The terms light source and illuminant have precise and different meanings. A light source is a physical emitter of radiation such as a candle, a tungsten bulb, and natural daylight. An illuminant is the specification for a potential light source. All light sources can be specified as an illuminant, but not all illuminants can be physically realized as a light source.

Illuminants are normally specified in terms of the relative energy tabulated for each wavelength or wavelength band. There are several illuminants that are widely used by the colour industry and these include A, C, D65, and TL84. Illuminants A and C were defined by the CIE in 1931 to represent tungsten light and natural daylight respectively. Illuminant C was found to be a poor representation of daylight in that it contains insufficient energy at the lower wavelengths and it has generally been replaced by a class of illuminants known as the D illuminants.

D class illuminants
The D class of illuminants specify relative energy distributions that closely correspond to the radiation emitted by a so-called black-body. As the tempertature of a black body is increased there is a shift in the emitted radiation to longer wavelengths. A specific D illuminant is therefore notated with reference to the temperature (in Kelvin) of the black-body which it most closely matches. For example, the illuminant D65 has a spectral energy distribution that closely matches that of a black-body at 6500K. Illuminant D65 also closely resembles the relative spectral energy distribution of north-sky daylight and is accordingly important for colour specification in northern Europe. Other D illuminants, notably D55, are important in other parts of the world.

TL84
There are a number of illuminants that specify light sources used in specific industries and sometimes by specific companies. An example of this is illuminant TL84.

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