This is a question as much about color perception as the physics of light. Since I have normal vision, without color blindness, I will draw on my experiences in formulating my question. I will add that this is similar to an existing question, but I am not satisfied with the answer. Why does adding red light with blue light give purple light?

Our eyes have sensitive rod cells that roughly act as our black and white vision, as well as less sensitive cone cells which distinguish color. Three sets of cones exist, responding to red, green, and blue light. Their frequency responses, pictured below, explain how we can perceive colors like yellow and cyan. If yellow light reaches our eye, the red and green cones will both respond, perceived as yellow. If both red and green light reaches our eyes, the red and green cones will both respond, so while the physical phenomenon is different, two wavelengths of light instead of just one, the perception is the same, yellow. The same is true for cyan with green and blue light. There seems to be a lot of confusion concerning pigmentation. My understanding is that a cyan pigment will absorb a range of wavelengths around red light and a yellow pigment will absorb a range of wavelengths around blue light, so what is reflected back is a mixture of green and blue for cyan or red and green for yellow. A red pigment, then, absorbs wavelengths covering across green and blue, similar is true for other primary color pigments.

Things get confusing with magenta however. A magenta pigment will reflect light that stimulates our red and blue cones, therefore magenta pigments must absorb wavelengths around green. This makes good sense so far. However, what about purple light? Purple light is weird because it's of an even shorter wavelength than blue light. You'd expect it to just be blue, and maybe a bit darker than you were expecting.

My experience with a violet laser pointer: My friend has a violet laser pointer, a relatively pure single wavelength. It's a very strong laser, similar power to his green one which is extremely bright, but the spot is very faint, yet definitely purple. It seems like my suspicions about short wavelengths appearing darker than expected are true, but it isn't a dark blue, it's a dark purple.

Obviously there's some difference between violet and magenta other than how bright they are. If I were to guess, I'd say the violet light stimulates the blue cone most and the red one little to not at all, while magenta colors (wavelength mixtures) stimulate blue and red simultaneously. Okay, but then how is the violet light reproduced on a computer screen with blue and a little bit of red? If the slight amount of red light is important to our perception of violet, then it seems like blue light might seem more violet (red/blue mixed) to us than violet (single wavelength).

Now, you see, my head is melting. Can we finally get a satisfying answer on how we perceive purple light and how that fits in with the physics of light and pigments?

Rod and cone frequency responses


marked as duplicate by David Hammen, Brandon Enright, ACuriousMind, Kyle Kanos, Neuneck Nov 12 '14 at 9:03

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