Basic principles of projection

Part two : Film transport

[Projector] The largest challenge in projection of a 35mm film is the requirement to fix the image during the exposure phase and achieve a very smooth unified velocity during the reading of the optical sound track.
These two requirements are almost contradictory.
On the left you see a schematic representation of a projector.

Film from the upper reel is taken by the upper take-up sprocket with a continuous velocity. In order to get a fixed image during the projection a special sprocket transports the film shock-wise with 24 frames per second through the film gate. A shutter, synchronized with the sprocket takes care for the exposure of the filmframe, at the moment the frame is not transported through the film gate. From there on the film is brought back to a unanimous speed. The sound needs to be read from the film at an unified velocity. There are only 18 frames between the exposure of the film to the projection light beam and the place where the sound is read. (the so called sound head is not included in this schematic) Also for the take up winding it is mandatory to have the machine running at an uniform speed.

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