International projectionist (Jan-Dec 1945)

VOLUME XX NOVEMBER 1945 NUMBER 11 THE PROJECTION LIFE OF FILM By R. H. TALBOT EASTMAN KODAK COMPANY THERE is no simple answer to the question "How many times can film be run through a projector?" It has been demonstrated many times that whereas one sample of film may become completely unserviceable after relatively few projections, another sample from the same roll of film may still he in serviceable condition after several thousand projections, depending on the conditions of operation. It is true that there are certain qualities of the film stock itself which influence its wear life. Much could be wrritten on the relation of the physical properties of the film (tensile strength, tear resistance, brittleness, etc. ) to its durability. This relation, however, is one with which the film manufacturer is primarily concerned. The user of film is more concerned with those factors which greatly influence the wear life of the film he has at hand. This article deals with some of these factors. If motion picture film were as dimensionally stable as steel, there would be no problem of the relative pitches of film perforations and sprocket teeth; the 2 pitches would always be equal and. consequently, several sprocket teeth would be in contact with the edges of the perforations at the same time. Actually, the pitch of the film perforations is changing more or less continually, owing to the loss of volatile materials and t .'. Sec. .1/..:. Pic. Eng., Auu. 1945. also to the dimensional changes which accompany variations in moisture content of the film. What then should be the pitch of the intermittent sprocket teeth? The projector manufacturer has several choices in the design of the intermittent sprocket. He may make the pitch of the teeth such that there will be almost perfect mesh between film and sprocket when the film is freshly processed and less and less close mesh as the film shrinks, or he may make the pitch of the teeth such that there will be closer and closer mesh as the film ages. He may even choose a compromise pitch so that the film fits the sprocket best after the film has shrunk a portion of the total amount. In order to decide which course is the best to follow from the standpoint of him wear, it might be of interest to review the behavior or movement of film on a sprocket in each of 2 cases. Case I I he pitch of the film perforations is greater than that of the sprocket teeth, and in Case II. the pitch of the film perforations is less than that of the sprocket teeth. Figure 1 shows the behavior or move ment ol film on a sprocket when the pitch of the film is greater than that of the sprocket. Note that the entering tooth strikes the edge of the perforation and shifts the film forward, the film traveling faster than the surface of the -procket. This wedging-on of the film on the sprocket teeth produces a severe strain on and eventually a tearing of the perforations, and decreases the life of the -film, This is: the .condition whie4i exists today with the 0.935-in. diameter intermittent sprocket which was designed for films of much higher shrinkage than those in use at the present time. Figure 2 shows the action of film on a sprocket when the pitch of the film is less than that of the sprocket. Here, the tooth that i about to leave the film does the driving, and the entering tooth engages the film without touching the edge of the perforation. The film is well seated at the base of the tooth when it makes contact with the driving face. Theoretically, this condition should cause less wear on the film than the previous case, in which the pitches of the film and the sprocket are reversed. Actually, experiments show that lower wear may occur on either side of perfect With the present scarcity of rate print stock, there is need for procedures which ivilt substantially increase the useful life of the film on hand. Prints are rendered unserviceable by damage to the perforations, by mutilation of the edges in shipment, and by abrasion of the surfaces. The means by which each of these types of film damage can be minimized (and in some cases almost entirely eliminated) are described herewith. The adoption of any one of these film conservation measures should double the useful life of a print. Universal adoption of all these measures could make possible several hundred bookings of a print. NOVEMBER 194