sophiecentaur said:
The CIE chart only shows chrominance, which can actually produce very different perceived 'colours' when Luminance changes. Two areas with the same XY co ordinates can often look very different with different Z. For, high and low luminanceareas can look yellow and brown. Pale 'European' skins tend to have very similar chrominance values to darker skins because the dark pigment is a pretty good neutral grey. Hardly surprising really because the tissues , fat, blood etc. are much the same for all skins.
When the eye sees coloured light it also takes into account the other light sources around and 'integrates to grey' , which is the best it can do so the actual chrominances in a scene can look almost the same in bright light or dim light There are hundreds of made-up images (illusions) that totally confuse the eye because surrounding areas affect what you 'see'.
I don't know that you have completely followed my explanation of the chart. There is the capital ## (X,Y,Z) ##, and the small ## (x,y,z) ##. The chart itself is small ## (x,y,z) ##, and for a given ## (x,y) ##, there is only one ##z ## which is computed to be ##z=1-x-y ##.
From what I can tell, I think the chart has been somewhat misunderstood or not fully understood by many. If we go to a different brightness level, we do scale up the ##(X,Y,Z) ##, but the ##(x,y,z) ## stays the same. (The ##(x,y,z) ## is found by where the vector ##(X,Y,Z ) ## crosses the plane ## x+y+z=1 ##.) That of course is then what you are saying, that the ##(x,y,z) ## for the scaled up case should really take on a different position on the chart=the chart assumes linearity of the system at all light levels, and that is only good to a very rough approximation at best. In any case, I find the chart to be based on some very good mathematics.
Edit: Computing where the ##(X,Y,Z) ## crosses the plane ## x+y+z=1 ##, it is given by ##x=X/(X+Y+Z) ##, ## y=Y/(X+Y+Z) ##, and ##z=Z/(X+Y+Z) ##. The ##(x,y,z) ## are the color coordinates, where the ##(x,y) ## is found on the chart, and it is simple arithmetic to compute the ##z ## which is ##z=1-x-y ##. The chart is a map on the plane ## x+y+z=1 ## of the color that is found for each vector ##(X,Y,Z) ## where each vector's color/direction is determined by where it crosses the plane ##x+y+z=1 ##.
(They really IMO would do well to footnote that ##z =1-x-y ## when showing the chart. The chart is actually a view from above of the plane ##x+y+z=1 ## that is colored in showing the colors of each ##(x,y,z) ##). .
Scaling up the ##(X,Y,Z) ## gives the same ## (x,y,z) ## on the color chart, which is really the best we can do with a linear model. It is not expected to be perfect, but it is reasonably accurate for many cases.