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Overhead projector

An overhead projector is a display system that is used to display images to an audience. They typically consist of a large box containing a very bright lamp and a fan to cool it, on top of which is a large lens that collimates the light. Above the box, typically on a long arm, is a mirror that redirects the light forward instead of up.

Transparencies are placed on top of the lens for display. The light from the lamp travels though the transparency and into the mirror where it is shone forward onto a screen for display. The mirror allows both the presenter and the audience to see the image at the same time, the presenter looking down as if writing, the audience looking forward at the screen.

A modification of the overhead projector allows it to display opaque materials by shining the light down onto the surface from above. Such opaque projectors are not as common as the traditional "overhead".

Overhead projectors were a common fixture at most schools and businesses, but today are being replaced by larger computer monitors and dedicated computer projection systems.