In order for a film image to be reproduced, a light source is required to pass through the film. A film projector contains a powerful lamp element. These lamps produce so much heat, they will burn the film should it become stuck in one position for more than a second or two.
The lamp is extinguished by a mechanical device known as a douser. The douser does not remove power to the lamp, but rather places a physical barrier between the lamp and the film to avoid damage to the film should the film strip become stuck in front of the lamp.
The light produced by the lamp must be focused on a specific plane (the projection screen) for the film image to be viewable. Different lenses are required to produce different screen dimensions, whether the film shown is full or wide-screen.
A single image from the film must be held precisely in place for a fraction of a second in order to be exposed to the light from the lamp. The gate holds the individual film frame flat and in proper position for the light to pass through.
For the illusion of motion created by film to be perceived by the human eye, each individual frame must be held stationary for a fraction of a second and be exposed to a cycle of light, then dark, as the next frame is positioned for viewing. This cycle of light and dark is managed by a device called a shutter. Even though motion pictures are individual images, the human brain “fills in” the gaps of darkness through a perception characteristic known as “persistence of vision.” If the shutter speed is fast enough, you do not notice the flicker created by this cycle of light and darkness.
Feature-length films are spooled onto a large reel. The reel that threads the film into the projector is called a supply reel. The empty reel that the film spools onto is known as the take-up reel. Modern theatrical projectors may use a single reel system that integrate the supply and take-up reels into a single platter system, similar to the way 8-track audio tape cartridges once worked.