In 1931 the Commission Internationale de l’Eclairage (CIE) recommended, for the unambiguous determination of colorimetric measures, the use of 3 spectral evaluation functions x(λ), y(λ) and z(λ), which are derived from the measurements made by Guild and Wright for a 2° field of view in humans with normal color vision.

Strictly speaking, these three evaluation functions only apply to theoretical vision, with its 2° field of view. Here the x(λ) and z(λ) functions make no contribution to the brightness of the color stimulus specifications, and the y(λ) function is identical to the luminous efficiency function V(λ) of the human eye in daylight vision.

Since the spectral sensitivity CIE 1931 chromaticity diagram for the definition of color type with the x,y chromaticity value



functions of the eye vary with the size of the illuminated area of the retina, and in practical visual color comparisons significantly wider fields of view are observed, there are, necessarily, differences between visual evaluation and evaluation based on the 2° standard spectral evaluation functions. In order to describe color equivalencies quantitatively, the color values X, Y, and Z are introduced:

The normalized color value components x, y, and z are calculated as follows from the standard color values X, Y, and Z:

 x = X / (X+Y+Z);
 y = Y / (X+Y+Z);
 z = Z / (X+Y+Z)

The type of color of an object is fully and unambiguously given by the two normalized color value components, x and y. The color type is sufficient to characterize the color of primary light sources. The brightness is then described by the luminance L or the illuminance E.
Since in the standard color stimulus specification system, x-y-z, the geometrical distances differ quite markedly from the subjectively experienced color differences, the CIE has defined a number of color spaces, one of which is the u’- v’ color space:

u´= 4X / (X+15Y+3Z)
v´= 9Y / (X+15Y+3Z)


Information for Gigahertz-Optik color measurement systems can be found under:

 Products section:  HCT-99, Color Meter,
                            P-9801, 8-Channel Optometer with CT-3701 Detector Head,
                       
 Tutorials section: Theory and Application

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