Overhead projector
These transparent sheets are placed on the glass platen of the projector, which has a light source below it and a projecting mirror and lens assembly above it (hence, "overhead").
In order to provide sufficient light on the screen, a high intensity bulb is used which often requires fan cooling.
Increasing the projection distance also means that the same amount of light is spread over a larger screen, resulting in a dimmer image.
Using the projector near its recommended projection distance allows a focusing position where this is avoided and the intensity across the screen is approximately uniform.
[4] In contrast, a modern LCD or DLP projector often uses an Ultra-high-performance lamp which has a higher luminous efficacy and lasts for thousands of hours.
German Jesuit scholar Athanasius Kircher's 1645 book Ars Magna Lucis et Umbrae included a description of his invention, the "Steganographic Mirror": a primitive projection system with a focusing lens and text or pictures painted on a concave mirror reflecting sunlight, mostly intended for long-distance communication.
The "solar microscope" was employed in early photographic experiments with photosensitive silver nitrate by Thomas Wedgwood and Humphry Davy in making the first, but impermanent, enlargements of minute objects.
[10][11] Subsequently, in 1857 Baltimore painter David Acheson Woodward' patented a solar enlarging camera, a large instrument operated out-of-doors.
[11] Portrait artists found it a boon that provided a guide in making accurate likenesses which they would paint in oils, watercolour or pastel over the enlargement, often made at life size.
[13] 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 corporations.
This, coupled with a wide angle lens and an overhead reflector, directs the light vertically upward through the stage.
This optical arrangement provides the instructor with a number of advantages, among which are: the projector may be placed in front of the class; the brilliant light permits excellent screen visibility without darkening a room; the instructor may place transparent images on the horizontal stage or diagram extemporaneously on it without turning away from his class.
A further advantage of this visual medium lies in its adaptability to the use of transparent pictorials locally prepared...[16]Allied to the US Navy development of the improved lightweight overhead projector was its adaptation of the Ozalid dry printing process, developed in Germany in 1923, to copy training documents and illustrations on projection transparencies, a process simple enough to be carried out in the field and which ensured uniformity of instructional material used.
[23] 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.
Teaching materials can be pre-printed on plastic sheets, upon which the educator can directly write using a non-permanent, washable color marking pen.
Following the class period, the transparencies are easily restored to their original unused state by washing off with soap and water.
The first of these LCD panels were monochrome-only, and could display NTSC video output such as from an Apple II computer or VCR.
Overhead projectors were once a common fixture in most classrooms and business conference rooms in the United States, but in the 2000s they were slowly replaced by document cameras, dedicated computer projection systems and interactive whiteboards.
