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Scotopic vision

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File:CIE 1951 scotopic luminosity function.svg

Scotopic vision 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.[1] In the human eye cone cells are nonfunctional in low light – scotopic vision is produced exclusively through rod cells which are most sensitive to wavelengths of light around 498 nm (green-blue) and are insensitive to wavelengths longer than about 640 nm (red).

Scotopic vision occurs at luminance levels of 10−2 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.[2]

Mesopic vision occurs in intermediate lighting conditions (luminance level 10−2 to 1 cd/m²) and is effectively a combination of scotopic and photopic vision. This however gives inaccurate visual acuity and color discrimination.

In normal light (luminance level 1 to 106 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.[3]

The normal human observer’s relative wavelength sensitivity will not change due to background illumination change under scotopic vision. The wavelength sensitivity is determined by the rhodopsin photopigment. This is a red pigment and can be seen at the back of eye of animals that have a white background to their eye called Tapetum lucidum. The pigment is not noticeable under photopic and mesopic conditions. The principle that the wavelength sensitivity does not change during scotopic vision led to the ability to detect two functional cone classes in individuals. If two cone classes are present, then their relative sensitivity will change the behavioral wavelength sensitivity. Therefore, experimentation can determine “the presence of two cone classes by measuring wavelength sensitivity on two different backgrounds and noting a change in the observer’s relative wavelength sensitivity.” For adaption to occur at very low levels, the human eye needs to have a large sample of light across the signal in order to get a reliable image. This leads to the human eye being unable to resolve high spatial frequencies in low light since the observer is spatially averaging the light signal.[4]

The behavior of the rhodopsin photopigment explains why the human eye cannot resolve lights with different spectral power distributions under low light. The reaction of this single photopigment will give the same quanta for 400nm light and 700nm light. Therefore, this photopigment only maps the rate of absorption and does not encode information about the relative spectral composition of the light.[5]

Another reason that vision is poor under scotopic vision is that rods, which are the only cells active under scotopic vision, converge to a smaller number of neurons in the retina. This many-to-one ratio leads to poor spatial frequency sensitivity.[6]

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