Further evidences of the effect of gradient on lightness

Recently, Agostini and Galmonte (submitted for publication) reported a new effect of gradient on lightness. When two identical mid-gray squares are placed at the centre (white and black, respectively) of two crosses, the first cross having arms filled by a constant achromatic gradient from black (outer part) to white (inner part), and the second cross being the negative of the first one, the gray target surrounded by white appears much darker than that surrounded by black. In a simultaneous comparison, this phenomenon is much stronger than the classical lightness contrast effect. Further observations showed that the effect is quite robust also when the amount of black area is reduced and the gradient is discontinuous. When the gray targets are enlarged to fill the centres of the crosses, the effect is still stronger than that observed in the classical simultaneous lightness contrast display. Furthermore, the effect is independent of the shape of the configuration. The phenomenon can be explained by assuming that the gradient is perceived as a change of the level of illumination rather than as a change of the reflectance of the surfaces.