Illuminant D65 - Why 6504 K?

Why 6504 K?

The name D₆₅ suggests that the correlated color temperature (CCT) should be 6500 K, while in truth it is closer to 6504 K. This discrepancy is due to the scientific community's revision of the constants in Planck's law after the definition of the illuminant. This shifted the Planckian locus, affecting all CCTs, which are calculated by finding the nearest point on the locus to the white point. The same discrepancy applies to all standard illuminants in the D series—D₅₀, D₅₅, D₆₅, D₇₅—and can be "rectified" by multiplying the nominal color temperature by ; for example for D₆₅.