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Subject: Re: ACR v3.2 D2x white balance still not correct

The CIE definition of CCT (coordinated color temperature) in Kelvin degrees
is based on spectral measurements of various light sources. This is not
technically a blue/yellow and green/magenta axis, but when plotted the
Kelvin line forms a curve across both axes on a gamut diagram. In other
words, 3200K and 6500K each define a unique point on each of these axes.


I don't think your explanation is quite correct. D65 defines a unique point on the Planckian locus with a unique SPD (spectral power distribution), but there are an infinite number of spectral power distributions having a correlated color temperature (CCT) of 6500K as shown xy chromaticity diagram in this link:

As you can see, the lines of CCT are roughly perpendicular to the Planckian locus (which runs roughly in the yellow-blue direction). The lines of CCT run roughly in the green-magenta axis.

This explanation is most likely highly simplistic, but at least is consistent with the temperature and tint controls of ACR and explains why a color temperature in degrees Kelvin is not sufficient for white balance with a non black body radiator.


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