Mesopic
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 to 3 cd m−2. 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.
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