Motion Pictures

At the end of the process, every motion picture goes through a projector. From the advent of sound until the mid-1970s projectors changed little. The system a projector uses for moving the film is similar to that used by a motion picture camera. A pull-down mechanism moves the film through the projector, while a rotating shutter only emits light while a frame is in position. The primary problem in making a projector was to provide a light source bright enough to enlarge a film frame enough to fill a theater screen—as much as 300,000 times—yet small enough to fit inside a projector.

The solution found to this problem, the carbon arc lamp, was used until the 1970s. These lamps used two carbon rods with a small gap between them. A strong electric current jumped the gap, creating a strong white light. These lamps needed constant adjustment, however, as well as a ventilation system. Most were replaced in the 1970s and 1980s by lamps using the inert gas xenon.

The projector also reads the soundtrack through a separate reader placed immediately after the lens. The soundtrack can be a magnetic strip or light pattern that runs along the side of the film. This light pattern, called optical sound, was the only system of sound reproduction until advances in magnetic tape recording in the 1950s. While soundtracks are now recorded and edited with magnetic tape or digitally, optical sound is often used on motion pictures because it can be printed right along with the images, and because so many theaters only have optical-sound equipment.

To record optical sound, sound waves are translated into electrical impulses, which in turn control a light beam that creates a photograph on a piece of film. When the optical soundtrack is played back, it passes before a lamp that projects its patterns onto a photoelectric cell. These intensities of light are converted into electrical impulses, then sound, which is amplified and sent to the speakers.