An overhead projector (often abbreviated to OHP), like a film or slide projector, uses light to project an enlarged image on a screen, allowing the view of a small document or picture to be shared with a large audience.
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In the overhead projector, the source of the image is a page-sized sheet of transparent plastic film (also known as "foils" or "transparencies") with the image to be projected either printed or hand-written/drawn. These are placed on the glass platen of the projector, which has a light source below it and a projecting mirror and lens assembly above it (hence, "overhead"). They were widely used in education and business before the advent of video projectors.