Fig. 14. Scotopic (left panel) and photopic (right panel) increment thresholds for and their relationship to the fraction of pigment bleached. Rods are saturated at retinal illuminance levels insufficient to provide significant bleaching. The cone system obeys Weber's law (linear portion of curve) across 6 log units of background. The reduction in pigment at high light level makes it difficult to saturate the cone system, because the bleached pigment reduces the effective background. (Enoch JM: The two-color threshold technique of Stiles and derived component color mechanisms. In Jameson D, Hurvich I [eds]: Visual Psyhophysics, Handbook of Sensory Physiology. Vol VII/4. Berlin: Springer, 1972.)