Photosensor dimming, or daylight harvesting, increases energy savings by taking advantage of the natural light entering a space through windows or skylights. Daylight harvesting technology includes photosensors and dimming ballasts. The photosensor generates a signal based on a sampling of the light in a space and the electronics that define its control algorithm. The degree to which the sampling of light represents the overall illumination level in a space depends on the photosensor's location and its optics. The photosensor control algorithm transforms raw light readings into a signal that is sent to the dimming ballasts to adjust the electric lighting level. This algorithm either dims or switches the ballast. In general, the daylight takes the place of the electric light to save energy, and ideally the dimming should go unnoticed by the occupant.

Photosensor dimming sounds simple, but several significant barriers impede its pervasive use. Garnering energy savings in occupied spaces is difficult because photosensor technologies have a variety of possible limitations, market costs are usually high, and little is known by specifiers about the technology and human factors issues involved with dimming. New research and technology developments, however, are making inroads to combat these issues.

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