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When I looked into my projector when it was on the blue screen it left a red spot in my vision. I should not have tried it but all the colors left a red spot. Why not a blue or yellow spot was left?

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    $\begingroup$ Did looking at a bright red light also leave a red after-image? Usually if you expose your eyes to a color, the eye becomes less sensitive to that color, and then shortly thereafter things will look more like the inverse of that color. If we're talking long-term damage, maybe it has something to do with the inside of the eye being red. $\endgroup$ – A L Jun 2 '16 at 20:04
  • $\begingroup$ Without more details this is nothing more than an anecdotal report and your perception may have had nothing to do with that beamer in the first place. Personally, I would close this Q on the basis of that, but since your Q has received so many upvotes apparently it is appreciated by others. $\endgroup$ – AliceD Sep 27 '18 at 8:36
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You are describing the phenomenon of negative afterimages, a type of physiological afterimage, which is a result of how the cells in the retina work. Specifically, it is a result of the adaptation of photoreceptors.

According to the opponent process theory of color vision, our perception of color is controlled by two opposing systems: a magenta-green system and a blue-yellow system. For example, the color red serves as an antagonistic to the color green so that when you stare too long at a magenta image you will then see a green afterimage. The magenta color fatigues the magenta photoreceptors so that they produce a weaker signal. Since magenta's opposing color is green, we then interpret the afterimage as green.

There is a well-known illusion, called the negative photo illusion, which exploits exactly that. It is required not to move the eyes, because you need to bleach the photoreceptors in an exact place on your retina. This is why you have to fixate on a dot in the middle of a picture, otherwise the bleaching will not occur.

EDIT: I mention a two-axis (magenta-green system, blue-yellow) system, but the exact color system - which would predict which afterimage you would see - is most likely a trichromatic system, which includes red, green and blue channels, based on the principle of the three types of retinal cone cells. It is important to remember that this is a phenomenon in perception, and thus may, in some cases, not be reducible to the fact that there are three types of cone cells, or that bleaching of a photoreceptor has occurred. Please see the comment thread below for some clarification.

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    $\begingroup$ So OP should've seen a yellow afterimage. They, reportedly, didn't. $\endgroup$ – AliceD Sep 27 '18 at 12:11
  • $\begingroup$ Of course you don't need to. I provided a detailed explanation as a comment under your answer. Would you prefer me to remove my downvote just to earn bounty points? That wouldn't be right in the spirit of science. @AliceD If the blue bleaching light was lighter, slightly green or cyan, it's opponent color would be red, as observed. See this illumination circle for reference $\endgroup$ – S Pr Sep 28 '18 at 11:59
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    $\begingroup$ Of course a green light would have a red afterimage, but that's not what the question says. $\endgroup$ – AliceD Sep 28 '18 at 14:04
  • $\begingroup$ Hi there. I'm sorry if I incorrectly interpret your comments as smug or highly pedantic. I'm not sure why such unnecessary and spiteful comments are being left, especially by a mod. The question states, why a red afterimage? to which my response, which happens to be scientifically accurate and concise, was because of cyan photobleaching and the opponent color phenomenon in visual perception. The question Why not blue or yellow? is well addressed. Demystifyingly, the questioner stared directly at a projector lamp giving off a light blue hue of light, which is common with projectors. $\endgroup$ – S Pr Oct 2 '18 at 13:47
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    $\begingroup$ That's clear then. The phenomenon really seems to be based on an opponent color phenomenon. Whether it is the Herring two-axis theory, or a trichromatic one (the Young–Helmholtz theory, which better captures the triadic nature of retinal cone cells) is a more detailed discussion, I'm sure you'll agree. I would explain it simply though: if you bleach blue/green photoreceptors in the fovea, the negative image is based on the immediate and relative hyperactivity of red cone cells. It's possible OP confused red for orange in the afterimage, which would be the opponent color of blue. Cheers! $\endgroup$ – S Pr Oct 4 '18 at 11:15

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