50% Gray in RGB, Lab ang Gray-gamma 2.2: why are different?

Started Jan 15, 2013 | Discussions thread
ronzie Senior Member • Posts: 1,288
Re: Here's Why


Kodak in its description of the current gray card R-27 mentions it uses the Munsell Lab/Gretag-MacBeth color checker standards for Middle Gray.



offers free .csv format (Excel or database) tables and a .jpg of the chart and data comprising it in several coordinate color space values.

The Color Checker data table gives the values for the color checker chart. The Munsell Conversion table are cross references in color space coordinates for I think all of the defined Munsell color references in their full catalog but it is a handy conversion chart across color spaces. Munsell has their own set of coordinates in addition to CIE and LAB, and they include RGB. Patch names are their own and ISCC/NBS ("white" is white, "medium gray" is "neutral 5") same standards bodies with old names.

With regard to the Color Checker Cart data, patch 22 is defined as medium gray as described on the physical version I have. Patch 19 is defined as white. Medium gray has a Munsell luminance value of 5, L* of 51.57. White has a luminance value of 9.5, L* of 96 which is close to the 90% reflectance claimed for the white side of the Kodak Gray Card. White RGB is defined as 243, 243, 242. Medium Gray is defined as 122, 122, 121 allowing for rounding half og the white RGB values. These are measured values from their laboratory standard patches under "Illuminant C 2 degree observer  H. Van Aken  March 12 2002".

I requested a description of the illuminant C 2 and I got way back a very pleasant e-mail from a Munsell scientist describing it as I recall an illumination falling on the target at noon local time, summer solstice, clear sky, with the target facing north. I forgot the latitude used. I no longer have that e-mail application so I do not have the text to quote.

Also in the description of the physical chart it mentions the chips are adjusted to reflect all colors of the visible spectrum. An example is given as looking at a photograph of the chart and the chart itself under the same illumination for comparison. That's what I did back in my film days looking at a photograph of the chart taken almost at the described date and time for the illumination standard printed to Fuji Crystal Archive Paper on a Fuji wet process system (with no operator intervention), the chart itself, and an ink jet printed chart after I digitized the negative, Fuji Superia color negative ASA 400 35mm. The wet process print was very close for hue and I could make my inkjet print very close as well.

If you stayed with this long winded response, congratulations, but I thought I would get into the standard and the numbers across color spaces.

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Ron Ginsberg
Minneapolis, MN
Land of 10,000 Puddles

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