Exhibitors Herald and Moving Picture World (Apr-Jun 1930)

April 12, 1930 Exhibitors Herald-World 139 cut off in a horizontal plane from top and bottom simultaneously during the full film movement period, and again during what might be termed the flicker interception at that period when the film is stationary before the aperture. Thus the heat from the beam is prevented from reaching the film during the entire period of film movement, and again during a portion of the time while the film is at rest, it is being allowed to reach the film only at two short intervals, once when the film has come to a complete stop before the aperture, and again after the flicker interception and until the film starts in motion again. The second purpose of reducing the heat of the metal parts of the projector adjacent to and including the aperture is accomplished by construction of the horizontal shutter itself, and the construction of its housing. By “heat at the aperture” is meant the heat on the aperture plate itself and the adjacent metal CYLINDRICAL type Fig. 3 parts, whereas “heat on the film” is the actual heat applied to the film. Reference to Figure 2 will show that the design of the rotating shutter is such that its ends are angled similar to the blades of a fan, but each end is of a different pitch. In addition are shown air disseminating vanes, all of which are designed to function in connection with the shutter housing to provide a forced air circulation for cooling of the aperture and the adjacent metal parts. This forced air circulation, it is pointed out, does not blmo air on the film, since any such air blown on the film would be drawn through the heated light beam; it has, in fact, an exact opposite effect. The shutter housing is attached to the film gate in a manner to provide air ports between it and the film gate. In the film gate itself are other air ports. The construction of the horizontal shutter with its fan ends and air disseminators, in conjunction with its housing, operates in a manner similar to the familiar “squirrel cage” ventilating fans. A forced current of air is established which draws cool air through the ports in the film gate and between the film gate and shutter housing in such a manner as to draw cool air over and away from the film, and over the aperture and its adjacent metal parts. In a test, the old type of front shutter DISC TYPE Fig. 4 was subjected to the heat of a light beam for a period of 30 minutes. At the conclusion of the test, the heat adjacent to the aperture registered 270 degrees. The same test repeated with a Model H mechanism with horizontal shutter in operation, registered a temperature of only 100 degrees — only slightly more than room temperature! In addition to the cooling effect, this method of air circulation tends to draw away from the film any dust or dirt in suspension in the air. The third purpose, to provide an in crease in screen illumination, is achieved also through the design made possible by the horizontal cylindrical formation of the rotating shutter. Rotating on a horizontal axis, it is possible to arrange two vanes of circular formation so that one intercepts the light beam from the top toward the center, while the other vane simultaneously intercepts the light beam from the bottom toward the center. This action is illustrated in Figure 3. Reference to the illustration will also show why this is a much more efficient cut-off than that of the front shutter, which in action cuts off only in one direction, either diagonally or nearly so, across the long dimension of the aperture. This is illustrated in Figure 4. In effect, the horizontal cylindrical shutter provides a double cut-off across the narrow dimension of the aperture. In addition to the efficiency already gained by the double cut-off, the horizontal cylindrical shutter made possible a more efficient design of shutter blade proportions. In most old type of rotating disc shutters of the two blade construction, the area represented by the shutter blades is approximately 60 per cent, and the light openings 40 per cent. In other words, the shutter cuts off about 60 per cent of the light, allowing 40 per cent to reach the screen. In the horizontal cylindrical shutter the blade dimensions are such as to provide 45 per cent blade area, with 55 per cent light opening, therefore, allowing only 45 per cent of cut-off, and passing 55 per cent of the light to the screen — a gain of 15 per cent in screen illumination. This is illustrated in Figure 5. The action of the shutter, resulting in a quicker cut-off of the light beam, together with the two blades, one cutting off near the film, and the other at a point farther from the focal plane, provides a blending or diffusing effect during the cut-off and flicker interception, a highly desirable quality which insures a smoothness of operation resulting in flickerless projection at normal projection speed. The horizontal cylindrical shutter housing functions also as an eye shield, Fig. 5