Scotopic vision

In the study of human visual perception, scotopic vision (or scotopia) is the vision of the eye under low-light conditions. The term comes from Greek skotos, meaning "darkness", and -opia, meaning "a condition of sight". In the human eye, cone cells are nonfunctional in low visible light. Scotopic vision is produced exclusively through rod cells, which are most sensitive to wavelengths of around 498 nm (blue-green) and are insensitive to wavelengths longer than about 640 nm (red-orange). This condition is called the Purkinje effect. Of the two types of photoreceptor cells in the retina, rods dominate scotopic vision. This is caused by increased sensitivity of the photopigment molecule expressed in rods, as opposed to that in cones. Rods signal light increments to rod bipolar cells, which, unlike most bipolar cell types, do not form direct connections with retinal ganglion cells - the output neuron of the retina. Instead, two types of amacrine cell - AII and A17 - allow lateral information flow from rod bipolar cells to cone bipolar cells, which in turn contact ganglion cells. Rod signals, mediated by amacrine cells, therefore dominate scotopic vision. Scotopic vision occurs at luminance levels of 10−3 to 10−6 cd/m2. Other species are not universally color blind in low-light conditions. The elephant hawk-moth (Deilephila elpenor) displays advanced color discrimination even in dim starlight. Mesopic vision occurs in intermediate lighting conditions (luminance level 10−3 to 100.5 cd/m2) and is effectively a combination of scotopic and photopic vision. This gives inaccurate visual acuity and color discrimination. In normal light (luminance level 10 to 108 cd/m2), the vision of cone cells dominates and is photopic vision. There is good visual acuity (VA) and color discrimination. In scientific literature, one occasionally encounters the term scotopic lux which corresponds to photopic lux, but uses instead the scotopic visibility weighting function. The normal human observer's relative wavelength sensitivity will not change due to background illumination change under scotopic vision.