Overhead projector
An overhead projector (OHP) is a device that enlarges and projects an image from a page-sized transparent sheet onto a screen. The image comes from clear plastic sheets called transparencies (viewfoils or acetates) that may be printed or drawn on. These sheets rest on a glass plate above a bright light; a mirror and lens assembly above the plate projects the image toward the screen.
The transparency is placed face up so the presenter can read it, and the mirror helps fold the optical path so the final image is right-side up on the screen. A Fresnel lens acts as a condenser to send most of the light toward the focusing lens, making the projection brighter without needing a huge lens. A high-intensity lamp and a cooling fan provide the light, and the focusing mechanism lets the instructor adjust the distance between the sheet and the lens to change magnification.
Projection distance affects brightness and edge quality; moving the lens for different distances can cause some color fringe at the edges if the optics aren’t at their optimal position. OHPs were popular in schools and offices because they are simple and inexpensive: you can pre-print materials on transparencies and write on them with washable markers during class.
Today, many classrooms use computer projectors, document cameras, or interactive whiteboards, which project directly from a computer and often show color more vividly without printing transparencies.
This page was last edited on 2 February 2026, at 16:22 (CET).