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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.
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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. Critics feel that there are some downsides as those are more prone to failure and have a much steeper learning curve than a standard overhead projector. While a computer projection system eliminates the need to create hard copy transparencies of the slide show presentation; many presenters make both in case the computer hardware fails.
History
The overhead projector was developed during World War II to facilitate the training of servicemen. 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.
A 1944 invention – the overhead projection system was destined to make an impact on schools around the world. Every classroom in America as well as industrial training rooms normally has an overhead projector.
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. As
the demand for projectors grew, Buhl Industries founded in
1953, became the leading contributor for several optical refinements
for the overhead projector and its projection lens. In 1957
the nation’s first Federal Aid to Education program
stimulated overhead sales which were boost to a high throughout
the late 1990’s and into the 21st Century.
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