Most nighttime outdoor and street lighting conditions are in the mesopic range.
At many nighttime levels, a combination of both cones and rods supports vision.
In most nighttime environments, enough ambient light prevents true scotopic 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'.
[3]As a result of gradually switching from cones to rods in processing light, a number of visual effects occur:[4] Cinematographers intentionally emulate mesopic effects to make scenes look darker than a display can actually achieve.
[4]: 1 The traditional method of measuring light assumes photopic vision and is often a poor predictor of how a person sees at night.
[5] In 1951, the International Commission on Illumination (CIE) established the scotopic luminous efficiency function, V'(λ).
Due to this deficiency, the CIE established a special technical committee (TC 1-58) for collecting the results of mesopic visual performance research.
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.