International projectionist (Jan 1943-Dec 1944)

The lens system was calibrated by attaching an indicator to the system as shown in Figure 1. The lower part of the indicator hand is attached directly to the lens barrel, then to a fixed fulcrum which magnifies the lens displacement at the tip of the pointer so that lens movements of 0.01 inch are read plainly and movements of the order of 0.0025 inch can be estimated. Film Movement in Aperture The focus indicator was calibrated in the following manner: A strip of film was held in a perfectly flat position in the gate by means of a piece of steel 35-mm in width. A small hole drilled in the center of this metal strip allowed a portion of the test image to be projected upon the screen. Since the film was not in motion a faint source of light was used at a considerable distance from the film in order to eliminate any heat effect. Thus the lens setting for best visual focus for perfectly flat film in the gate was established. This we shall call "zero focus." In a like manner, the lens setting for film that has been displaced a known amount was obtained by mounting the film on metal shims of known thickness and placing the shims against the film trap. Thus if a shim 0.01 inch in thickness were used, the film would be moved axially toward the lens 0.01 inch. The best visual focus was again obtained by projection of the image and the focus indicator calibrated. FIGURE 2. E-7 gate cut away for aperture pictures As is well known, film that curls so that the emulsion side is concave is referred to as having "positive curl," and film that curls in the oppositive direction is referred to as having "negative curl." Since the film in the gate has its emulsion side toward the arc, and has its edges pressed against the gate, it will be seen that if film has positive curl, the image plane at the center of the film will be shifted toward the projector lens. The shift of the lens in order to correct for this displacement will be referred to as a positive focus. In like manner, if the film in the gate of the projector has a negative curl, the image plane at the center of the film will be displaced in the opposite direction or toward the lamp. The shift of the lens to correct for this displacement will be referred to as a negative focus. Thus it may be seen that the effective position of the film in the gate, so far as the screen image is concerned, may be arrived at during projection simply by setting the lens at the best visible focus and reading the displacement of the lens in hundredths of an inch on the dial. It was discovered immediately that almost without exception new films projected at the customary heat intensity of about 850°F or higher assumed a negative curl in the aperture. This was viewed at first as a rather disconcerting discovery inasmuch as such films are almost always in a state of slight positive curl. In fact, film entering and leaving the gate was observed to have slight positive curl, and yet the focus indicator showed plainly that the film in the aperture, while the image was being projected upon the screen, was negative in curl to the extent of. in many cases, at least 0.01 inch. On the other hand, it is quite logical to assume that temperatures of this magnitude, even though operating but for an instant, could effect this change in the physical state of the film. One has. in effect, a situation analogous to a bimetallic strip such as is used in many thermostats. This consists of two bonded metal strips, one having a greater coefficient of expansion than the other. When heated, the strip is forced to assume a curvature convex to the more rapidly expanding element. In our case, the emulsion layer and the support form the two members of the strip. The expansion takes place almost wholly in the emulsion layer since the support absorbs practically none of the heat. Expansion of the emulsion layer would force the strip of film to be convex to the emulsion or negative in curl. In order to determine at what point in the pull-down cycle the reversal of curl took place, high-speed motion pictures were taken of the film as it passed through the aperture. In most of this work, a Simplex E-7 projector with a McAuley Hy-Candescent Lamp and New Super H. I. National carbons were used. However, certain phases of the work were repeated with other projectors and other lamps and the same results were obtained. It was necessary to cut away a portion of the E-7 gate, as shown in Figure 2, in order to obtain the pictures of the film as it passed by the aperture. A reference bar was attached to the gate so that slight movements of the film in relation to this bar could be observed (Figure 3). Figure 4 shows this special gate in place on the projector, and Figure 5 shows the high-speed Cine Kodak in position to take the pictures. The pictures were taken at an angle of about 15 degrees from the plane of the film. By this means pictures have been taken of film in the aperture of projectors at a rate of about 1500 frames per second. In other words, with film traveling through the 35-mm projector at the normal rate of 24 frames per second and with the film remaining stationary in the aperture for 1/32 second, about sixty 16mm exposures were made between successive pull-downs. As is known, there are two blades on the shutter of a standard 35-mm projec FIGURE 3. E-7 gate with reference bar attached SEPTEMBER 1943 11