Many D65 values to choose from

Posted by Steve Eddins, August 25, 2015

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I am a crazy person.

Late last week I was chasing down some numerical precision questions I had about certain color-space computations. I found myself trying to determine, once and for all, what tristimulus values to use for the CIE standard illuminant called D65.

CIE defines D65 using a table of spectral values, 1 nm spacing, over the range 300 nm to 830 nm.

So how do you get the corresponding tristimulus values for the CIE 1931 standard color observer? The CIE technical report on colorimetry, CIE 15:2004, suggests that the “rigorous” way to do it is using a dot product of the 1 nm spectral values with the 1 nm xbar, ybar, and zbar tables for the standard observer. But the technical report contains written tristimulus values that do not quite agree with that computation. And there are other values in use out there as well.

This table shows everything I was able to track down:

If you round all these values to five fractional digits, there are 10 unique triples among the 13 rows shown above. Even if you round to just four fractional digits, there are 6 unique triples.

Sigh.