An overhead projector is a very basic but reliable tool used to display 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.
This type of projector can be used to enlarge images onto the screen or wall for audiences to view. Transparencies can be placed onto the base to be viewed by both the audience and the speaker. The device was once a common feature in both classrooms and business meetings, although it has seen a decline in use as more sophisticated computer based projectors are favored.
Overhead projectors were used during World War II as a tool to train groups 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. It reached its sales peak in the 1990s, when almost every classroom and business place in America had one installed.
The projector is best used when the screen is approximately 6 to 12 feet (about 2 to 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 projectors is the type of optics used.
There are three main types of optics that can be used in the overhead projector. A direct optics projector puts the optics and light source in a straight line, allowing the image to be reflected from a mirror onto the screen. It's a very common form, and relatively inexpensive, but it can get quite hot.
Folded optics or chamber optics have a mirror placed in between the lamp and the lens, and a light source that is surrounded by an elliptical faceted reflector. This has a number of benefits, including making the resolution quite clear, and making more light available to project the image. This model is usually cooler than direct optic versions.
Another type is the reflected light optic, which has a specially designed lens that is backed by a mirror. The light source directs light to the lens and 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, these devices are on the decline. Some critics still favor them for their reliability, however, and claim that they are far easier to use than the more sophisticated digital versions.