Technical description of the core section of a 35mm film projector

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Advances in technology have significantly enhanced theatrical motion picture presentation over the past 120 years, culminating most recently in the switch from film to digital projectors. It may come as a surprise, then, that a time-traveling projectionist from the beginning of the 20th century would not only recognize, but probably quickly learn to operate a modern 35mm film projector.

The core of a contemporary film projector still operates on exactly the same principles as the earliest models designed by the Lumiere Brothers, Edison and others in the late 19th century.

Some things have changed.

• Individual 20-minute reels (approximately 2,000 feet of film) can now be spliced together and run continuously through a single projector.
• Analog and digital optical sound readers have been added.
• Carbon-arc lamps have been replaced by cleaner, more efficient Xenon bulbs.
• And hand-cranks have been exchanged for electric power.

But the motion picture itself is still cast upon the screen in the same way it was when moving black and white images first awed crowds over a century ago.

At the most fundamental level, film travels from the top of the projector to the bottom, facilitated by spinning wheels with sprockets on each end that fit into holes on the edges of the film. The frames containing the pictures pass in front of a light source, which projects the images through a lens onto the screen.

The light source, which is housed in a protective case behind the projector, shines through an aperture plate, which frames the image so the soundtrack and edges of the film are not visible on the screen. In order to keep the film taut and prevent image distortion, the film is clamped into a metal trap as it passes the aperture plate.

During early experiments with motion pictures, inventors quickly discovered that persistence of vision only works if each image pauses for a moment before continuing to the next; continuous movement is perceived as a blur on the screen. Thus, all of the sprocket wheels on a projector spin continuously except one.

In order to allow the audience to view each frame long enough for the optical input to reach the brain, the sprocket wheel directly below the trap, which pulls each frame into place, does not turn continuously like the others. This is the intermittent sprocket, and as its name implies, it spins intermittently, turning to advance the film, then stopping briefly to allow viewers’ eyes to process the projected image before the next frame is moved into place.

Because a blur appears when film is in motion, a shutter behind the aperture plate is carefully timed to allow light to pass through only when the image is static. When the intermittent sprocket is advancing the frame, the shutter blocks the light. Although it happens so quickly that they do not realize it, moviegoers are technically looking at a dark screen every time the intermittent sprocket is in motion.

Early projectors were hand cranked, which meant the speed at which the images moved could vary depending upon the projectionist’s stamina and whim. When electricity took over as a power source, it was discovered that the optimal length of time for one’s eyes to process an image before moving to the next one was 1/24 of a second. Thus, all modern 35mm projectors run at a speed of 24 frames per second, which allows the image on each frame to pause on the screen long enough for the audience to see it, but still perceive a continuous moving picture.

While the equipment surrounding it has changed dramatically, the 35mm projector itself has consistently delivered moving pictures to audiences worldwide for over 100 years with no significant adaptations. As digital cinema revolutionizes exhibition practices, this relatively simple piece of machinery retires with a clean record and a history of remarkable service.

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