Re: Colorimetric Accuracy in the Field
Re: Colorimetric Accuracy in the Field
- Subject: Re: Colorimetric Accuracy in the Field
- From: Robin Myers <email@hidden>
- Date: Wed, 05 Jun 2013 13:30:26 -0700
it is not only the white patch on a ColorChecker that has a slightly yellowish appearance, all the patches do except the black patch. This is due to a slightly increased yellow spectral reflectance (combined with a high absorption of violet wavelengths) of the titanium dioxide pigment used in varying proportion to make the patch paints. The black patch does not show this yellowness because it is comprised of only the black pigment, carbon black, which reflects light evenly across the spectrum (has no dominant hue). Thus the resulting neutral patch axis is fixed at the black patch end but shifted at the white patch end. The yellowish affect of the titanium dioxide on the gray mixture decreases with the lightness but it is there in all the gray patches.
If the ColorChecker neutral patches reflected light evenly across the spectrum, then their measured XYZ values would be identical to those on the D50 axis. Since they are slightly yellow, their XYZ values lie on an axis defined by D50 but shifted slightly by their amount of yellow reflectance.
Robin Myers
On Jun 5, 2013, at 12:53 PM, Ben Goren wrote:
> On Jun 5, 2013, at 12:41 PM, Robin Myers <email@hidden> wrote:
>
>> Since the RGB values of a chart image defines a sampling in RGB space, and when using a color chart the neutral patches are used by color engineers to represent the RGB space's neutral axis, then the chart neutral patches can be envisioned as a sampling of the neutral axis.
>
> I'm with you on that, but then we come back to your statement that caused me the confusion in the first place:
>
>>>> 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).
>
> I think we're agreed that the illuminant and chart neutral axes are synonymous, and that the chart may have patches that lie very close to but aren't actually on the neutral axis.
>
> So...without me putting words in your fingers, could you help clarify what you mean by the ColorChecker neutral axis being skewed from the D50 neutral axis?
>
> Thanks,
>
> b&
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