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A PARALLEL OP 16 AND 35 MM. FILMS 341
It is known that motion pictures could not exist if nature had not favored us with the phenomenon of persistence of vision.
Each single picture frame is successively brought in exact position at the projector aperture at a rate of speed of 16 frames per second for silent, and 24 frames per second for pictures synchronized with sound. A shutter intercepts the projection light during the periods of film motion and allows the passage of the light through the film onto the screen during the periods at which the film is stationary.
During the periods of light occultation, the screen image is still visible by the human eye through the faculty it has to retain image for a certain length of time.
It is obvious that the periods of occultation and visibility must be sufficiently rapid so that the projector's shutter uncovers a frame before the image of the preceding one has ceased to "persist" in the retina of the eye.
Experimentation has disclosed that forty-eight occultations and consequent exposures per second are necessary for the eye to have the impression of looking at a constantly illuminated screen.
Since only 16 or 24 picture frames are taken per second through the camera the necessary number of occultations can be reached by controlling the rotative motion of the shutter in such manner that it covers and uncovers each film image three times for speed of 16 and twice for 24 speed while the film is stationary at the gate and displace the film during one period of occultation.
Prior to sound pictures, when 35 millimeter pictures were photographed at a speed of 16 frames per second, the shutter of the professional projector was usually in the form of a disc with three blades. It was made to revolve at 16 revolutions per second and the film mechanism was so synchronized with its rotation that the film would be displaced at the aperture during the occultation period of one of the three blades.
Since sound pictures have standardized a photographic speed of 24 picture frames per second, shutters with only two blades are used rotating at a speed of twenty-four revolutions per second.
As opposed to the above, 16 millimeter projectors are usually equipped with shutters rotating at high speed. The film remains stationary during three complete revolutions of the shutter.
The flux of light transmitted for each frame depends upon the angular aperture of the shutter.
It is evident that the shorter the periods of occultation, the greater is the time during which the eye is perceiving the screen image, and shorter the time during which "persistence of vision" is dependent upon.
The drawing illustrates a complete cycle of the shutter of the Filmo projector which, as shown, has an angular aperture of 220°, which insures absolute absence of "flicker" even when the projector is operated at a speed much lower than 16 pictures per second.