Photosensors are electronic control units that automatically adjust the output level of electric lights based on the amount of light detected. Lighting control devices enable occupants to control their lighting environment by either dimming the lights or switching them on and off.

Some control devices, such as light switches, manual dimmers, and window blinds, can be directly accessed and controlled by occupants. Others, such as occupancy sensors, timers, and photosensors, often are designed to take the place of occupant actions.

Photosensors are a form of automatic control that replaces or accompanies occupant control. The main reason for installing control devices is to conserve energy by switching off or dimming the electric lights when full output is not needed.

The benefit of automatic control is that energy savings can occur throughout the day without human intervention. When combined with dimming electronic ballasts, photosensors can dim lights based on the amount of daylight entering a room.

Problems easily arise, however, when occupants are disturbed by insufficient illumination levels or by sudden light level fluctuations. Meeting the occupants' visual and comfort needs as well as their expectations about good lighting is of primary importance for automatic lighting controls.