International projectionist (Oct 1931-Sept 1933)

INTERNATIONAL PROJECTIONIST VOLUME III NUMBER 1 JULY, 1932 LUBRICATION OF MOTION PICTURE FILM J. /. Crahtree and C, E. Ives*^ WHEN freshly developed or socalled "green" motion picture film is passed through a projector, there is a tendency for an incrustation to accumulate on the aperture plate and tension springs, which retards the free passage of the film through the machine. Chemical analysis has shown that this incrustation consists largely of gelatin with more or less silver, dirt, carbon dust, and oil, but it contains usually only a trace of the metal or alloy of which the gate is composed. The effect of the incrustation is twofold, namely: (a) It increases the friction between the metal parts of the gate and the gelatin coated surface of the film. This causes excessive strains on the edges of the perforations at the pulldown sprocket which ultimately results in torn perforations and, therefore, a diminishing projection life of the film, and (6) The film is no longer held flat in the gate but oscillates in and out of the focal plane, producing the well known "in-and-out-of-focus" effects. Also, since the film is in a condition of varying strain between the intermittent sprocket and the aperture, the projected picture is unsteady. A similar action at the sound aperture causes a flutter in the * Kodak Research Laboratories, Rochester, N. Y. volume and frequency of the reproduced sound. If developed motion picture film is examined under a microscope by reflected light, it is seen that the gelatin surface is covered with innumerable extrusions (see Fig. 1, magnification 540), which impart a definite degree of roughness to the film. It is possible to smooth the film surface either by grinding away or burnishing down the minute projections or by filling-up the crater-like depressions. The effect of filling-up the depressions with wax and then polishing is shown in Figure 2, the left-hand side of which shows the surface of untreated film (magnification 540), while the right-hand side shows the same film after applying wax and burnishing. The Burnishing Effect Tests have shown that the mere act of burnishing or polishing the film surface without the application of a lubricant, such as wax or oil, does not appreciably facilitate the passage of the film through the projector gate. It is well known, however, that film which has been projected once or twice has a much less tendency to produce an incrustation on the gate than "green" film, and this is usually attributed to the burnishing or polishing action of the aperture plate or [7] pressure springs on the gelatin coating of the film. The burnishing effect produced by projecting film in a Simplex projector ten times is very slight as shown in Figure 3 (magnification 540). This is a photomicrograph of the film surface in the region between the perforations. The lower half of the figure shows a portion of the film surface which was in contact with the aperture plate, and it is apparent that the burnishing effect on the film surface was almost negligible. Effect of Oil It is considered that traces of oil which are transferred to the film surface during the first projection are chiefly responsible for the increased ease of passage of the film on subsequent projection. It is obvious also that the moisture content and degree of hardening of the gelatin coating are important factors which determine the rate of form.ation of the incrustation on the gate. If the gelatin coating of the film contains an excess of moisture, it tends to soften and become "tacky" much more readily in the hot projector gate than is the case with dry film. This tendency of the gelatin coating to soften under the action of heat can be diminished by hardening