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The use of transparent sheets for overhead projection, called viewfoils or viewgraphs, was largely developed in the United States. Overhead projectors were introduced into U.S. military training during World War II as early as 1940 and were quickly being taken up by tertiary educators, [14] and within the decade they were being used in ...
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.
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.
All enlargers consist of a light source, normally an incandescent light bulb shining though a condenser or translucent screen to provide even illumination, a holder for the negative or transparency, and a specialized lens for projection, though some, such as the Rapid Rectilinear or Aplanat [citation needed] could be used in both camera and enlarger.
Reversal film is produced in various sizes, from 35 mm to roll film to 8×10 inch sheet film. A slide is a specially mounted individual transparency intended for projection onto a screen using a slide projector. This allows the photograph to be viewed by a large audience at once.
The discolored portion is oxidation of the stainless steel caused by the high temperature of the lamp, but the gobo is still usable. A glass gobo of the Earth, projected using a halogen projector A gobo being projected with beams of smoke. Gobos are used with projectors and simpler light sources to create lighting scenes in theatrical applications.
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