Photograph of a handheld digital ambient light meter, showing an f-stop of 5.6 for 24 frame/s 500 ISO filming

A 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.

Several system of light meter have been used the three most common being selenium, CdS , and silicon light meters.

Selenium and silicon light meters use sensors that are photovoltaic. These sensors generate a voltage proportional to light exposure. Selenium sensors generate enough voltage for direct connection to a meter. Silicon sensors need an amplification circuit and require a power source like a batteries to operate. CdS light meters use a sensor based on photoresistance . These also require a battery to operate. Most modern light meters use silicon or CdS sensors.

Many 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.