White Point
The white point is the colour value of pure white in a colour management system. Two different white points are involved:
- Illuminant white point
This represents the colour of the light that is used to view a set of colour samples. - Media white point
This represents the colour of a non-printed patch of the medium on which the colour samples are shown.
Theory is that our eyes are adapted to the illuminant white point, and view samples relative to this white point. ICC profiles are always standardised to D50 as illuminant, so the illuminant white point does not play a role in transforms based on ICC profiles. Display profiles, which more typically have D65 as illuminant, are transformed to D50 using a Bradford transform when represented as an ICC profile.
In practice our eyes also adapt to the media white point, and for this reason relative colorimetric transforms are often used, which also apply a Bradform transform to the media white point.
A major problem is that for display profiles media white point and illuminant white point are the same. According to V4 of the ICC specification, the media white point of a display profile should be set to D50, but many profiles are based on V2 of the standard and have the media white point set to D65. This has as consequence that many colour management systems will shift display colours in a wrong way when transforming according to an absolute colorimetric rendering intent.