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One question to ask at this point is, how do I know I matched the luminance of the color with the luminance of the gray, when I was adjusting the control points, and not the brightness? And besides, what actually would be the consequence of doing brightness matching instead of luminance matching? Let me answer the second question to lead into the first one.

Suppose we were to directly match the brightness of this patch of red with this gray, just by looking at the two colors, making a subjective judgement; we might match the red with this gray. If we did it for this blue, we might choose this gray.

Now you might think if we just superimpose the two experiments, summing the spectra on both sides, we'd get a magenta which matches this gray. Does that happen?

Actually, no, the magenta will look a little dimmer. This is called additivity failure, and its a characteristic of direct brightness matching; its also known as the Helmholtz-Kohlrousch effect. The effect is that saturated colors "glow", with a brightness out of proportion to their actual luminance.