2006.12.06 22:19 "Re: [Tiff] Re: Tiff Digest, Vol 31, Issue 3", by Graeme Gill
If you colour model goes as far as to be based on actual physical properties of light, it's extremely complicated to convert RGB to "greyscale". You'll have to dig into spectra and whatnot, for all I know.
I can't agree with you on this. Taking the weighted sum of an RGB output in linear light is not an approximation, it's actually correct, equivalent to CIE Y value if the right weightings are used. The weightings simply depend on the primary locations and the white point.
This is because any additive space is a 3x3 transform from CIE XYZ space, so it only takes one row of the inverse matrix to compute Y again.