Fechner color

The Fechner color effect is an illusion of color seen when looking at certain rapidly changing or moving black-and-white patterns. They are also called pattern induced flicker colors (PIFCs). Not everyone sees the same colors.

The effect is most commonly demonstrated with a device known as Benham's top. It can also be seen in stroboscopic lights when flashes are set at certain critical speeds. Rotating fan blades, particularly aluminum ones, can also demonstrate the effect; as the fan accelerates or decelerates, the colors appear, drift, change and disappear. The stable running speed of the fan does not (normally) produce colors, suggesting that it is not an interference effect with the frequency of the illumination flicker.