What is the difference between a light source and an illuminant?

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a poor representation of daylight in that it contains insufficient energy at the lower wavelengths and it has generally been replaced by a class of illuminants known as the D illuminants.