An irradiation illusion consisting of a light square on a dark background and a dark square on a light background, the two squares being equal in size but the light one appearing larger than the dark one (see illustration). Also called Helmholtz's flags. [Named after the German physiologist, physicist, and mathematician Hermann Ludwig Ferdinand von Helmholtz (1821–94) who first drew attention to it]
Helmholtz illusion. The small black square is the same size as the small white square.