Photoelectric controls are designed to strategically use daylight to reduce the need for artificial lighting, a process called daylight harvesting. They may be located in perimeter offices, atriums, hallways, or in areas with skylights. Ambient light sensors measure natural and ambient light, and based on the amount of natural light, adjust the lighting to maintain a constant light level. In some spaces, manual or automatic blinds, or other means of reducing the direct solar exposure glare, excessive light levels, and heat gain, can be used to supplement the photoelectric control. These may include motorized window shades or blackout shutters.
Proper daylight harvesting design not only includes providing enough daylight to an area, but doing so without any undesirable side effects, such as heat gain and glare. Successful daylight harvesting designs will incorporate shading devices to reduce glare and excess contrast in an area. Window size and spacing, glass type, and the reflectance of interior finishes must be taken into account as well. Despite all of these design considerations, daylight harvesting provides little benefit without an integrated electric lighting system, because of the increased thermal loads from the sun. The electric lighting and thermal loads must be reduced while simultaneously increasing daylight to an area.