• 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.