Where can I find X-Rite ColorChecker Classic patch values, and which patch is the 18% gray reference?
Asked 11/3/2019
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I have an X-Rite ColorChecker Classic and want the reference values for each patch so I can use them in color-correction software. Are the official patch specifications available from X-Rite, and in what color space or format are they provided? Also, on the bottom grayscale row, which patch corresponds to an 18% gray card equivalent?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
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After further research, here is a partial answer taking as source Blog-Couleur: Quelles sont les valeurs RVB de la colorchecker ? and BabelColor PDF and the Xrite website.
The last row seems to be white 95%, neutral 80%, neutral 65%, neutral 50%, neutral 35%, black 20%.
On trouve entre le blanc et le noir, une série de 4 gris : neutral 0.35, neutral 0.50, neutral 0.65 et neutral 0.80. Le neutral 0.50 de luminosité 50 % est le fameux gris moyen de luminance 18 %.
Thus, the fourth square of the last row seems to be the 50% neutral gray (18% reflectance) of a gray card. In sRGB, it should be close to RGB(119,119,119) according to some sources (color #777777 in hex) or RGB(122,122,121) or RGB(121,121,122) according to other sources.
More about this here.
More generally, the website BabelColor / Danny Pascale gives a lot of data (RGB, Lab) related to various versions of Colorchecker:
Here (PDF) is the data given by Xrite themselves:
Originally by user60130. Source · Licensed CC BY-SA 4.0
user60130
6y ago
0
Generated from our catalog & community — verify before relying on it.
Yes. X-Rite publishes official ColorChecker Classic reference data on its support site. The patch specifications are provided as measured values in CIE Lab, referenced to illuminant D50; X-Rite notes there are different datasets for charts made before vs. after the November 2014 revision.
For software development, those official Lab values are the right starting point. You can convert them to other spaces such as XYZ or RGB as needed for your workflow.
For the grayscale row, the patches are commonly identified as: white 95%, neutral 80%, neutral 65%, neutral 50%, neutral 35%, and black 20%.
The 18% gray-card equivalent is the neutral 50 patch, which is the 4th patch from the left on the bottom row. In sRGB terms it is often approximated around RGB 119–122 per channel, but for accurate work you should rely on the official Lab data rather than approximate RGB numbers.
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UniqueBot
AI6y ago
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