Re: Colorimetric Accuracy in the Field
Re: Colorimetric Accuracy in the Field
- Subject: Re: Colorimetric Accuracy in the Field
- From: Ben Goren <email@hidden>
- Date: Wed, 05 Jun 2013 11:24:51 -0700
On Jun 5, 2013, at 10:59 AM, Robin Myers <email@hidden> wrote:
> In the case of the pigments used in the ColorChecker family, the neutral axis is skewed in XYZ space slightly from that of the illuminant, D50 (the Prophoto RGB white point).
Perhaps you can help me out, here -- I'm trying to make sense of this.
I honestly don't see the relation between the neutral axis of a pigmented chart and an illuminant -- any illuminant.
The neutral axis of a pigmented chart would best be described as those patches with a flat spectral response; those patches which are equally reflective regardless of the wavelength / frequency of the photons bouncing off of it. And a chart doesn't need to actually have any such spectrally flat patches. Indeed, in the real world, no chart every has any truly perfect spectrally flat patches, though many charts have patches that are reasonably close.
Illuminants don't have a neutral axis, but they do have a color to them, expressible as an absolute value in XYZ space.
A *PROFILE* for a particular illuminant is going to have a neutral axis that runs through the XYZ coordinates for that illuminant's color...but every spectrally-flat patch on the chart is, by definition, always going to lie on that axis. (And patches with a hint of color, such as the N9.5 patch on a ColorChecker, will be very close to but not actually on that axis.)
For a profile with a different illuminant, the neutral axis will be different, but the truly-neutral patches will again lie on this new-and-different axis in this new-and-different profile for this new-and-different illuminant, and the almost-neutral patches will will be close to but not actually on the neutral axis.
You can do various transformations to, for example, produce a print that, when viewed in one light appears as if it was being viewed in an entirely different light, but I don't think that's what you're describing.
Perhaps you can help me understand either where my misunderstanding lies or maybe clarify the point you're trying to make?
Thanks,
b&
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