The lilac chaser is a very popular optical illusion: it consists of 12 lilac (or pink, Rose color, or magenta), blurred spots arranged in a circle (like the numbers on a clock), around a small black, central cross on a grey (or silver) background. One of the discs disappears briefly (for about 0.1 seconds), then the next (about 0.125 seconds later), and the next, and so on, in a clockwise direction. When one stares at the cross for about 20 seconds or so, one sees three different things:
- A gap running around the circle of lilac spots;
- A green disc running around the circle of lilac discs in place of the gap;
- The green disc running around on the grey background, with the lilac discs having disappeared in sequence.
The chaser effect results from the "phi phenomenon" illusion, combined with an afterimage effect in which an opposite, complementary, color—green—appears when each lilac spot disappears (if the discs were blue, one would see yellow), and Troxler's fading of the lilac spots.
The illusion was discovered by Jeremy Hinton, a visual expert, when experimenting on how to devise stimuli for visual motion experiments.
In one version of a program to move a disc around a central point, he mistakenly neglected to erase the preceding disc, which created the appearance of a moving gap. On noticing the moving green-disc afterimage, he adjusted foreground and background colours, number of discs, and timing to optimise the effect.