RE: D50 definition

Thanks Phil for the explanation. This was also pointed out by Mike Rodriguez when he was ICC Vice Chair (around 2004?). I agree that it would be confusing to change the PCS definition but in cases where the precision matters, it may be useful to refer to ‘ICC D50’ and ‘CIE D50’. I think this may cause significant (or at least numeric) differences in cases where colour conversions that are defined mathematically are compared with ICC colour conversions.

Lars has also alluded to another revision for D50 from 5000 K to 5003 K – in this case due to a revision of one of the constant factors in Planck’s law (I’m not sure which one) after the standard was defined.

Chris – you raised this issue; was there some concern or just intellectual curiosity?

Best regards,
_Craig

From: Phil Green <green@colourspace.demon.co.uk>
Sent: 02 April 2018 22:04
To: public-colorweb@w3.org
Subject: 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

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