CIE Illuminant Series D
CIE illuminants DThese is a series of illuminants, that has been statistically defined in 1964 upon numerous measurements of real daylight. Although mathematically described, they can hardly be realized artificially. The correlated color temperatures (CCT) of the commonly used illuminants D50, D55 and D65 are slightly different to the values suggested by their names. Due to the revision of an estimate of one of the constant factors in Planck’s law after the standards were defined, the correlated color temperature was shifted a little. For example, the CCT of D50 is 5003K and that of D65 is 6504 K.
“[CIE standard illuminant D65] is intended to represent average daylight and has a correlated colour temperature of approximately 6500 K. CIE standard illuminant D65 should be used in all colorimetric calculations requiring representative daylight, unless there are specific reasons for using a different illuminant. Variations in the relative spectral power distribution of daylight are known to occur, particularly in the ultraviolet spectral region, as a function of season, time of day, and geographic location. However, CIE standard illuminant D65 should be used pending the availability of additional information on these variations.” [CIE Standard Illuminants for Colorimetry, 1999]