ABSTRACT
The response of the human eye changes with the luminance level. Photopic conditions are met by levels above 10 cd.m-2, mesopic conditions are between 0.01 and 10 cd.m-2 and scotopic conditions apply when levels are below 0.01 cd.m-2. The peak of the human eye sensitivity curve lies at 555 nm for photopic conditions and shifts towards shorter wavelengths as the luminance levels decrease. Under scotopic conditions, the peak response of the human eye is at 507 nm. Normal daylight represents photopic conditions whilst moonlight is typical of scotopic conditions and roadway lighting is usually in the mesopic region. An outline of the principles and regulations for roadway lighting is given by Simons [1].
