Movie projector

A movie projector is an opto-mechanical device for displaying moving pictures by projecting them on a projection screen. Most of the optical and mechanical elements, except for the illumination and sound devices, are present in movie cameras.

Projection elements

As in a slide projector there are essential optical elements:

Light source

An incandescent lamp or an electric arc light produces light. The traditional carbon arc or modern xenon arc light source produces sufficient heat to burn the film should the film remain stationary for more than a fraction of a second. Xenons were introduced in the 1950s and are now the more common source, being easier and safer to maintain for the most part.

Reflector and condenser lens

A curved reflector redirects light that would otherwise be wasted toward the condensing lens.

A positive curvature lens concentrates the reflected and direct light toward the film gate.

Douser

(Also spelled dowser.)

A metal blade which cuts off light before it can get to the film - usually this is part of the lamphouse, and may be manually or automatically operated. Some projectors have a second, electrically-controlled douser that is used for changeovers (sometimes called a "changeover douser" or "changeover shutter"). Some projectors have a third, mechanically-controlled douser that automatically closes when the projector slows down (called a "fire shutter" or "fire douser"), to protect the film if a failsafe causes the motor to shutdown. Dousers protect the film when the lamp is on but the film is not moving, preventing the film from melting from prolonged exposure to the direct heat of the lamp. It also prevents the lens from scarring or cracking from excessive heat.