Overhead projector

An overhead projector (OHP), like a film or slide projector, uses light to project an enlarged image on a screen. In the overhead projector, the source of the image is a page-sized sheet of transparent plastic film (also known as 'foils') with the image to be projected either printed or hand-written/drawn. These are placed on the glass surface 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 computer-based projection.