Top: the brightness measured across the picture: flat, a straight ramp, flat. Bottom: how bright it looks, with a dark and a light band that aren’t there.
Lighter or darker?
Mach bands
Look where the fade starts and where it ends. What do you see?
A dark stripe where the fade starts and a light stripe where it ends. Neither is there.
Why? The same cells that sharpen edges, turning down their neighbors, add a band on each side of the fade. Ernst Mach found them in 1865, and doctors still have to watch for them on X-rays, where they can look like a crack.
Picture: Drawn by Science Playground · Wikipedia(opens in a new tab)