Illuminant (colorimetry)

A defined spectral power distribution of light, tabulated by the CIE and used in color calculations, independent of any real light source.

Spectral power distribution: The tabulated radiance or relative power of a light at each wavelength across the visible spectrum. An illuminant is a specified SPD; the tables are its entire content.
D65: The CIE standard daylight illuminant at about 6500 K correlated color temperature; the default for displays and sRGB. The most widely used illuminant and the reference white of most imaging standards.
Light source: A physical lamp or emitter actually producing light; it can approximate an illuminant but never match it exactly. The distinction between a paper table and a real lamp is the definition's whole point.
CIE 1931 color space: The foundational CIE system of tristimulus values and chromaticity coordinates defining color numerically. Illuminants were introduced within this framework and are meaningless outside it.
ICC color management: The profile system embedding illuminant and white point data to convert colors across devices. Every ICC profile records its measurement illuminant, usually D50 or D65.
CIE standard observer: Color-matching functions describing average human trichromatic vision, defined by the CIE in 1931. Tristimulus values are computed by weighting the illuminant spectrum with these functions.
D50: A CIE daylight illuminant at about 5000 K, used as the reference white in graphic arts and printing. The printing industry's standard viewing condition, distinct from D65 displays.
Metamerism: Spectrally different stimuli matching in color under one illuminant but not another. Changing the illuminant is the standard test that exposes metameric pairs.
Spectroradiometry: The measurement of radiant power as a function of wavelength, the empirical basis of illuminant tables. Daylight illuminant tables were derived from spectroradiometric measurements of real daylight.
sRGB color space: The default web and display color space, defined with D65 as its reference white. Its D65 white point fixes how encoded RGB numbers map to actual color.

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