Three standard illuminants were recommended by the CIE (1986) in 1931, denoted by the letters A, B and C. CIE standard illuminant A represents light from the full radiator with a correlated colour temperature of 2856K in accordance with "The International Practical Temperature Scale, 1968". Although the spectral radiant power distribution of illuminant A has remained constant the correlated colour temperature has changed due to amendments in the value of the radiation constant used in Planck's radiation formula from which the spectral radiant power distribution is calculated. By amending the temperature when adopting the new value of the radiation constant the spectral radiant power distribution is unchanged. Originally, the correlated colour temperature was 2848K and remained so up until a few years prior to 1971 during which the correlated colour temperature of illuminant A was re-calculated to be 2854K. CIE standard illuminant B is intended to represent direct sunlight and has a correlated colour temperature of approximately 4874K. This illuminant has now been declared obsolete by the CIE and has largely fallen into disuse in most applications. Average daylight was intended to be represented by CIE standard illuminant C with a correlated colour temperature of 6774K.
In 1964 the CIE recommended a further four standard illuminants D50, D55, D65 and D75, with correlated colour temperatures of approximately 5003K, 5503K, 6504K and 7504K, respectively. All four of these illuminants are designed to represent average natural daylight, however, illuminant D65 is by far the most commonly used and has largely replaced illuminants B and C. Tables of data containing the relative spectral radiant power distribution of all of these CIE standard illuminants (with the exception of illuminant B) for the wavelengths between 300nm to 830nm in 5nm steps have been published by the CIE (1986).