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An overhead projector works on the same principle as a slide projector, in which a focusing lens projects light from an illuminated slide onto a projection screen where a real image is formed. However some differences are necessitated by the much larger size of the transparencies used (generally the size of a printed page), and the requirement ...
Overhead projector in operation, with a transparency being flashed. A transparency, also known variously as a viewfoil or foil (from the French word "feuille" or sheet), or viewgraph, is a thin sheet of transparent flexible material, typically polyester (historically cellulose acetate), onto which figures can be drawn.
A projector or image projector is an optical device that projects an image (or moving images) onto a surface, commonly a projection screen. Most projectors create an image by shining a light through a small transparent lens , but some newer types of projectors can project the image directly, by using lasers .
The Kinodrome 35-mm projector mechanism, introduced in 1907, steadied projected images and reduced the flicker that can occur during motion-picture projection. [4] The 35-mm perforator, introduced in 1910, set the standard throughout the industry as to the expected distance and width of the sprocket holes running on each side of the 35-mm film ...
As a subsidiary of Mast Development Company, Keystone produced telebinoculars, eye training products and overhead projectors. In 1972 Gifford Mast closed down the Meadville manufacturing site, although the name of Keystone View Company continues to be used on eye training equipment.
As copying technology advanced, Thermofax machines were subsequently marketed as a method of producing transparencies (viewgraphs) for overhead projector presentations. A sheet of heat-sensitive clear stock was placed on top of the original, and passed through a ThermoFax, producing a black image on the clear stock.
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At a basic level, visual content can be provided by drawing directly onto a blackboard or whiteboard during the presentation. At a more advanced level, flip charts, slide projectors, and overhead projectors can be used for displaying prepared content. The use of prepared material often results in a neater and more accessible presentation.