Figure 9.

The human retina functions over a very wide range of light intensities. Scotopic vision is light detection by the rod photoreceptors of the eye under low light conditions. In the human eye cone photoreceptors are nonfunctional in low light and rods mediate scotopic vision. Photopic vision is light detection by the cone photoreceptors of the eye under bright light conditions. In humans and many other animals, photopic vision allows color perception, mediated by cone cells, and a significantly higher visual acuity and temporal resolution than available with scotopic vision. Mesopic vision is a combination of photopic vision and scotopic vision in low but not quite dark lighting situations and involves an input from both rod and cone photoreceptors. As light levels increase, and as rods become saturated, melanopsin photoreception is activated. Whilst this diagram gives some sense of the sensory thresholds for the different photoreceptor classes, it is also misleading in that it fails to take into consideration the differences in effective stimulus durations for the rods, cones and melanopsin-pRGCs. Rods and cones detect light over the millisecond range whilst melanopsin-based pRGCs require long duration exposure to light to elicit a biological response. See text for details.