I have a question about color space terminology. When referring to "XYZ tristimulus values" and "xy chromaticity coordinates", does this always refer to the CIE 1931 XYZ color space, and never to other color spaces like CIELab?

  • Obviously not Lab as coordinates are named L, a and b not x, y, z – joojaa Jan 17 '17 at 10:01
  • Thanks! I was confused because I am used to generic 2D plots in which x and y are coordinates that could correspond to any variable. – KAE Jan 17 '17 at 14:59

I was confused when I wrote this question; it doesn't make sense. Now I know the 1931 CIE color space refers to a 2 degree observer, and the 1964 CIE color space refers to a 10 degree observer, see here. These two color spaces have different XYZ tristimulus values so you should specify the observer (2 degrees or 10 degrees) when you use XYZ values. The chromaticity coordinates x,y are derived from the XYZ values. Lab values are derived from the XYZ values but depend on the illuminant (for example D50 or D65), so you should specify both the illuminant and observer when using Lab values. In graphic arts, D50/2 is common. A good intro to all this is here.

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