Simultaneous lightness contrast under different illuminations

We investigated lightness perception under different illuminations with observers adapted to the ambient light. Four simultaneous lightness contrast paper displays were sequentially presented in random order; one display was the classic textbook illusion (1), the other three were contrast displays with four squares per background arranged in such a way to form a cross with a central gap around the target. The crosses for the three modified contrast displays were characterized as follows: white cross on black background and black cross on white background (2); dark grey cross on black background and light grey cross on white background (3); crosses with luminance ramps (4). Participants were randomly assigned to one of three illumination conditions – scotopic, normal, and Gelb, which yielded for the identical targets the following luminance: 0.82, 6.81, and 560 cd/m2 – and were asked to find for each target the corresponding “color” on a 16-step achromatic Munsell scale inserted inside a box whose illumination was unaffected by stimulus illumination. A main effect of illumination on lightness matching was found (p<0.0001), with targets appearing lighter as stimulus luminance is increased, regardless the type of configuration. Results are discussed with reference to existing theories in lightness perception.