Color matching functions

Colorimeters. Also known as tristimulus colorimeters, these are instniments that do not measure spectral data but typically use four broad-band filters to approximate the jy, and the two peaks of the x color-matching functions of the standard observer curves of Figure 7. They may have lower accuracy and be less expensive, but they can serve adequately for most industrial color control functions. Examples of colorimeters are the BYK-Gardner Co. XL-835 the Hunter Lab D25 series the Minolta CA, CL, CS, CT, and CR series (the last of these is portable with an interface) and the portable X-Rite 918. 

The C1E color coordinates are obtained by a similar procedure, repeating the convolution for each of the tabulated color matching functions that describe the response of the ideal observer s blue, green, and red cones (b, rA). 

In Eqs. (7)—(10), 5(A) is the spectral power distribution of the illuminant, and R A) is the spectral reflectance factor of the object. Jc(A), y(A), and 5(A) are the color-matching functions of the observer. In the usual practice, k is defined so that the tristimulus value, Y, for a perfect reflecting diffusor (the reference for R A)) equals 100. Using the functions proposed by the CIE in 1931, y(A) was made identical to the spectral photopic luminous efficiency function, and consequently its tristimulus value, Y, is a measure of the brightness of objects. The X and Z values describe aspects of color that permit identification with various spectral regions. 

The A(A), y(A), and z(A) terms were derived by the CIE from data obtained in visual experiments where observers matched colors obtained by the mixing of the blue, green, and red primary colors. The average result for human observers were defined as the CIE 1931 2° standard observer, and the wavelength dependencies of these color-matching functions are illustrated in Fig. 6. 

Fig. 6 Color-matching functions Jc(A), and z(A) of the CIE 1931 2° standard observer. (Data adapted from Ref. 40.)...

Tristimulus values X, Y, and Z are computed for a primary light source with power spectrum L(X) from the color-matching functions x, y, and z as follows ... 

The "tristimulus" variables X, Y, Z are defined by color-matching functions x(l), y(l), and z(l) (Fig. 10.4) that roughly correspond to the color sensitivity of the three types of human retina rods. For any "primary" color, with intensity 1(1), the tristimulus values X, Y, and Z are defined by... 

ESTEVEZ, o. (1982) A better colorimetric standard observer for color-vision studies. The Stiles and Burch 2° color-matching functions. Color Research and Application, 1, 131-134. 

This diagram shows schematically how the spectral power distribution of a CIE source, the spectral reflectance, R, of an object, and the spectral color-matching functions, x, y and z, combine by multiplication (each wavelength by each wavelength), followed by summation across the spectrum, to give the CIE tristunulus values. However, these analogues cU e... 

When we introduce the color response characteristics of the source, combined with the detector of our instrument, we find that we must drastically modify the transmission characteristics of our filters in order to duplicate the CIE color matching functions for the equal-energy spectrum. However, this is not an impossible task and we find that an excellent match can be obtained to the transmission functions of 6.7.20. This is typical for commercially available instruments. Now, we have an instrument, called a Colorimeter, capable of measuring reflective color. 

Color matching functions n. Relative amounts of three additive primaries required to match each wavelength of light. The term is generally used to refer to the CIE Standard Observer color matching functions designated x + y + z. 

Tristimulus computation data n. Products of relative spectral-energy distribution of an illuminant multiplied by each of the three color matching functions in the CIE system designated as EcY, EcT e.g., for Illuminant C and the color mixture data for the standard observer at designated wavelengths. 

X n. Special color matching functions of the CIE standard observer used for calculating the X tristimulus value. McDonald R (ed) (1997) Colour physics for industry, 2nd edn. Society of Dyers and Colourists, West Yorkshire, England. 

See tristimulus computation data, tristimulus value, and color matching functions. 

Y n. Special color matching function of the CIE standard observed used for calculating... 

Where R is the reflectance of the object S AX the spectral distribution of the flux irradiating the object and Xx, yx, zx, the CIE color-matching functions [30],... 

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