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An overhead projector (OHP) is a display system that is used to display images to an audience.

Contents 1 Mechanism 2 History 3 Decline in use 4 Pop culture references 5 References 6 External links 7 See also

An overhead projector typically consists of a large box containing a very bright lamp and a fan to cool it off, 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 at the transparency as if writing, the audience looking forward at the screen.

The first overhead projector was used for police identification work. It used a cellophane roll over a 9-inch stage allowing facial characteristics to be rolled across the stage. The U.S. Army in 1945 was the first to use it in quantity for training as World War II wound down. It began to be widely used in schools and businesses in the late 1950s and early 1960s.

A major manufacturer of overhead projectors in this early period was the company 3M. As the demand for projectors grew, Buhl Industries was founded in 1953, and became the leading US contributor for several optical refinements for the overhead projector and its projection lens. In 1957 the United States' first Federal Aid to Education program stimulated overhead sales which remained high up to the late 1990s and into the 21st Century.

Overhead projectors were once a common fixture in most classrooms and business conference rooms, but today are slowly being replaced by larger computer monitors, dedicated computer projection systems and interactive whiteboards. Such systems allow to make presentations, typically using software like Microsoft PowerPoint.

Critics feel that there are some downsides as these technologies are more prone to failure and have a much steeper learning curve for the user than a standard overhead projector. While a computer projection system eliminates the need to create hard copy transparencies (which can be quite expensive, particularly if made in colour) of the slide show presentation, many presenters make both in case the computer hardware fails. Furthermore, the overhead projector allows a more direct interaction through live writing on the transparency.

• The Art of the Overhead

An art and media festival dedicated to this almost outdated technology was arranged for the first time in Copenhagen, October 2005. The project spawned a row of overhead installations and performances, like the song "Farewell to the Overhead" (see below).

The festival is an ongoing project. [1]

• Farewell to Overhead

In 2005 the group Monochrom created a melancholic electro pop song about the "dead medium" overhead projector and adolescence/socialisation. [2]

501 Ways to Use the Overhead Projector by Lee Green (ISBN 0-87287-339-0)

• Overhead Projector In Depth Analysis of types, components, history etc.
• Buhl Industries Manufacturer of Overhead Projectors merged with Hamilton Electronics.
• Overhead Projector Information Resource

• A related type of the projector is the opaque projector.
• Projector for a directory of projector types
• Transparency (projection)