The most common white points in digital color workflows are D50 and D65. Both D50 and D65 are D-Illuminants with xy chromaticity coordinates specified by the CIE (Commission Internationale de l’Eclairage). The “D” stands for daylight. The number is a loose reference to the correlated color temperature of the white point on the Kelvin scale. This is a loose reference because the current correlated color temperature of D65 is 6504 K, not 6500 K. To quote Wyszecki and Stiles, “CIE standard illuminant D65 represents a phase of natural daylight with a correlated color temperature of approximately 6504 K.”
Let me share with you the details on why the D65 white point has a correlated color temperature of 6504 K.
The xy chromaticity coordinates for D65, based on the relative spectral radiant power distribution for D65 and the CIE 1931 color-matching functions, are x=0.3127 and y=0.3290. These xy coordinates are on the CIE daylight locus in the CIE 1931 xy chromaticity space.
When the CIE defined the relative spectral radiant power distribution for D65, the correlated color temperature of the CIE xy coordinates of D65 on the Planckian locus for blackbody radiation was 6500 K.
In 1968 the scientific community changed the second radiation constant (c2), which is used in Planck’s radiation formula for blackbody radiation, from 0.014380 to 0.014388 (Reference: The International Practical Temperature Scale of 1968, IPTS-68). This change in the second radiation constant changed the location of the Planckian locus for blackbody radiation in the CIE 1931 xy chromaticity space. The shift of the Planckian locus away from the CIE daylight locus changed the correlated color temperatures of the CIE D-Illuminants. Thus the correlated color temperature of D65 shifted from 6500 K to 6504 K.
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