chromaticity

Chromaticity is a property of color that describes its quality as perceived by the human eye independent of how bright the color appears. It conveys hue and saturation while separating out luminance or brightness. In color science, chromaticity is derived from the spectral power distribution of a light source or from a reflectance spectrum, but it does not specify how light energy is arranged in intensity.

Chromaticity coordinates are obtained from the tristimulus values X, Y, Z used in the CIE color system. The chromaticity coordinates are defined as x = X/(X+Y+Z) and y = Y/(X+Y+Z) (with z = Z/(X+Y+Z) = 1 − x − y). Because the sum X+Y+Z scales with luminance, the coordinates x and y depend only on the spectral composition, not on absolute brightness. The Y component carries luminance information.

In a chromaticity diagram, such as the CIE 1931 xy diagram, the spectrum locus marks the pure spectral colors along a curved boundary. Colors inside the membrane represent mixtures of wavelengths, while white points lie near the center depending on the illuminant. The diagram is a tool for visualizing hue and saturation relationships, comparing colors across devices, and guiding color matching and reproduction. Other chromaticity representations include the CIE 1960 UCS and CIE 1976 u′v′ spaces, which aim to reduce perceptual distortions.

Limitations include dependence on the chosen observer and illuminant, and the fact that chromaticity alone omits luminance information which is essential for accurate color reproduction in lighting and display systems.