Gelb lighting, SLC, and the Benary effect

If a standard simultaneous-lightness-contrast display (SLC) (two mid-grey targets on adjacent black and white surrounds) is placed under Gelb lighting (an illumination border is made to coincide with the outer border of the display and the rest of the visual field is left in near darkness), the size of the contrast effect increases significantly relative to measurements made under homogeneous illumination. A similar increase is found when the direction of the illumination border is inverted so that the display is now in shadow and the rest of the field is brighter. We studied the effect of the Gelb lighting on SLC, on the Benary display, and on related displays to control for different spatial configurations and shapes of the targets. Areas of black, white, and mid-grey regions where kept constant. Observers chose the closest match for a given target on a Munsell scale presented next to the display and also placed under gelb lighting. Each observer performed one match in one condition only. Results indicate that the increase of contrast due to Gelb lighting is maximal for a standard SLC display, whereas it is much reduced in the Benary configuration. Results have implications for theories of lightness contrast and the spatial integration of luminance ratios.