Photosensors automatically adjust the light output of a lighting system based on detected illuminance. The technology behind Photosensors is the photocell. A photocell is a light-responding silicon chip that converts incident radiant energy into electrical current.

While some Photosensors just turn lights off and on, others can also dim lights. Automatic dimming can help with lumen maintenance. Lumen maintenance involves dimming luminaires when they are new, which minimizes the wasteful effects of over-design. The power supplied to them is gradually increased to compensate for light loss over the life of the lamp.

Nearly all Photosensors are used to decrease the electric power demand for lighting. In addition to lowering the electric power demand, dimming the lights also reduces the thermal load on a building's cooling system. Any solar heat gain that occurs in a building during the day must be taken into account for a whole building energy usage analysis.

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Figure 115: Outdoor lights with photo sensors are controlled by the sun.

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