Light meterA light meter is a device used to measure the intensity of light. In photography, light meters are used to determine proper exposure. Given a film speed and shutter speed, the meter will show the f-stop which would give a neutral exposure of an ideal gray slate.
Selenium light meters are based on a photopile and thus have a limited life expectancy, while CdS light meters and silicon light meters are based on a photo-resistance, and thus needs batteries to operate.
Most modern consumer still and video cameras include a built-in meter that measures a scene-wide light level and are able to make an approximate measure of appropriate exposure based on that. Professional photographers and motion picture cinematographers generally use handheld ambient light meters to precisely measure the light falling on various parts of their subjects, and build up lighting to produce the desired exposure levels.