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30 PRINCIPLES OF CINEMATOGRAPHY
The British Kinematograph Society has been very energetic in co-operating with the Regent Street Polytechnic to offer a course of instruction in cinematograph technique. This course is very comprehensive and, until recently, was the only one of its kind in the country.
The Manufacture of Motion Picture Film
There are five main operations necessary to produce a roll of
cinematograph film, these are: (1) Preparing the emulsions,
(2) Coating the emulsions onto the support, (3) Drying the film,
(4) slitting the film to the required width, (5) perforating the final
roll.
The preparation of the emulsion is essentially a chemical operation of exceptionally high standard and is carried out under very exacting conditions. Although the types of emulsions used throughout the motion picture industry will be described when consideration is given to sensitometry and film processing, the actual chemistry of photographic emulsion making does not come within the scope of this work.
However, brief mention may be made of the manner in which the emulsion is applied to the film base. Rolls of film base are usually 40-inches (101.6-cms) in width and anything up to 2,000feet (609.6-metres) in length and are coated while still at these dimensions. Figure 5 shows in outline the basic requirements of a film coating machine. The roll of uncoated film base is mounted in a position as shown at 'A' and the leading edge of the roll is threaded under a coating roller 'B', which is then lowered into a trough of warm emulsion as shown at *C\ The film then passes vertically upwards and over a chilled roller 'D' by which the emulsion is 'set'. From this point the film is passed to the first of a series of rollers shown at 4E' which are continually moving very slowly down a long drying tunnel and in a direction away from the coating head.
The thickness of the emulsion will depend, amongst other considerations, upon the speed at which the film passes through the emulsion trough, and this must be relatively fast. On the other hand, it is necessary to allow the film to dry out very thoroughly and exceptionally slowly. Because of this the mechanism which carries the coated film down the drying tunnel must move at a slower speed than that which carries it through the coating head. It is therefore obvious that some intermediate mechanism will be required to overcome this speed difference.
Of the several mechanisms capable of performing this operation,