An overhead projector is a very basic but reliable form of projector. The overhead projector displays images onto a screen or wall. It consists of a large box containing a cooling fan and an extremely bright light, with a long arm extended above it. At the end of the arm is a mirror that catches and redirects the light towards the screen.

An overhead projector can be used to enlarge images onto the screen or wall for audiences to view. Transparencies can be placed onto the projector to be viewed by both the audience and the speaker. The overhead projector was once a common feature in both classrooms and business meetings. Recently, it has seen a decline in use, as more sophisticated computer based projectors are favored.

The overhead projector was first seen during World War II. It was used as a tool to train large numbers of servicemen. In the 1950s and 60s, it crossed over into the classroom as an educational tool, and then into the business world as a training tool. The overhead projector reached its sales peak in the 1990s, when almost every classroom and business place in America had one installed.

The overhead projector is best used when the screen is approximately 6 to 12 feet (about 2-4 meters) from the projector. There are overheads that can increase the distance between projector and screen, but these are usually custom built and the cost is greatly increased. The biggest difference that can be seen between overhead projectors is the type of optics used.

There are three main types of optics that can be used in the overhead projector. Folded optics or chamber optics have a mirror placed in between the lamp and the lens. There are a number of benefits in using a folded optic projector as opposed to the more common direct optic projector. The resolution is a lot clearer, and there is more light used to project the image. There is approximately 50% more light available with a folded optic than with a direct optic projector.

Another type of optic is the reflected light optic. This has a specially designed lens that is backed by a mirror. The light source directs light to the lens, and the light is reflected by the mirror. The reflected light is directed back through the lens and onto the main projective lens. The light is then focused to a second mirror and onto the screen.

The reflected light optic is used more in portable overhead projectors, and the cooling system, light source and main projection optics are all placed in one unit. The entire projector can be folded down into a case for portability. Although still used in many classrooms around the world, the overhead projector may sadly be on the decline. However, some critics still favor the overhead projector for reliability and claim it is far easier to use than the more sophisticated projectors.