Several system of light meter have been used the three most common being selenium, CdS, and silicon light meters.
Selenium light meters are based on a photopile (that has a limited life expectancy), while CdS light meters and silicon light meters are based on photo-resistance and needsbatteries 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.