Originally Posted by Wouter73
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.
This information is from the CIE's work to quantify and define a standard for daylight simulation when judging colored surfaces. They published their findings in CIE 51.2-1999, titled: 'A Method For Assessing The Quality Of Daylight Simulators For Colorimetry.'
There may have been a revision since that date. The context of this thread is more specifically in reference to video applications. In video, D65 is a specific location in color space with tighter tolerances for white balance (loosely referred to as color temperature) than tolerances for surface colorimetry.
The CIE specifications for D65 daylight simulation allow for white point tolerances within a broad radius (CIE 51.2-1999, pt. I, sec. 3.3, Figure 1). Those tolerances encompass D55 and D75. Much more attention is devoted to spectral power distribution (as it relates to metameric spectral reflection radiance factor) in the above document than specific white point. No one who is familiar with video white balance would want to settle for a gray scale that deviated between D55 (5500K+) and D75 (7500K+) when calibrating a video display. There is a clear difference in how the term "CIE D65" should be understood and applied between video performance versus judging surface colors.
Best regards and beautiful pictures,
G. Alan Brown, President
A Lion AV Consultants affiliate
"Advancing the art and science of electronic imaging"Edited by GeorgeAB - 2/23/13 at 11:27am