Re: D50 definition

Hi Chris

This discrepancy has been known for some time, and was discussed in a 
paper I did at Electronic Imaging in about 2004.

The values in the ICC.1 specification [96.42, 100, 82.49] were those for 
the 1931 observer and D50 illuminant as originally published in CIE 
Publication 15. Unfortunately there was an error in the Z value, which 
was corrected in CIE 15:2004 to 82.51. These values and the precision of 
two significant places remain the same in the new version currently in 
press.

ICC has discussed this on a number of occasions and it has been decided 
that it would not be appropriate to change the D50 PCS illuminant value 
for ICC.1, since this would require a change in CMMs, extensive 
modification of installed code, and potential interoperability problems 
with existing profiles. ICC.2, however, supports the use of the 
corrected values as a custom PCS.

Note that values [0.9642,...] arise since by convention ICC encodes CIE 
XYZ values normalised so that Y=1.0 rather than 100.

Phil Green

ICC Technical Secretary


On 02/04/2018 21:19, Chris Lilley wrote:
>
> Hi folks,
>
> It was recently pointed out[0] that the XYZ values for the D50 
> illuminant given on, for example, Matlab[1] or Bruce Lindbloom's site[2]
>
> |[0.96422, 1.00000, 0.82521]|
>
> differ from the once specified by the ICC
>
> |[0.9642, 1.0000, 0.8249]|
>
> ||
>
> "In ICC v4, the requirement was introduced that the media white point 
> of a Display class profile shall be equal to D50 (i.e. [96.42, 100, 
> 82.49])" [3].
>
> I assume the authoritative source is the CIE. I will check my books 
> this evening, but can anyone shed light on the discrepancy (and the 
> correct value)?
>
> [0] https://github.com/w3c/csswg-drafts/issues/2492#issuecomment-377913660
> [1] 
> https://www.mathworks.com/help/images/ref/whitepoint.html?s_tid=gn_loc_drop
> [2] http://www.brucelindbloom.com/index.html?Eqn_ChromAdapt.html
> [3] http://www.color.org/whyd50.xalter
>
> -- 
> Chris Lilley
> @svgeesus
> Technical Director @ W3C
> W3C Strategy Team, Core Web Design
> W3C Architecture & Technology Team, Core Web & Media