Tally light
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In a television studio, a tally light is a small signal-lamp on a television camera or monitor. On a camera, it is usually located just above the lens and indicates, for the benefit of the actors being filmed, that the camera is 'live' - ie: its signal is being recorded or transmitted at that moment. Tally lights are typically red, although some cameras and video-switchers accept a preview tally signal, which is typically green.