||This article needs additional citations for verification. (February 2008)|
An overhead projector is a variant of slide projector that is used to display images to an audience.
An overhead projector typically consists of a large box containing a very bright lamp and a fan to cool it. On top of the box is a large fresnel lens that collimates the light. Above the box, typically on a long arm, is a mirror and lens that focusses and redirects the light forward instead of up.
Transparencies are placed on top of the lens for display. The light from the lamp travels through 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 at the transparency as if writing, the audience looking forward at the screen. The height of the mirror can be adjusted, to both focus the image and to make the image larger or smaller depending on how close the projector is to the screen.
Focal-length adjustment 
Better-quality overhead projectors offer an adjustment wheel or screw on the body of the projector, to move the lamp towards or away from the fresnel lens. When the mirror above the lens is moved too high or too low, it moves out of the best focal distance for an evenly white image, resulting in a projected image with either blue or brown color fringing around the outside edge of the screen. Turning the adjustment wheel moves the lamp to correct the focal distance and restores the all-white projected image.
The lamp technology of an overhead projector is typically very simple compared to a modern LCD or DLP video projector. Most overheads use an extremely high-power halogen lamp that may consume up to 750 watts yet produces a fairly dim, yellowed image. A high-flow blower is required to keep the bulb from melting itself due to the heat output. Further, the intense heat usually causes the halogen lamp to fail quickly, often lasting less than 100 hours before failing and requiring replacement. A modern LCD or DLP uses an arc lamp which has a higher luminous efficacy and lasts for thousands of hours. A negative to LCD/DLP technology is the warm up time required for arc lamps.
Older overhead projectors used a tubular quartz lamp body containing the filament only, which mounted above a bowl-shaped polished reflector. However because the lamp was suspended above and outside the reflector, a large amount of light was cast to the sides inside the projector body that was wasted and required a very large lamp for sufficient screen illumination. More recent projectors use an integrated lamp and conical reflector assembly that allows the lamp to be located deep within the reflector so that more light is focused towards the fresnel lens, allowing for a lower-power lamp.
The most recent innovation for overhead projectors with integrated lamps/reflectors is the quick-swap dual-lamp control, allowing two lamps to be installed in the projector in movable sockets. If one lamp fails during a presentation, the presenter can merely move a lever to slide the spare into position and continue with the presentation, without needing to open the projection unit or waiting for the failed bulb to cool before replacing it.
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. 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. As the demand for projectors grew, Buhl Industries was founded in 1953, and became the leading US contributor for several optical refinements for the overhead projector and its projection lens. 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.
Use in education 
|This section does not cite any references or sources. (March 2010)|
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 his 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.
LCD overhead displays 
In the early 1980s–1990s, overhead projectors were used as part of a classroom computer display/projection system. 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.
The first of these LCD panels were monochrome-only, and could display NTSC video output such as from an Apple II computer or VCR. In the late 1980s color models became available, capable of "thousands" of colors (16-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.
Decline in use 
Overhead projectors were once a common fixture in most classrooms and business conference rooms, but today are slowly being replaced by document cameras, dedicated computer projection systems and interactive whiteboards. Such systems allow users to make animated, interactive presentations with movement and video, typically using software like Microsoft PowerPoint.
The primary reason 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 have fallen out of favor due to the limited resolution available and relatively dim, fuzzy image produced by the overhead.
The standards of users have also increased, 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 primarily due to the plastic fresnel lens, which can only approximate what would normally be an extremely large and heavy glass lens.
Video projectors utilize extremely small picture generation mechanisms, allowing for precision optics that far exceed the plastic fresnel lens' optical performance. They also include additional optics that eliminate the hotspot in the center of the screen directly above the light source, so that the brightness is uniform everywhere on the projection screen.
Critics feel that there are some downsides as these technologies are more prone to failure and have a much steeper learning curve for the user than a standard overhead projector. While a computer projection system eliminates the need to create hard copy transparencies (which can be quite expensive, particularly if made in color) of the slide show presentation, many presenters make both in case the computer hardware fails. Furthermore, the overhead projector allows a more direct interaction through live writing on the transparency.
See also 
|Wikimedia Commons has media related to: Overhead projectors|
- Opaque projector, a related type of the projector
- Projector (disambiguation)
- Slide projector
- Transparency (projection)
- 501 Ways to Use the Overhead Projector by Lee Green (ISBN 0-87287-339-0)