Inthe study ofvisual perception,scotopic vision (orscotopia) is the vision of theeye under low-light conditions.[1] The term comes from theGreekskotos, meaning 'darkness', and-opia, meaning 'a condition of sight'.[2] In thehuman eye,cone cells are nonfunctional in lowvisible light. Scotopic vision is produced exclusively throughrod cells, which are mostsensitive towavelengths of around 498 nm (blue-green)[3] and are insensitive to wavelengths longer than about 640 nm.[4] Under scotopic conditions, light incident on the retina is not encoded in terms of thespectral power distribution. Higher visual perception occurs under scotopic vision than it does underphotopic vision.[5]
Of the two types ofphotoreceptor cells in theretina,rods dominate scotopic vision. This dominance is due to the increased sensitivity of thephotopigment molecule expressed in rods, as opposed to those incones. Rods signal light increments torod bipolar cells which, unlike most types ofbipolar cells, do not form direct connections withretinal ganglion cells – the output neurons of the retina. Instead, two types ofamacrine cell –AII and A17 – allow lateral information flow from rod bipolar cells to cone bipolar cells, which in turn contact ganglion cells. Thus, rod signals, mediated by amacrine cells, dominate scotopic vision.[6]
Scotopic vision occurs atluminance levels of 10−3[7] to 10−6[citation needed]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.[8]
Mesopic vision occurs in intermediate lighting conditions (luminance level 10−3 to 100.5cd/m2)[citation needed] and is effectively a combination of scotopic andphotopic vision. This gives inaccuratevisual acuity and color discrimination.
In normal light (luminance level 10 to 108cd/m2), the vision of cone cells dominates and isphotopic vision. There is good visual acuity (VA) and color discrimination.
The normal human observer's relative wavelength sensitivity will not change due to background illumination under scotopic vision. The wavelength sensitivity is determined by therhodopsinphotopigment. This is a red pigment seen at the back of the eye in animals that have a white background to their eye calledTapetum lucidum. The pigment is not noticeable underphotopic andmesopic 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."[9]
The behavior of therhodopsinphotopigment 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 400 nm light and 700 nm light. Therefore, this photopigment only maps the rate of absorption and does not encode information about the relative spectral composition of the light.[9]
In scientific literature, one occasionally encounters the termscotopic lux which corresponds tophotopic lux, but uses instead the scotopic visibility weighting function.[10] The scotopic luminosity function is a standard function established by theCommission Internationale de l'Éclairage (CIE) and standardized in collaboration with theISO.[11]
The maximum scotopic efficacy is 1700lm/W at 507 nm (compared with 683 lm/W at 555 nm for maximum photopic efficacy).[12] While the ratio between scotopic and photopic efficacies is only around 2.5 counted at peak sensitivity the ratio increases strongly below 500 nm.
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 highspatial frequencies in low light since the observer is spatially averaging the light signal.[9]
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 poorspatial frequency sensitivity.[9]
High-level visual perception is similar with scotopic as with photopic sight with peoplereading with unimpaired accurately (though twice as longfixations), able torecognize faces, and show aface inversion effect.[5]