Mesopic vision

Mesopic vision is an intermediate vision (intermediate between photopic and scotopic) that occurs in lighting situations, which without reaching total darkness, are not the light of a day in full sun. This is mainly the type of vision used in artificial light conditions, where both cones and rods come into play.

The human vision system has two types of receptors in the retina, cones and rods, which it uses to send visual signals to the brain. The current photometry system for determining the amount of light needed to perform a task, regardless of lighting conditions or time of day, is based on how cones work. Cones are the dominant visual receptors under photopic lighting conditions (daytime). Rods function primarily under dark (scotopic) conditions. Under mesopic lighting conditions, typically found outdoors at night, the combination of rods and cones are part of human visual function. Therefore, outdoor luminaires that are adjusted according to the way human vision behaves under mesopic lighting conditions can be used to reduce street surface glare and at the same time time provide better visibility.

The majority of outdoor nighttime scenarios and street lighting are in the mesopic range.

The traditional method of light measurement considers only photopic vision and is commonly inaccurate in predicting how humans perceive light at night. Research in this area typically focuses on improving outdoor lighting, as well as aviation lighting. The visual effect considers the measurement of mesopic illumination and is used for lighting tests in outdoor luminaires and public lighting.