A light meter is test device used to measure the amount of ambient light present in an environment. The light measurement is expressed in foot-candles or lux, which are the non-SI and SI units of illuminance cast on a surface by a light source.
Light meters are commonly used to increase the energy efficiency of an interior lighting system. A photoelectric sensor measures the amount of light in a room, and designers can adjust output of luminaires to suit the environment.
Light meters offer a great deal of flexibility and accuracy, depending on the model. Testers can measure luminance up to 999,900 lux with a resolution up to 0.1 lux. They are accurate within 3 to 5 percent at distances up to several meters.