On the right you can see an arrangement of lozenges or elongated diamonds.
Question: Where are the diamonds darker, at the top or at the bottom?
Answer: All diamonds are identical, but it sure doesn't look like it.
What to do?
Start as follows: Pick any diamond with the mouse while the ∆contrast slider is in its upper range. Now you can move this diamond around and compare it to all the others. You also can use the button “Rotate 45°”; when you have rotated by 180° it becomes very obvious that the diamonds are not equal in lightness at their top compared to their bottom.
There is a “luminance gradient” along the diamonds: lighter at top, darker at bottom. The slider allows to adjust the strength of this gradient.
Shallow changes in lightness are already suppressed by neural processing in the retina. What is transmitted from the retina to the visual centers of the brain is the contrast step from the dark bottom end of each diamond to the lighter top end of the diamond in the next lower row. So the brain receives information on a lightness step, not of the slow return of lightness, and “concludes” the every row is lighter then the one above it.
I thankful to Akiyoshi Kitaoka for digging up the original source for me. Not only did he do that, he also promptly produced this beautiful variant.
Watanabe I, Cavanagh P, Anstis S, Shrira I (1995) Shaded diamonds give an illusion of brightness. ARVO Annual Meeting, Fort Lauderdale, Florida. [According to him, the illusion originates from Pat Cavanagh.]
I interprete this effect as a variant of the Craik-O'Brien-Cornsweet illusion
The present demonstration was inspired by this cute movie at “how stuff works”