standard illuminants

(optics) Three standard sources of light, designated A, B, and C, used in specifying the light used when colors are matched; A is light from a filament at a color temperature of 2575°C, and B and C, representing noon sunlight and normal daylight respectively, are obtained by modifying A with rigorously specified filters.

engineering, colorimetry These are reference illuminants of specific spectral composition, established to avoid the problems familiar through the use of fluorescent lights that distort colours relative to natural light. Standard A is the light of an incandescent filament at 2 848 K (4666.73°F). Standards B, representing overhead sunlight, and C, representing normal sunlight, are defined from A via filters, each being a double cell made of colourless glass holding in each compartment a solution of specified composition. Standard illuminant B is used along with the three cardinal illuminants red, green, and blue in the CIE system.
[Judd D. B. J. Opt. Soc. Amer. Vol. 23, 359-74 (1933)]

Technically these are standard sources, i.e. lights, but being defined by spectral composition, they are illuminants.