CIEXYZ

CIEXYZ, or CIE XYZ, is a color space defined by the International Commission on Illumination (CIE) in 1931 as a standard observer-based model of human color vision. It is a device-independent reference space used to describe colors in a way that approximates human perception and to serve as an intermediate step in many color conversions.

The tristimulus values X, Y, and Z are obtained from a light's spectral power distribution S(λ) by integrating with the CIE color matching functions x̄(λ), ȳ(λ), and z̄(λ): X = ∫ S(λ) x̄(λ) dλ, Y = ∫ S(λ) ȳ(λ) dλ, Z = ∫ S(λ) z̄(λ) dλ. The Y value is designed to correspond to luminance, making Y a convenient proxy for perceived brightness, while X and Z carry chromatic information.

Chromaticity coordinates x and y are derived from X, Y, Z as x = X/(X+Y+Z) and y = Y/(X+Y+Z), with z = 1 − x − y. The pair (x, y) defines a color’s position on the CIE 1931 chromaticity diagram, while X, Y, Z retain absolute tristimulus values.

The standard observer historically used is the 1931 two-degree color matching functions, with later revisions (e.g., 1964 ten-degree). Illuminants such as D65 or D50 provide reference whites for normalization. Typical white points under D65 yield reference values around X=95.047, Y=100.000, Z=108.883.

Applications include serving as the foundational, device-independent space for converting spectral data to display and print color spaces (such as sRGB, Adobe RGB, and CMYK) and underpinning ICC profiles. While practical, CIEXYZ is not perceptually uniform; perceptual spaces like CIELAB or CIELUV are often used for perceptual color differences.