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Overhead projectors were introduced into U. Military Academy. The recent adaptation of plastic to the manufacture of condenser lenses has permitted a revolutionary redesign of a lecture-demonstration projector, now commonly referred to as an overhead projector. The flat lightweight plastic lens makes possible the use of a large horizontal stage aperture. This, coupled with a wide angle lens and an overhead reflector, directs the light vertically upward through the stage.
Next, it is reflected horizontally to the screen. 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.
Early interest in the use of 'vertical' projection for demonstrating science phenomena is evidenced by the publication of an article on this subject in Previous limitations in weight and aperture size have been overcome by the substitution of plastic for the heavier glass condenser.
Thus, transparencies slides up to 10 x 10 inches in size may be used. As a lecture-demonstration instrument, the overhead projector is destined to gain popularity with university instructors because it facilitates the instructor's command of the psychological advantages of visual presentation without sacrifice of his identity as the instructor and class leader.
A further advantage of this visual medium lies in its adaptability to the use of transparent pictorials locally prepared 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 , 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.
Overhead projectors were used early on for police work with a cellophane roll over a 9-inch stage, allowing facial characteristics to be rolled across the stage. As the demand for projectors grew, Buhl Industries was founded in , and became the leading US contributor for several optical refinements for the overhead projector and its projection lens.
Overhead projectors began to be widely used in schools and businesses in the late s and early s,  beside the contemporaneously-developed carousel slide projectors with a horizontally mounted tray manufactured by Kodak. In the late s Roger Appeldorn was challenged by his boss at 3M to find a use for the transparencies that were the waste of their color copy process. Appeldorn developed a process for the projection of transparent sheets that led to 3M's first marketable transparency film.
The Strategic Air Command base in Omaha was one of the first big clients, using circa 20, sheets per month. It took several prototypes before a cost-effective, small and foldable version could be presented on January 15, It had a new fresnel lens made with a structured-surface plastic, much better than other plastic lenses and much cheaper than glass. Overhead projectors were widely used in education and business before the advent of computer-based projection.
The overhead projector facilitates an easy low-cost interactive environment for educators. Teaching materials can be pre-printed on plastic sheets, upon which the educator can directly write using a non-permanent, washable color marking pen.
This saves time, since the transparency can be pre-printed and used repetitively, rather than having materials written manually before each class. The overhead is typically placed at a comfortable writing height for the educator and allows the educator to face the class, facilitating better communication between the students and teacher. The enlarging features of the projector allow the educator to write in a comfortable small script in a natural writing position rather than writing in an overly large script on a blackboard and having to constantly hold their arm out in midair to write on the blackboard.
When the transparency sheet is full of written or drawn material, it can simply be replaced with a new, fresh sheet with more pre-printed material, again saving class time vs a blackboard that would need to be erased and teaching materials rewritten by the educator.
Following the class period, the transparencies are easily restored to their original unused state by washing off with soap and water. A liquid-crystal panel mounted in a plastic frame was placed on top of the overhead projector and connected to the video output of the computer, often splitting off the normal monitor output. A cooling fan in the frame of the LCD panel would blow cooling air across the LCD to prevent overheating that would fog the image. In the late s color models became available, capable of "thousands" of colors bit color , for the color Macintosh and VGA PCs.
The displays were never particularly fast to refresh or update, resulting in the smearing of fast-moving images, but it was acceptable when nothing else was available. The Do-It-Yourself community has started using this idea to make low-cost home theater projectors.
By removing the casing and backlight assembly of a common LCD monitor, one can use the exposed LCD screen in conjunction with the overhead projector to project the contents of the LCD screen to the wall at a much lower cost than with standard LCD projectors. Due to the mirroring of the image in the head of the overhead projector, the image on the wall is "re-flipped" to where it would be if one was looking at the LCD screen normally.
Overhead projectors were once a common fixture in most classrooms and business conference rooms in the United States, but in the s they were slowly being replaced by document cameras , dedicated computer projection systems and interactive whiteboards.
Such presentations can also include animations, interactive components, or even video clips, with ease of paging between slides. The relatively expensive printing or photocopying of color transparencies is eliminated. The primary reason [ dubious — discuss ] for this gradual replacement is the deeply ingrained use of computing technology in modern society and the inability of overheads to easily support the features that modern users demand.
While an overhead can display static images fairly well, it performs poorly at displaying moving images. The LCD video display panels that were once used as an add-on to an overhead projector have become obsolete, with that combination of display technology and projection optics now optimally integrated into a modern video projector. The standards of users have also increased [ citation needed ] , so that a dim, fuzzy overhead projection that is too bright in the center and too dim around the edges is no longer acceptable.
The optical focus, linearity, brightness and clarity of an overhead generally cannot match that of a video projector [ citation needed ]. Video projectors use extremely small picture generation mechanisms, allowing for precision optics [ dubious — discuss ] that far exceed the plastic fresnel lens' optical performance. They also include additional optics that eliminate the hotspot in the center of the screen [ citation needed ] directly above the light source, so that the brightness is uniform everywhere on the projection screen.
Critics [ who? From Wikipedia, the free encyclopedia. Device that projects a transparent image. This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. Main article: LCD projector. This section possibly contains original research. Please improve it by verifying the claims made and adding inline citations. Statements consisting only of original research should be removed.
October Learn how and when to remove this template message. ISBN Designing Communication and Learning Environments. Educational Technology. Teaching of Social Studies. PHI Learning. Retrieved November 2, Journal of Physics D: Applied Physics. It lights up the glass cover and any transparencies on it.
The light will pass through the clear transparency sheet and project any image or text printed on it. A reflector in the base directs the lamplight towards a mirror. It prevents the light from scattering and producing a dim image. The mirror in the base serves to reflect the light upwards and through the glass cover. This enables it to capture light in its horizontal path and direct it upwards. Related post: Opaque Projector vs.
Overhead Projector. The fan in the projector is also of great importance. The fan is located in the base. It removes the heat that lamplight produces. If a projector overheats, it can cause damage to the system. Overheating can also make your projector shutdown while projecting. This is one of the two lenses used by an overhead projector. It is a circular lens located under the glass cover.
The fresnel lens focuses light to one spot. In this case, it focuses the light in the direction of the head. It bends the rays of light that spread outward from the glass cover. That way, all the rays from one spot on the transparency come back together onto one spot in the head.
The projector has an arm which is connected to the base. The function of the arm is to hold up the head. This arm can be adjusted to help focus the image on your projection screen. Moving the head unit up or down can improve focus.
There is a mirror in the head of an overhead projector. The bottom of the head is open, allowing light to enter. On the inside of the head is an objective lens. Light passes through the transparency and into the head. The image on the transparency is projected through the lens and onto the mirror.
The mirror then flips the image and directs it onto a projection screen. The image would be displayed on whatever the mirror is pointing towards. Overhead projectors cannot work without transparencies. Anything to be projected has to be printed on the transparency. This is because transparencies support the light mechanism of overhead projectors. Light can pass through them.
For your projector to work , it must first be connected to a power source. You can then turn it on and proceed to use it. The lamp in the base comes on and illuminates the transparency placed on the glass cover. The fresnel lens directs the light. It directs the light from the transparency to the objective lens in the head.
Next, the image reaches the mirror in the head. The mirror flips the image and points the light in the direction of the screen. Read more: LCD Projector vs.
An overhead projector (often abbreviated to OHP), like a film or slide projector, uses light to project an enlarged image on a screen, allowing the view of. Find great deals on used overhead projector for Sale in South Africa. Browse Gumtree Free Online Classifieds for second hand Electronics from sellers in. Find Great Deals on overhead projector | Compare Prices & Shop Online | PriceCheck. Nobo Overhead Projector Replacement Lamp Watts. R R