Re: D50 definition

Hi Craig

I agree 'ICC D50' would be unambiguous, although where precision matters 
you may wish to sub-divide CIE D50 into 'the value recommended by CIE in 
Publication 15' and 'the value computed according to the procedure in 
CIE 15 at x precision' - for example I get [96.4198655760898330   
100.0000000000000000 82.5116483221039090] when doing the calculation at 
double precision.

In my 2006 paper I showed that there are also inconsistencies in the 
ASTM method used in ISO 13655 (some of which remain in ISO 13655:2017), 
but that for most practical purposes the impact of these issues is 
negligible.

Phil


On 03/04/2018 08:51, Craig Revie wrote:
>
> 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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