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312 A. S. HOWEJLL AND J. A. DUBRAY [J. S. M. P. B
that an excessive width of perforation would tend to dangerously weaken the film.
The above considerations lead to the conclusion that a critical perforation width can be determined with sufficient accuracy to assure satisfactory film registration and shrinkage accommodation with a minimum of film wear.
With reference to film registration, the shape of the perforation plays an extremely important part. In consideration of the fact that a change in film dimensional standards demands alteration and rebuilding of all machinery used by the motion picture industry an improvement in the shape of film perforation can be considered without fear of encountering too many difficulties in its application.
Commercial reasons have imposed upon the industry the present practice of two different sizes and shapes of perforation, one known as the Bell & Howell Standard, for negative films; and the other as the rectangular, for positive films; the first having a height of 0.073 in. and the second a height of 0.078 in. for an equal width of 0.1 10 in.
Such reasons disappear with the creation of a new film standard width, and it seems reasonable to conclude that only one pitch size and shape of perforation should be determined for both negative and positive films.
This question has been discussed in papers previously presented to this Society and it has been conceded that the rectangular shape of perforation, with rounded corners, is the one most adaptable and that it presents the greatest opportunity of accurate film registration, especially inasmuch as film control can be established from the film perforation instead of from the film edge. This procedure compensates whatever possible errors may be encountered in the different processes of film manipulation, for it is possible to register the film entirely from the perforation in both transverse and longitudinal control.
Film control from the perforation also materially reduces the width of the surface to be registered and, by nearing the ideal condition of perfect registration, reduces to a minimum the necessary allowances for shrinkage accommodation.
At C in Fig. 1, the dimensions of 70 mm. unshrunk film are shown, as well as an enlarged view of the rectangular perforation and its dimensions as accepted in the present practice, 0.080 by 0.130 in.