What is a light meter? You know those black things with the white domes on them that the director or DP holds up to whatever it is they're going to shoot? How exactly do they work, and what exactly do they do? Are they necessary?
It measures the light: they're mostly used for film, as they're calibrated for film's response to light, but could be used for video.
However, it's a bit pointless when you can see what you're getting in the viewfinder of a video camera.
Sekonic, which manufactures light meters, has a pretty good explanation of how they work:
http://www.sekonic.com/IncidentVsReflect.html
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