Illusory lightness in the Delboeuf figure

A white disk presented inside a small concentric circle (a display like the Delboeuf figure) looks lighter than when inside a larger one. The magnitude of the effect was measured with 20 observers (10 women, 10 men; ages 19 to 26 years). The decrements in luminance of the disk inside the smaller circle, in order to match the lightness of the disk inside the larger circle, are reported for three different grey backgrounds and correspond to 9.13, 10.33, and 10.46%.