Mesopic vision is a combination of photopic vision and scotopic vision in low but not quite dark lighting situations. Mesopic light levels range from luminances of approximately 0.001 cd/m2 to 3 cd/m2. Most night-time outdoor and traffic lighting scenarios are in the mesopic range.
Humans see differently at different light levels. This is because under high light levels typical during the day (photopic vision), the eye uses cones to process light. Under very low light levels, corresponding to moonless nights without electric lighting (scotopic vision), the eye uses rods to process light. At many night-time levels, a combination of both cones and rods supports vision. Photopic vision facilitates excellent color discrimination ability, whereas colors are indiscriminable under scotopic vision. Mesopic vision falls between these two extremes. In most night-time environments, there is enough ambient light at night to prevent true scotopic vision.
In the words of Duco Schreuder,
There is not one single luminescence value where photopic vision and scotopic vision meet. To the contrary, there is a wide zone of transition between them. Because it is between photopic and scotopic vision, it is usually called the zone of mesopic vision. The reason that the zone of mesopic vision exists is because the activities of neither cones nor rods is simply switched 'on' or 'off'. There are reasons to believe that the cones and the rods both operate in all luminescence conditions.
The effect of switching from cones to rods in processing light is called the "Purkinje shift". During photopic vision, people are most sensitive to light that is greenish-yellow. In scotopic vision, people are more sensitive to light which would appear greenish-blue.
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 (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 (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,
where is the standard photopic weighting function (peaking at 613 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 . 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.
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- CIE Publication No. 41. Light as a true visual quantity: principles of measurement. 1978.
- Schreuder, D., Outdoor Lighting: Physics, Vision and Perception (Berlin & New York: Springer, 2008), p. 237.
- "Mesopic Vision and Photometry" (PDF). Retrieved 9 June 2011.
- CIE Publication No. 81. Mesopic photometry: history, special problems and practical solutions. 1989.
- Yandan Lin, Dahua Chen, Wencheng Chen. "The significance of mesopic visual performance and its use in developing a mesopic photometry system", Building and Environment, Volume 41, Issue 2, February 2006, Pages 117–125.
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- "Driver Response to Peripheral Moving Targets under Mesopic Light Levels" (PDF). Retrieved 9 June 2011.
- "Mesopic Street Lighting Demonstration and Evaluation Final Report for Groton Utilities Groton, Connecticut Prepared by Peter Morante, Lighting Research Center Rensselaer Polytechnic Institute" (PDF).
- Photopic and Scotopic lumens - 4: When the photopic lumen fails us