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An overhead projector (OHP), like a film or slide projector, uses light to project an enlarged image on a screen, allowing the view of a small document or picture to be shared with a large audience.

In the overhead projector, the source of the image is a page-sized sheet of transparent plastic film (also known as 'foils') with the image to be projected either printed or hand-written/drawn. These 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 (hence, 'overhead'). They were widely used in education and business before the advent of video projectors.

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 (generally the size of a printed page), and the requirement that the transparency be placed face up (and readable to the presenter). 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 (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.

A related invention for enlarging transparent images is the solar camera, but a similar purpose for opaque materials is served by the epidiascope.

Condenser

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.[1] 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. The Fresnel lens is located at (or is part of) 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.[2] Without such a condenser at that point, most of the light would miss the focusing lens (or else the focusing lens would have to be very large and prohibitively expensive). 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 (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. This permits a range of projection distances.

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). 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 (Fresnel lens) 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.

Source of illumination

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.[3] 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.

Further, the intense heat accelerates failure of the high intensity lamp, often burning out in less than 100 hours, requiring replacement, which is often the mos

In the overhead projector, the source of the image is a page-sized sheet of transparent plastic film (also known as 'foils') with the image to be projected either printed or hand-written/drawn. These 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 (hence, 'overhead'). They were widely used in education and business before the advent of video projectors.

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 (generally the size of a printed page), and the requirement that the transparency be placed face up (and readable to the presenter). 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 (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.

A related invention for enlarging transparent images is the solar camera, but a similar purpose for opaque materials is served by the epidiascope.

Condenser

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.[1] 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. The Fresnel lens is located at (or is part of) 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.[2] Without such a condenser at that point, most of the light would miss the focusing lens (or else the focusing lens would have to be very large and prohibitively expensive). 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 (including the folding mirror) in order to adjust the object distance (optical distance between the slide and the lens) to fo

A related invention for enlarging transparent images is the solar camera, but a similar purpose for opaque materials is served by the epidiascope.

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.[1] 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. The Fresnel lens is located at (or is part of) 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.[2] Without such a condenser at that point, most of the light would miss the focusing lens (or else the focusing lens would have to be very large and prohibitively expensive). 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