http://compresscafe.i2p/resources.html
RGB/XYZ matrices tells you how to convert between any color space and CIE XYZ. Completely painless programmer’s guide to XYZ, RGB, ICC, xyY, and TRCs The luminance of an sRGB color , among other things, answers the question of “Why is there a Y
channel in the xyY components of some primaries, but not others?”