Overhead projector

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An overhead projector (OHP), like a film or slide projector, uses light to project an enlarged image on a screen.wikipedia
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Projection screen

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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.
Different markets exist for screens targeted for use with digital projectors, movie projectors, overhead projectors and slide projectors, although the basic idea for each of them is very much the same: front projection screens work on diffusely reflecting the light projected on to them, whereas back projection screens work by diffusely transmitting the light through them.

Slide projector

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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. An overhead projector (OHP), like a film or slide projector, uses light to project an enlarged image on a screen.

LCD projector

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The lamp technology of an overhead projector is typically very simple compared to a modern LCD or DLP video projector.
It is a modern equivalent of the slide projector or overhead projector.

Projector

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Some ancient projectors like the magic lantern can be regarded as predecessors of the overhead projector.
Video projectors are digital replacements for earlier types of projectors such as slide projectors and overhead projectors.

Transparency (projection)

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However some differences are necessitated by the much larger size of the transparencies used (generally the size of a printed page), and the requirement that the transparency be placed face up (and readable to the presenter).
These are then placed on an overhead projector for display to an audience.

Fresnel lens

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Since this requires a large optical lens (at least the size of the transparency) but may be of poor optical quality (since the sharpness of the image does not depend on it), a Fresnel lens is employed.
Cheap Fresnel lenses can be stamped or molded of transparent plastic and are used in overhead projectors and projection televisions.

Document camera

document camerasVisualiser (education)visualizer
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.
Document cameras replaced epidiascopes and overhead projectors, which were formerly used for this purpose.

Slide show

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Slide shows are sometimes still conducted by a presenter using an apparatus such as a carousel slide projector or an overhead projector, but now the use of an electronic video display device and a computer running presentation software is typical.

Microsoft PowerPoint

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Such systems allow the presenter to project video directly from a computer file, typically produced using software such as Microsoft PowerPoint and LibreOffice.
The earliest version of PowerPoint (1987 for Macintosh) could be used to print black and white pages to be photocopied onto sheets of transparent film for projection from overhead projectors, and to print speaker's notes and audience handouts; the next version (1988 for Macintosh, 1990 for Windows) was extended to also produce color 35mm slides by communicating a file over a modem to a Genigraphics imaging center with slides returned by overnight delivery for projection from slide projectors.

Opaque projector

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Because they must project the reflected light, opaque projectors require brighter bulbs and larger lenses than overhead projectors.

Real image

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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.

Mirror image

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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.

Movie projector

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Therefore, the transparency is placed face up (toward the mirror and focusing lens), in contrast with a 35mm slide projector or film projector (which lack such a mirror) where the slide's image is non-reversed on the side opposite the focusing lens.

Belshazzar's feast

the writing on the wallDaniel 5writing on the wall
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 (optics)

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Because the focusing lens (typically less than 10 cm [4 in] in diameter) is much smaller than the transparency, a crucial role is played by the optical condenser which illuminates the transparency.

Forced convection

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In order to provide sufficient light on the screen, a high intensity bulb is used which often requires fan cooling.

Thin lens

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Overhead projectors normally include a manual focusing mechanism which raises and lowers the position of the focusing lens (including the folding mirror) in order 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.

Focal length

effective focal lengthfocal distancelimiting the focal range of the lens
Overhead projectors normally include a manual focusing mechanism which raises and lowers the position of the focusing lens (including the folding mirror) in order 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.

Magnification

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Increasing (or decreasing) the projection distance increases (or decreases) the focusing system's magnification in order to fit the projection screen in use (or sometimes just to accommodate the room setup).

Digital Light Processing

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The lamp technology of an overhead projector is typically very simple compared to a modern LCD or DLP video projector.

Arc lamp

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In contrast, a modern LCD or DLP projector uses an arc lamp which has a higher luminous efficacy and lasts for thousands of hours.

Luminous efficacy

efficacyefficientLuminous efficiency
In contrast, a modern LCD or DLP projector uses an arc lamp which has a higher luminous efficacy and lasts for thousands of hours.

Magic lantern

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Some ancient projectors like the magic lantern can be regarded as predecessors of the overhead projector.

Athanasius Kircher

KircherMusurgia universalisKircher, Athanasius
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.