Illuminant D65

Illuminant D65

CIE Standard Illuminant D65 (sometimes written D₆₅) is a commonly-used standard illuminant defined by the International Commission on Illumination (CIE). It is part of the D series of illuminants that try to portray standard illumination conditions at open-air in different parts of the world.

D65 corresponds roughly to a midday sun in Western Europe / Northern Europe, hence it is also called a daylight illuminant. As any standard illuminant is represented as a table of averaged spectrophotometric data, any light source which statistically has the same relative spectral power distribution (SPD) can be considered a D65 light source. There are no actual D65 light sources, only simulators. The quality of a simulator can be assessed with the CIE Metamerism Index.

The CIE positions D65 as the standard daylight illuminant:

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. —ISO 10526:1999/CIE S005/E-1998, CIE Standard Illuminants for Colorimetry