The first three standard illuminants were introduced in 1931. Illuminant A represents an incandescent tungsten filament lamp.
“[CIE standard illuminant A] is intended to represent typical, domestic, tungsten-filament lighting. Its relative spectral power distribution is that of a Planckian radiator at a temperature of approximately 2856 K. CIE standard illuminant A should be used in all applications of colorimetry involving the use of incandescent lighting, unless there are specific reasons for using a different illuminant.” [CIE Standard Illuminants for Colorimetry, 1999]
Illuminants B and C represent direct and shady daylight respectively. They can be derived from illuminant A using liquid conversion filters with high absorbance in the red part of the spectrum. Due to their deficiency at wavelengths below 400 nm, that are important when e.g. working with fluorescent optical brighteners, illuminants B and C are considered deprecated in favor of the CIE D-series of illuminants. Practical realization of CIE illuminants A, B and C is possible since it is defined in the standard.