How Movie Projectors Work

Moving the Film

Once a projectionist splices the film and loads it on the feed platter, he threads the film through the platter's payout assembly and into the top of the projector. A strip of film has small square holes along each side called sprocket holes. These holes fit over the teeth of special gear-like wheels called sprockets. The sprockets, driven by an electric motor, pull the film through the projector. Cambers, small spring-loaded rollers, provide tension to keep the film from bunching up or slipping off the sprockets.

The film needs to advance one frame, pause for a fraction of a second and then advance to the next frame. This is accomplished using one of two mechanisms. The first one uses a small lever known as the claw, which is mounted on a bar next to the film's path. The claw is connected to the outer edge of a wheel that acts as the crank. The circular motion of the crank makes the claw lift up and out to come out of a sprocket hole and then down and in to catch onto another sprocket hole. This causes the film to advance one frame. The speed of the sprockets is closely synchronized with the lever action of the claw to make sure that the claw is consistently advancing the film at a rate of 24 frames per second.

The second type uses another sprocket wheel mounted just below the aperture gate. This intermittent sprocket rotates just far enough to pull the film down one frame, pauses and then rotates again. Intermittent sprockets provide more reliable performance and do not wear out the sprocket holes as quickly as the claw.

The film is stretched over a couple of bars as it passes in front of the lens. The bars serve to keep the film tight and properly aligned. Depending on the projector's configuration and the sound format used, the film will pass through an optical audio decoder mounted before or after the lens assembly. For digital sound, the film will travel through a special digital decoder attached to the top of the projector. As the film leaves the projector (or the digital-audio decoder), it is carried on a series of rollers back to the platter's payout assembly and spooled to a take-up platter.