Overhead projector


An overhead projector , like a film or slide projector, uses light to project an enlarged image on a screen. In the overhead projector, the source of the image is a page-sized sheet of transparent plastic film with the image to be projected either printed or hand-written/drawn. These are placed on the glass surface of the projector, which has a light source below it and a projecting mirror and lens assembly above it. They were widely used in education and business before the advent of computer-based projection.

Optical system

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, and the requirement that the transparency be placed face up. For the latter purpose, the projector includes a mirror just before or after the focusing lens to fold the optical system toward the horizontal. That mirror also accomplishes a reversal of the image in order that the image projected onto the screen corresponds to that of the slide as seen by the presenter looking down at it, rather than a mirror image thereof. Therefore, the transparency is placed face up, in contrast with a 35mm slide projector or film projector where the slide's image is non-reversed on the side opposite the focusing lens.
The device has sometimes been called a "Belshazzar", after Belshazzar's feast: "In the same hour came forth fingers of a man's hand, and wrote over against the candlestick upon the plaister of the wall of the king's palace: and the king saw the part of the hand that wrote".

Condenser

Because the focusing lens is much smaller than the transparency, a crucial role is played by the optical condenser which illuminates the transparency. Since this requires a large optical lens but may be of poor optical quality, a Fresnel lens is employed. The Fresnel lens is located at the glass plate on which the transparency is placed, and serves to redirect most of the light hitting it into a converging cone toward the focusing lens. Without such a condenser at that point, most of the light would miss the focusing lens. Additionally, mirrors or other condensing elements below the Fresnel lens serve to increase the portion of the light bulb's output which reaches the Fresnel lens in the first place. In order to provide sufficient light on the screen, a high intensity bulb is used which often requires fan cooling.

Focus adjustment

Overhead projectors normally include a manual focusing mechanism which raises and lowers the position of the focusing lens in order to adjust the object distance to focus at the chosen image distance given the fixed focal length of the focusing lens. This permits a range of projection distances.
Increasing the projection distance increases the focusing system's magnification in order to fit the projection screen in use. Increasing the projection distance also means that the same amount of light is spread over a larger screen, resulting in a dimmer image. With a change in the projection distance, the focusing must be readjusted for a sharp image. However, the condensing optics is optimized for one particular vertical position of the lens, corresponding to one projection distance. Therefore, when it is focused for a greatly different projection distance, part of the light cone projected by the Fresnel lens towards the focusing lens misses that lens. This has the greatest effect towards the outer edges of the projected image, so that one typically sees either blue or brown fringing at the edge of the screen when the focus is towards an extreme. 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.

History

Some ancient projectors like the magic lantern can be regarded as predecessors of the overhead projector. The steganographic mirror possibly came closest to how the overhead projector was used.
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. In 1654 Belgian Jesuit mathematician André Tacquet used Kircher's technique to show the journey from China to Belgium of Italian Jesuit missionary Martino Martini. It is unknown how exactly Tacquet used Kircher's system, but it is imaginable that he drew pictures on the projecting mirror while details of the journey were explained.
French physicist Edmond Becquerel developed the first known overhead projection apparatus in 1853. It was demonstrated by French instrument maker and inventor Jules Duboscq in 1866.
An overhead projector designed by American scientist Henry Morton was marketed around 1880 as a "vertical lantern".
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. After the war they were used at schools like the U.S. Military Academy.
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 1953, 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 1950s and early 1960s. In the early 60s, carousel slide projectors with a horizontally mounted tray was released by Kodak.
In the late 1950s 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 20,000 sheets per month. 3M then decided to develop their own overhead projector instead of the one they had been selling until then, which was produced by an outside manufacturer. It took several prototypes before a cost-effective, small and foldable version could be presented on January 15, 1962. It had a new fresnel lens made with a structured-surface plastic, much better than other plastic lenses and much cheaper than glass.
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

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.

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, 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 in the United States, but in 2000s they were slowly being replaced by document cameras, dedicated computer projection systems and interactive whiteboards. Such systems allow the presenter to project video directly from a computer file, typically produced using software such as Microsoft PowerPoint and LibreOffice. 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 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, 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. Video projectors use 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.