A film projector shines light through a strip of film and focuses the image onto a screen, while a mechanism pulls each frame into place, holds it still for an instant, then moves to the next. Done fast enough, it produces the illusion of motion we talked about earlier.
The Light Path
A bright lamp (historically a carbon arc, now usually a xenon bulb) sits behind the film. A reflector and condenser lenses gather its light and concentrate it on a single frame. The light passes through the transparent film, picking up the image, then goes through a projection lens that focuses and enlarges it onto the screen.
The Stop-and-go Problem
Here’s the tricky part: each frame has to be completely still while it’s being projected, but the film has to keep advancing. If the film moved continuously through the light, you’d see a smeared blur. So the projector moves the film in a jerky rhythm: pause, advance, pause, advance, 24 times per second.
The Intermittent Mechanism
Two main parts handle this:
- Claw mechanism: A small hook engages the perforations (sprocket holes) along the film’s edge, pulls the film down exactly one frame, then withdraws and resets.
- Geneva drive (Maltese cross): An alternative in many projectors. A rotating pin drives a cross-shaped wheel that turns in quarter-steps, converting smooth rotation into precise stop-and-go movement.
Sprocket wheels above and below this gate feed the film in and take it up smoothly. Loops of slack film on either side absorb the jerky motion so the reels themselves can turn steadily.
The Shutter
While the film is being pulled to the next frame, the light has to be blocked, or you’d see the movement as a vertical smear. A rotating shutter, a disc with cut-out sections, cuts off the light during each advance.
Most projectors use a two- or three-blade shutter, so each frame is flashed on screen two or three times before the next one arrives. At only 24 flashes per second, you’d perceive flicker. Flashing each frame twice gives 48 flashes per second, which is fast enough that the flicker disappears.
The Sound
Sound is read separately from the picture. Depending on the film format, a sound head reads an optical track printed along the film’s edge (a light sensor converts variations in the track into an electrical signal), or reads digital data printed between the sprocket holes, or syncs with a separate audio source. Because the sound reader sits at a different point along the film path than the picture gate, the audio is placed on the film slightly ahead or behind its matching image so everything lines up.
Digital Projection
Modern theaters mostly use digital projectors instead. These have no film or moving frames. A chip (such as a DLP chip with millions of tiny mirrors) or LCD panel forms each image directly from digital data, and light is bounced off or passed through it to the lens. The principle of projecting rapid successive images is the same, but the mechanical stop-and-go and shutter are gone.
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