Equivalent lightness of colored objects at illuminances from the scotopic to the photopic level

Equivalent lightness of four colored objects, blue, green, yellow, and red, was obtained for illuminance levels covering the range 0.01 to 1000 lx by brightness matching to a grey scale. Three field sizes, 20', 1° and 6° arc of visual angle, were investigated. With the smallest size, for which no rod response was expected, the equivalent lightness increased monotonically for incresing illuminance, showing the increasing effect of colorfulness upon brightness. The results obtained with the largest size showed influence from both rods and cones and were interpreted as due to two underlying lightnesses, one lightness based on the achromatic response of rods, cones, or both, and the other lightness based on the chromatic response of cones exhibited as colorfulness.