Mesopic vision, sometimes also called twilight vision, is a combination of
photopic
Photopic vision is the vision of the eye under well-lit conditions (luminance levels from 10 to 108 cd/m2). In humans and many other animals, photopic vision allows color perception, mediated by cone cells, and a significantly higher visua ...
and
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, ...
under low-light (but not necessarily dark) conditions. Mesopic levels range approximately from 0.01 to 3.0
cd/m2 in
luminance
Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls withi ...
. Most nighttime outdoor and street lighting conditions are in the mesopic range.
Human eyes respond to certain light levels differently. This is because under high light levels typical during daytime (photopic vision), the eye uses
cones
A cone is a three-dimensional geometric shape that tapers smoothly from a flat base (frequently, though not necessarily, circular) to a point called the apex or vertex.
A cone is formed by a set of line segments, half-lines, or lines conn ...
to process light. Under very low light levels, corresponding to moonless nights without artificial lighting (scotopic vision), the eye uses
rods to process light. At many nighttime levels, a combination of both cones and rods supports vision. Photopic vision facilitates excellent
color perception
Color vision, a feature of visual perception, is an ability to perceive differences between light composed of different wavelengths (i.e., different spectral power distributions) independently of light intensity. Color perception is a part of ...
, whereas colors are
barely perceptible under scotopic vision. Mesopic vision falls between these two extremes. In most nighttime environments, enough
ambient light prevents true scotopic vision.
In the words of Duco Schreuder:
The effect of switching from cones to rods in processing light is called the "
Purkinje shift
The Purkinje effect (; sometimes called the Purkinje shift, often incorrectly pronounced ) is the tendency for the peak luminance sensitivity of the eye to shift toward the blue end of the color spectrum at low illumination levels as part of da ...
". In photopic vision, people are most sensitive to light that is
greenish yellow. In scotopic vision, people are more sensitive to light that would appear
greenish blue
''Icaricia saepiolus'', the greenish blue, is a butterfly of the family Lycaenidae. It is found from the northwestern United States to southern Saskatchewan and Alberta.
The wingspan is 21–28 mm. Adults are on wing from June to August.< ...
. These are combinations of
primary color
A set (mathematics), set of primary colors or primary colours (see American and British English spelling differences#-our, -or, spelling differences) consists of colorants or colored lights that can be mixed in varying amounts to produce a gamu ...
s.
The traditional method of measuring light assumes photopic vision and is often a poor predictor of how a person sees at night. Typically research in this area has focused on improving street and outdoor lighting as well as
aviation lighting.
Prior to 1951, there was no standard for scotopic
photometry Photometry can refer to:
* Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision
* Photometry (astronomy), the measurement of the flux or intensity of an astronomical object's electrom ...
(light measurement); all measurements were based on the photopic spectral sensitivity function V(λ) which was defined in 1924. In 1951, the
International Commission on Illumination
The International Commission on Illumination (usually abbreviated CIE for its French name, Commission internationale de l'éclairage) is the international authority on light, illumination, colour, and colour spaces. It was established in 1913 a ...
(CIE) established the scotopic luminous efficiency function, V'(λ). However, there was still no system of mesopic photometry. This lack of a proper measurement system can lead to difficulties in relating light measurements under mesopic luminances to visibility. Due to this deficiency, the CIE established a special technical committee (TC 1-58) for collecting the results of mesopic visual performance research.
Two very similar measurement systems were created to bridge the scotopic and photopic luminous efficiency functions, creating a unified system of photometry. This new measurement has been well-received because the reliance on V(λ) alone for characterizing night-time light illumination can result in the use of more electric energy than might otherwise be needed. The energy-savings potential of using a new way to measure mesopic lighting scenarios is significant; superior performance could in certain cases be achieved with as much as 30 to 50% reduction in the energy use comparing to the high pressure sodium lights.
Mesopic weighting function
The mesoscopic weighting function at wavelength
can be written as the weighted sum,
[Photopic and Scotopic lumens - 4: When the photopic lumen fails us](_blank)
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:,
where is the standard photopic weighting function (peaking at 683 lm/W at 555 nm) and is the scotopic weighting function (peaking at approx. 1700 lm/W at 507 nm). The parameter is a function of the photopic luminance
Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls withi ...
. Various weighting functions are in use, for blue-heavy and red-heavy light sources, as proposed by two organizations, MOVE and Lighting Research Center (LRC). Some values for are shown in the table below.
References
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Eye
Vision