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I has nothing to do with adaptation to brightness, but adaptation to color. It depends on wether you indoor lighting is more incandescent-style (~2700K) or daylight (6500K), or if you go outside during sunset/sunrise or midday (the latter should be 6500k again). The colors of the objects that surround also taint your perception. There's a number of excellent demos here.

I has nothing to do with adaptation to brightness, but adaptation to color. It depends on wether you indoor lighting is more incandescent-style (~2700K) or daylight (6500K), or if you go outside during sunset/sunrise or midday (the latter should be 6500k again). The colors of the objects that surround also taint your perception. There's a number of excellent demos here.

biological neuro nets are inherently bad at making exact estimates, but they make up for it by being able to be sensitive to extremely small variations. compare perceived light levels indoors under lamps vs outdoors in sun, or perceived smell when a stench is first introduced to 6 hours later. look at the blue/yellow optical illusions, http://www.lottolab.org/illusiondemos/Demo%2012.html# when you activate the mask it will look like it is cheating and the squares are changing colors, confirm the honesty with ms paint, the squares really are all the same grey squares

There is some additional background material at Lottolab Studio on related research conducted by Dr. Beau Lotto.

Kudos to all involved.

this is very similar to this famous color constancy illusion.

That one still gets me. The first time I saw it I swore up and down that the colors were different. I opened it up in the GIMP and used to color picker to check it out, and didn't even totally believe it when I saw the exact same color values. I couldn't convince myself until I cropped part of square 'A' out and dragged it down to square 'B'. But the demo from today's article just doesn't do it for me. I looked at the discs for a while and couldn't tell what the illusion was supposed to be, then I read the description and the two center squares just don't look different at all to me. Not even a little bit.

I'm not trying to sound like I have perfect vision and am 'above' simple optical illusions; like I said, to this day the cylinder-on-checkerboard illusion still gets me even though I _know_ they're the same color. Is there anyone else who just doesn't see the illusion in this optical illusion?

Take a look at http://www.lottolab.org/Visual%20Demos/Demo%204.html

The site says the surfaces are "physically identical." I call BS. They are identical only in the sense that they have (assuming this is a faithful rendering of something) the same irradiance per unit solid angle hitting the viewer's eye. They are, in fact, physically different surfaces -- look at the top left corner of each piece, which are facing roughly the same directions and so are similarly lit. The top face is dark and lit more brightly, and the bottom face is light and lit dimly, and a robot that observes notices this (like humans do) is pretty impressive.

Demo 15 (the one in the article) is a bit more legitimate, but it's hard to tell whether the left disc is redder or is just lit with red light.

Look at this one. If you see more clearly for "true similarity" on the red pipe (?), then you will notice slightly a bullshit. :)