Photo-cells switch outdoor lights on at dusk and off at dawn, for example. Advanced designs gradually raise and lower fluorescent light levels with changing daylight levels. Mechanical or electronic time clocks automatically turn on and off indoor or outdoor lights for security, safety, and tasks such as janitorial work. Crank timers, which are spring-driven and similar to old oven timers, limit lights to short durations where the need for light is brief. Occupancy sensors activate lights when a person is in the area and then turn off the lights after the person has left.
Dimmers reduce the wattage and output of incandescent and fluorescent lamps.
Dimmers also increase the service life of incandescent lamps significantly. However, dimming incandescent lamps reduces their lumen output more than their wattage. This makes incandescent lamps less efficient as they are dimmed. Dimming fluorescents requires special dimming ballasts and lamp holders, but does not reduce their efficiency. The success of any lighting controls installation depends on careful tuning of the system and special attention to the needs of the space's occupants.