Projector

Projector is a device used to show pictures on a screen. One common type is a machine for showing photographic slides. The simplest projector consists of (1) a light, (2) a reflector that focuses light rays, (3) a condensing (focusing) lens, and (4) a projection lens.

A powerful light is needed to show pictures on a screen. A projector uses an incandescent bulb that glows with heat. Some projectors use bulbs as strong as 1,000 watts. The reflector, located behind the bulb, is a concave (inward curving) mirror. Light rays that strike it are reflected back through the bulb. These rays and rays coming directly from the bulb then enter a thick plano-convex lens that is flat on one side and outward curving on the other. The flat side of this lens faces the bulb. Some projectors use a double condenser, which consists of two condensing lenses. The light rays entering the condensing lens are bent inward. The rays then pass through a photographic slide that is placed upside down between the focusing and projection lenses. The projection lens turns the image right side up and enlarges it.

Some kinds of projectors can be used with both filmstrips and slides. Overhead projectors project pictures above and behind the operator, who can face the audience and use the projector at the same time. Some projectors can project drawings, maps, pages of books, and other material that is opaque--that is, neither transparent nor on transparent film. These projectors are called opaque projectors.

Contributor: Robert A. Sobieszek, M.Phil., Curator, Los Angeles County Museum of Art.

Projector

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