 Conference, Vancouver, Canada, Oct. 16-20, 2000 (Final manuscript) CIE Fundamentals for Color Measurements Yoshi Ohno Optical Technology Division National Institute of Standards and Technology Gaithersburg, MD 20899 USA Abstract The paper first overviews the CIE system of colorimetry, covering CIE 1931 color matching functions, XYZ tristimulus values, the x, y diagram, the 1976 u', v' diagram, and the evolvement of CIELUV and CIELAB color spaces and color difference formulae. The paper reviews the measurement of object colors introducing CIE standard illuminants and the CIE terminology for color and reflectance measurements, then the measurement of light- source colors (including displays) with calculation of correlated color temperature and color rendering indices. The paper also discusses practical aspects of color measurements for imaging applications using spectroreflectometers, spectroradiometers, and tristimulus colorimeters. Overview is given for calibration and verification of instruments’ accuracy, spectral irradiance and reflectance standards (available from national laboratories), and uncertainty components. Introduction The term color is used with different meanings in different technologies. To lamp engineers, color refers to a property of light sources. To graphics art engineers, color is a property of an object’s surface (under a given illumination). In each case, color must be physically measured in order to record it and reproduce the same color. The perception of color is a psychophysical phenomenon, and the measurement of color must be defined in such a way that the results correlate accurately with what the visual sensation of color is to a normal human observer. Colorimetry is the science and technology used to quantify and describe physically the human color perception. The basis for colorimetry was established by CIE (Commission Internationale de l clairage) in 1931 based on visual experiments. Even though limitations are well recognized, the CIE system of colorimetry remains the only internationally agreed metric for color measurement. All the official color-related international standards and specifications use the CIE System. The CIE system works well in most cases, but one should know the assumptions and limitations in visual conditions where the CIE system is defined. In this paper, the CIE system of colorimetry is briefly overviewed, and then practical aspects of color measurements and instruments – spectroreflectometers, spectroradiometers, and colorimeters – are discussed, with a focus on the calibration methods and standards. Uncertainty components and correction for errors are also discussed. For further details in colorimetry and color science, refer to official CIE publications 1-3 and many other appropriate references 4 .
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