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A color space defined by the Commission Internationale de l'Eclairage (see CIE). In 1931, CIE defined the CIE XYZ color space, which represented all possible colors based on human perception. Like RGB, CIE XYZ has three orthogonal dimensions; however, X, Y and Z do not correspond to real colors. They are simply mathematically convenient with Y carrying the luminance information. CIE is usually represented by a 2D "chromaticity diagram" obtained from the CIE XYZ model.

The CIE XYZ model and chromaticity diagram are not perceptually uniform. A more uniform version of CIE was defined in 1976, officially known as CIE L*a*b*. L* stands for luminance, a* is the red-green axis, and b* is the blue-yellow axis. The asterisks were added to differentiate CIE from another L,a,b model.

A Reference and Intermediary
Although CIE L*a*b* has a large color gamut and is considered the most accurate color model, it is often used as a reference only or as an intermediary for color space conversion. In practice, the RGB and CMYK color spaces are more widely used for display and printing, and HSB and HSL are used for color selection. However, all color spaces, including RGB, CMYK, HSB, HSL and YUV, are subsets of the entire human color gamut as measured and defined by CIE. See color space, RGB, CMYK, YUV, HSB and HSL.

CIE 1976 Chromaticity Diagram
In the original 1931 CIE diagram, the color green seemed to dominate. In 1976, changes were made that displayed perceived colors more uniformly. (Image courtesy of Photo Research, Inc.,