It seems to me that CIE X,Y,Z are the best choices; extend better than
densities (do not level off so quickly), are much less noisy... X for
Cyan, Y for Magenta, Z for Yellow, and any of them for Black.
Another way is the dot product with the vector to the 100% colorant
value to white (or some other choice related to the underlying
colorant vector direction) in a particular colorspace (XYZ, L*a*b* etc.).
This could be viewed as creating an "optimal" mix of the
co-ordinates as a measure, rather than just picking one
of the co-ordinates.
XYZ isn't a good choice as absolute measure, as it is
linear light, so the blacks steps will be too big.
A gamma or L* curve applied to XYZ is needed at least.
Density is a log scale, which is why it's not too bad,
although in the Colorbus calibration system I set
a target density curve corresponding to a typical
20% dot gain press curve.
Graeme Gill.
