Alexei Volkov
Plastic is a synthetic material composed primarily of polymers, which are chains of carbon atoms. Plastics are typically solid, durable, flexible, and low-cost, making them useful for a wide range of applications. One such application is in projection technology, where transparent plastic sheets, also known as viewfoils or transparencies, are used in overhead projectors to display printed or handwritten images. These projectors, widely used in education and business before the advent of video projectors, allowed for the projection of enlarged images onto a screen, facilitating better communication between teachers and students. The term plastic projection thing may also refer to a specific part in 3D printing called a belt tensioner, which controls belt spacing.
| Characteristics | Values |
|---|---|
| Name | Overhead Projector (OHP) |
| Other Names | Viewfoil Projector, Film Projector, Slide Projector, Spatial Light Modulator (SLM) |
| Function | Used to display material printed or written on transparent plastic sheets |
| Sheet Material | Transparent plastic film, also known as "viewfoils", "foils" or "transparencies" |
| Sheet Size | Generally the size of a printed page, up to 10 x 10 inches |
| Sheet Placement | Placed on the glass platen of the projector |
| Light Source | Light source is below the glass platen |
| Projection | Projects light from an illuminated slide onto a projection screen |
| Use Case | Widely used in education and business before the advent of video projectors |
| Advantages | Allows excellent screen visibility without darkening a room, enables the instructor to face the class, and provides a comfortable writing height |
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Overhead projectors
An overhead projector (OHP) uses light to project an enlarged image on a screen, allowing a small document or picture to be shared with a large audience. The source of the image is a page-sized sheet of transparent plastic film (also known as "viewfoils", "foils", or "transparencies") with the image to be projected either printed or handwritten/drawn. 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. Overhead projectors were widely used in education and business before the advent of video projectors.
The manual focusing mechanism of an overhead projector raises and lowers the position of the focusing lens (including the folding mirror) to adjust the object distance (optical distance between the slide and the lens) to focus at the chosen image distance (distance to the projection screen) given the fixed focal length of the focusing lens. This permits a range of projection distances. Increasing (or decreasing) the projection distance increases (or decreases) the focusing system's magnification to fit the projection screen in use or to accommodate the room setup.
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 or 1000 watts. A high-flow blower is required to keep the bulb from melting due to the heat generated, and this blower is often on a timer that keeps it running for a period after the light is extinguished. The intense heat accelerates the failure of the high-intensity lamp, which often burns out in less than 100 hours and requires replacement, which is often the most expensive part of owning a projector. In contrast, a modern LCD or DLP projector often uses an ultra-high-performance lamp that has a higher luminous efficacy and lasts for thousands of hours.
Some early-modern projectors, such as the magic lantern and steganographic mirror, can be regarded as predecessors of the overhead projector. 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, intended for long-distance communication.
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Slide projectors
A slide projector is a device used to display images or information onto a screen or other surface. It works by passing light through a transparent material, such as plastic or glass, with the image projected onto a wall or screen. Slide projectors were commonly used in classrooms and for business presentations before the widespread adoption of digital projectors and presentation software.
The traditional slide projector uses a light source, typically a halogen bulb, which shines through a small image or frame of film, magnifying the image and projecting it onto a larger surface. The slides themselves are often made from transparent plastic sheets, historically cellulose acetate, but more recently polyester PET film.
In recent years, digital alternatives have largely replaced traditional slide projectors. Digital projectors, such as LCD and DLP projectors, offer benefits such as higher resolution, easier image manipulation, and the ability to project directly from a computer or other digital source. However, slide projectors are still used in certain applications, like the display of vintage photographs or in artistic installations.
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Transparency film
In addition to polyester PET film, there are other types of transparency films available, such as those designed for use with inkjet printers. Inkjet transparency films are often waterproof and have a microporous top coating that allows for high-quality prints. They can be used with dye or pigment inks and are suitable for various applications, including screen printing, pad printing, and offset negatives.
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Viewfoils
The plastic projection thing is likely referring to a "viewfoil", also known as a "foil" or "transparency". These are page-sized sheets of transparent plastic film that are placed on the glass platen of an overhead projector, which has a light source below and a projecting mirror and lens assembly above it. The image to be projected is either printed or handwritten/drawn on the viewfoil.
Overhead projectors work on the same principle as slide projectors, projecting light from an illuminated slide onto a projection screen. However, they utilise larger transparencies, generally the size of a printed page, and require the transparency to be placed face up. The substitution of plastic for the heavier glass condenser in overhead projectors allowed for larger transparencies of up to 10 x 10 inches to be used.
The development of transparency film and overhead projectors is attributed to Roger Appeldorn of 3M in the late 1950s. The first major client was the Strategic Air Command base in Omaha, which used approximately 20,000 sheets per month. Overhead projectors gained popularity in education due to their low cost and interactive capabilities, with high sales stimulated by the Federal Aid to Education program in 1957.
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Fresnel lens
A Fresnel lens is a composite compact lens that reduces the amount of material required compared to a conventional lens. This is achieved by dividing the lens into a set of concentric annular sections. The lens was first proposed by Georges-Louis Leclerc, Comte de Buffon, and later reinvented by French physicist Augustin-Jean Fresnel (1788–1827) for use in lighthouses.
Fresnel's design, completed in 1822, used thin bull's-eye-shaped panels, which refracted light both horizontally and vertically, producing a stronger beam of light. This represented a significant advancement in lighthouse lighting technology and maritime safety. Hundreds of pieces of specially cut glass surround a lamp bulb in a Fresnel lens, intensifying the light and focusing rays that would normally scatter into a single, powerful beam.
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Frequently asked questions
It is called a belt tensioner.
The plastic projection thing in overhead projectors is called a transparency film, or simply "transparency", "viewfoil", "foil" or "viewfoil".
The world's first fully synthetic plastic was Bakelite, invented in New York in 1907 by Leo Baekeland, who coined the term "plastics".
