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In cinematography and photography, 'pre-flashing' is the exposure of the film or other photosensor to uniform light prior to exposing it to the scene to be imaged. This adds a bias to the overall light input recorded by the sensor.
It is sometimes used for artistic effects, as it can be used to add a colour cast to shadows without significantly affecting highlights.
It is also used in fields such as astronomy to bias CCD image sensors into a linear region, improving their low-light sensitivity.