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.
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.
Overhead projectors were once a common fixture in most classrooms and business conference rooms, but in the 2000s they were slowly being replaced by document cameras, dedicated computer projection systems and interactive whiteboards.Such systems allow the presenter to project video directly from a computer file, typically produced using software such as Microsoft PowerPoint and LibreOffice. Such presentations can also include animations, interactive components, or even video clips, with ease of paging between slides. The relatively expensive printing or photocopying of color transparencies is eliminated.
While an overhead can display static images fairly well, it performs poorly at displaying moving images. The LCD video display panels that were once used as an add-on to an overhead projector have become obsolete, with that combination of display technology and projection optics now optimally integrated into a modern video projector.
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