​FILM PROJECTOR
Yuxuan Xie
Film came in many sizes from the giant 70mm-popular in the 1960s for epics like Lawrence of Arabia- to 35mm used for most feature films, to 16mm for schools, and even 8mm used by home enthusiasts.
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This article would show the mechanisms of a film projector by the example of common 16mm projectors.
We’ll look at the shuttle that starts and stops the film, the shutter that strategically blocks light and the photo sensor that reads the sound – all of which operate in harmony.
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To create the illusion of movement, a series of still images – the film – is pulled off the supply reel, threaded in between the lamp and lens so the image can be projected, then run across the sound drum, and finally coiled onto the take-up reel.
First, the shuttle. The shuttle has three teeth which engage the sprocket holes in the film. The shuttle moves back to disengage from the film, then moves up, then forward to engage the film, then moves down pulling the film with it. The film is stationary most of the time and only moves when the shuttle is moving down. This is the intermittent motion of the film necessary to avoid blurring of the projected image.
The blur is eliminated by a shutter. The shutters had a single blade that covered the advancement of the film with an open section that showed the picture.The shutter rotates once every frame and is synced so that the shutter blade blocks light from the lamp while the shuttle is advancing the film. This prevents the projection of film motion on the screen.
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The film passes by the lamp at 24 frames per second. At that rate the human mind blends the still frames into fluid motion. This switching between a bright projected image and darkness is called flicker. If the flicker occurs at about sixty to seventy times per second, the bring flashes fuse together and appear-to the human eye-continuously bright with no periods of darkness. This rate is called the flicker fusion threshold. Modern shutters have three blades. The frame rate stays at 24 frames per second and the flicker rate increases to 72 flickers per second.
Sound in movies is recorded optically on the edge of the film. After the film runs past the lamp, it runs across the sound drum. To read this optical soundtrack, light shines through a tube with a slit. This concentrates the light on a small section of the film’s soundtrack. A photo sensor on the other side of the film measures the amount of light passing through the film at a given time. The photo sensor converts the amount of light transmitted into current and this current drive the speakers. Therefore, the sound of the film could be played.
