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16-MM SOUND-FILM PROJECTION 395
bent to any required shape in boiling water and, therefore, a 'pipe' of material is formed to follow the shortest convenient path from the scanning point to the photo-cell.
If a photo-cell is placed directly behind a small aperture some known distance from a light source and, in this position, the light transmission is considered to be 100 %, the introduction of a parallel plate of Perspex, 0-375-inch (9-252-mm) in thickness, between the aperture and the photo-cell, will only reduce the transmission to 90%. If a similar rod of Perspex 10-inches (25-4-cms) in length is placed between the aperture and the photo-cell, the transmission will only fall to 65%. When this same 10-inch (25-4-cms) rod is then bent to a radius of 1 • 0-inch (25 • 4-mm), and is so turned through a right-angle at a point mid-way along its length, the transmission is reduced from 65% to 57-5%. It is therefore possible to bend a rod of Perspex or similar material to any required shape so that it will fit around the shaft and bearings which support the sound scanning drum. Such a rod may then be introduced into the small cavity between the film and the drum support and at the point where the light beam passes through the film.
Since the final transmission through such a bent rod will increase as the length of the rod is reduced, the number of bends is kept to a minimum and the degree of bending is as low as possible. The light beam has only to be introduced into the rod to ensure that it will be carried to the photo-cell, so that it is not necessary for the rod to be mounted with the same precision as would be necessary if mirrors or prisms were used for this purpose.
General Requirements
In concluding this short description of the main features in the design of 16-mm sound projection mechanisms, the following points are collected together to form a basis of the more general requirements which should be satisfied by all such instruments. (1) The mechanism used to move the film through the picture gate should perform this operation at a rate of 24-pictures per second and to within 0-75 % of this speed it should also be capable of transporting films which contain two consecutive damaged perforations. (2) All sprocket wheels, rollers and guiding shoes used to move or guide the film through the entire machine should be so designed that neither the picture or sound track areas of the film come into contact with stationary parts which are likely to cause scratching after they become worn. (3) Any projector should be fitted with a power operated film rewinding mechanism capable of rewinding a full 1,600-ft (487-7-metres) reel of film within