Standard illuminant
A numerically defined spectral power distribution, published by the CIE, used to specify the light under which colors are measured.
Spectral power distribution: The curve giving a light's radiant power at each wavelength across the visible spectrum. An illuminant is nothing more than an agreed SPD, written as a table of numbers.
Illuminant D65: The CIE standard representing average mid-day north European daylight, about 6500 K. The default illuminant for color measurement, displays, and the sRGB standard.
Color matching functions: The CIE functions describing how the standard observer's three cone responses weight any spectrum. They convert an illuminant's SPD into tristimulus values and chromaticity coordinates.
sRGB: The default color space of the web, defined with D65 as its white point. Every web color is specified relative to a standard illuminant.
Light source: A physical emitter of light, as opposed to the CIE's theoretical spectral tables. An illuminant is defined on paper; a light source only approximates it.
CIE 1931 color space: The first mathematical system mapping physical spectra to human color perception via tristimulus values. The XYZ values of an illuminant define its position and serve as the reference white.
Illuminant A: The CIE standard for incandescent light, defined as a black-body radiator at 2856 K. The oldest standard illuminant, still used for testing color under tungsten light.
Black-body radiation: The spectrum emitted by an idealized hot body, described by Planck's law and set by temperature alone. Illuminant A is a black body, and daylight illuminants are anchored to the black-body locus.
ICC profile: A data file describing a device's color behavior so colors translate predictably between devices. Profiles record the illuminant under which device measurements were taken.
Color rendering index: A 0–100 metric scoring how faithfully a real light source reveals object colors against a reference. The CRI reference illuminants are standard illuminants; the metric grades departure from them.