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A system of mirrors, prisms and/or imaging lenses is used to focus an image of the material onto a viewing screen. Because they must project the reflected light, opaque projectors require brighter bulbs and larger lenses than overhead projectors. Care must be taken that the materials are not damaged by the heat generated by the light source.
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 .
The mirrors can be individually rotated ±10-12°, to an on or off state. In the on state, light from the projector bulb is reflected into the lens making the pixel appear bright on the screen. In the off state, the light is directed elsewhere (usually onto a heatsink), making the pixel appear dark.
The two surfaces of the reflector have different radii to correct the aberration of the spherical mirror. Light passes through the glass twice, making the overall system act like a triplet lens. [3] Mangin mirrors were used in searchlights, where they produced a nearly true parallel beam.
The collimator lens has the lower focal length and is placed closer to the light source, and the collector lens, which focuses the light into the triplet lens, is placed after the projection image (an active matrix LCD panel in LCD projectors). Fresnel lenses are also used as collimators in overhead projectors.
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The distance is not the same as from the object to the lenses. Real images may also be inspected by a second lens or lens system. This is the mechanism used by telescopes, binoculars and light microscopes. The objective lens gathers the light from the object and projects a real image within the structure of the optical instrument.
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