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breckenridgeback t1_jadkj55 wrote

Reply to comment by Mand125 in Eli5 why do we perceive red+blue as purple? by YYM7

> Each of the three has a color it’s best at detecting, and we name them based on that. One for red, one for green, one for blue.

This is incorrect. The three cones in your eye are most sensitive to violet, green, and yellow-green, and they're not called red, green, or blue cones. The usual name is S, M, and L, for short, medium, and long wavelengths.

> But for whatever reason, way down in violet, the red cone has a little bump in sensitivity again, after not seeing green or blue at all. So violet literally is the blue cone sending a strong signal, but a little bit of the red cone too.

Also incorrect. The L cone does not have any particular sensitivity to violet light.

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hh26 t1_jae6ady wrote

And yet if you mix red and blue light together my brain perceives it very similarly to violet, so if the rods and cones are not doing that, what is?

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breckenridgeback t1_jaeacic wrote

Well one, no, violet is quite a distinct color from magneta (the color you get by mixing blue and red light, i.e., "purple"). They're not hugely far apart, but e.g. on this chromaticity diagram, violet is at the bottom while mixtures of blue and red light form a magenta shade bottom-right of center. Magenta and violet are as different as red and yellow or green and cyan.

But the answer to your question is that you distinguish blues from purples by how different the signals from your M and L cones are. In both colors, the S cones are stimulated. If L > M, you see purples. If L ~ M, you see violet. If M > L, you see blues. At the far violet end of the spectrum, both L and M are near zero. At the far red end of the spectrum, L > M but both are weak, so combining that with blue produces high S, low-but-positive L-M (as opposed to high S, ~zero L-M for violet). The difference L - M is not hugely different between violet and purple, which is why they are similar-ish looking.

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Mand125 t1_jadlj4f wrote

I’ve seen different data presented, I’m not sure I can explain the discrepancy.

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breckenridgeback t1_jadn8ck wrote

My guess is that you're thinking of some sort of opponent-process model or something like the CIELAB space. But neither of those corresponds to the physical response curves of cones.

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