Exposure meter
From The Glossary of Digital Photography
Also called a light meter. A device, often built in to cameras, which tells the photographer the exposure to use. Some exposure meters are incident meters, which indicate the amount of light falling on the meter. An incident meter is commonly used at the subject location and aimed back at the camera. Another type of meter is a reflected light meter. These meters are used at the camera position and measure the amount of light reflected back toward the camera from the subject. In both cases, the exposure meter assumes that the world is 18% gray and indicates the amount of exposure necessary to render the subject an 18% gray. Using a reflected light meter when the subject is not 18% gray (for example, in black or white), results in the meter's value having to be adjusted to provide the correct rendering of the
subject.
