British Kinematography (1950)

December, i 950 cricks : engineering aspects of film production 201 Since film is a dimensionally unstable material, and its liability to shrinkage is particularly marked in the lateral direction, it is the general practice to arrange for one register pin to fit the perforation in both directions, and the second pin to fit only vertically. Perforation Standards. Since the film perforations play such an important part in ensuring steadiness of the projected picture, one would have thought that their form and dimensions would have been closely standardised. Strange to say, two different forms of perforations are in current use, and a third is projected. That known as the negative perforation (A in Fig. 2) is based upon the original design of Edison, while the positive perforation B, designed by Kodak, is rectangular with radiused corners. Any hope of interchangeability in a register-pin mechanism is negatived by the fact that the former is 0.006 inch higher than the latter. Thus, all negative stocks and laboratory films are made with the original negative perforation, whilst positive release films — which are printed in a continuous machine in which the films are driven by a continuously rotating sprocket — have the rectangular perforation, which provides greater resistance to wear. The proposed third perforation form, known as the Howell & Dubray (C in Fig. 2), is an attempt to reconcile the differences between the two earlier types of perforation.12 The reduced height as compared with the original positive perforation is made possible by the lower shrinkage of modern film stocks. It is not yet an accepted standard. Wear-resisting Requirements. Film has a considerable abrasive action upon even the hardest of metals. In the case of the kinema projector this is hardly surprising, seeing that in an average day's programme well over half a million frames of film will be projected. Even in the studio camera exposing only a few hundred feet of film a day, the effect cannot be neglected. . Monel metal, hard grades of stainless steel, and hard chromium plating, are commonly used for parts subject to film wear, notably the gate components.13 Experiments are now in hand in the use of synthetic sapphire inserts for such components — a principle which Newman introduced many years ago.14 A factor which may have undesirable effects is that new film stock has a slight excess of free acid, which in course of time etches polished surfaces ; on polished pressure pads, for example, the shape of the perforations of the temporarily stationary film becomes visible. Yet another factor associated especially with high-speed cameras is the discharge of static electricity from the rapidly moving film ; it has been found that Monel is particularly resistant to pitting. CONCLUSION. It has been possible in this brief paper to deal only superficially with a very small proportion of the field of kinematograph engineering. The aspects chosen have been those most likely to be of practical interest to those engaged in other branches of engineering. One final point should be stressed. Ten years ago, most of the equipment in British studios was of American origin. To-day, the bulk of studio equipment, and almost the whole of kinema equipment, is British ; all the equipment here described is of British construction, and, with one or two exceptions, of British design. It is particularly gratifying that this country, which, American claims notwithstanding, can be credited with the invention of kinematography, should now be producing equipment at least the equal of, and in some respects superior to any in the world.