Visual Glossary - Incident Meter

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From moviola.com's exclusive Visual Glossary--
An incident meter is a device that reads the amount of light that falls upon it. It uses a hemispherical light collector, or “photosphere”, that collects light information from the top, bottom, sides and front. Typically, an incident meter is held in front of a subject, ball facing toward the camera, to measure the average amount of light at that point. This light reading assists the director of photography with determining the aperture of the shot.