Impossible Colors
July 5, 2011 6:14 PM   Subscribe

An optical illusion based on retinal fatigue allowing you to "see" an "impossible color"?

Herodios' recent comment on Impossible Colors reminded me of a link I'd seen here before and was hoping to get help finding it again.

Someone posted a link on MeFi within the past couple of years, I think, about "impossible colors" and linked to a page that had an optical test. In the test/illusion, I seem to recall that you stared at a color or pattern for a while and then closed your eyes. Alternately, you then stared at a white background.

The residual image left from the retinal fatigue was an "impossible color" that you couldn't normally see in nature. A kind of fluorescent greenish-yellowish-blue, if I recall.

Anyone remember that link? (Note: it's not the one on the "Impossible Colors" wikipedia article page for the yellow/blue overlap.) I'd love to share it with my students in our upcoming chemistry lecture dealing with spectroscopy.
posted by darkstar to Science & Nature (10 answers total) 21 users marked this as a favorite
Best answer: You are talking about The Eclipse of Mars. There are others like eclipse of Uranus, etc. :)
posted by smoke at 6:17 PM on July 5, 2011 [4 favorites]

Response by poster: Wow, that's it, and within 3 minutes! Incredible! And thank you, smoke. :)

Reading the link again, I can see now I misremembered the description of the illusion. It doesn't purport to show you an "impossible color", as described by the wikipedia article. Rather, it purports to show you a color which your computer monitor is incapable of displaying effectively.

It does leave the question whether this color of pure cyan can be seen directly by our eyes or if it can only be neurally inferred from the retinal fatigue that occurs by depleting the red-sensing photoreceptors in our retinas...
posted by darkstar at 6:24 PM on July 5, 2011

Response by poster: Also, a link which has the related ultra-green illusion, too.
posted by darkstar at 6:33 PM on July 5, 2011

You might also be interested in tetrachromacy in humans (short version: tetrachromats have four primary colors).
posted by anaelith at 6:44 PM on July 5, 2011 [2 favorites]

Response by poster: Fascinating, anaelith. As I'm considering the Eclipse of Mars illusion, I'm beginning to wonder whether that color that we sense as a result of the illusion isn't, indeed, impossible to directly see. We have cones that are blue-red sensitive. When we see a pure cyan (if we were able to encounter it in nature), would we register it as a pure cyan? Or would our cones be incapable of perceiving it that pure?
posted by darkstar at 7:01 PM on July 5, 2011

There was a story I remember reading on the blue, too, about a painting that consisted of just two red stripes with a green stripe in the middle. It was called "Sunrise" or something. The person in the story saw the painting and was unimpressed, until a security guard told her how to really see it: stare at the green stripe for a while, and then look at the red. The process of doing so created an experience of a wholly new intensity of "redness" in the viewer. I wish I could find the story now, or at least remember the name of the painting or the artist ...
posted by penduluum at 7:09 PM on July 5, 2011 [2 favorites]

Best answer: I'm pretty sure it was Octarine. A great post.
posted by Chicken Boolean at 7:15 PM on July 5, 2011 [2 favorites]

And the painting was Barnett Newman's Voice of Fire. Now I really want to see that painting in person.
posted by Chicken Boolean at 7:17 PM on July 5, 2011 [2 favorites]

Best answer: As discussed in the post Chicken Boolean links above, the color you perceive is "impossible" to see without exhausting some of your cones. If you look at the responsivity spectra for human cone cells, you can see that pure cyan light (around 500 nm) is also going to stimulate your red receptors, and so won't be as saturated as the color you could achieve by first exhausting your red receptors. Any mixture of blue and green wavelengths of light to make cyan would only be worse, with the green light stimulating the red receptors even more.

Are you really seeing a new color? Is this a more pure cyan? That depends on what you mean by color and cyan. But it's a more intense contrast between the signals from your blue and green receptors and the signals from your red receptors (perceived cyan saturation) than you would get by just looking at an object or light source.
posted by JiBB at 11:11 PM on July 5, 2011 [3 favorites]

Response by poster: Outstanding, Chicken Boolean!
posted by darkstar at 1:19 PM on July 6, 2011

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