International projectionist (Jan-Dec 1958)

Graphic derails of film behavior in straight and curved gates; the author is one of the major developers of this mechanism. Gate and Shutter Characteristics SINCE PUBLICATION of the article, "Modulated Air Blasts for Reducing Film Buckle," (IP, October 1952, p. 14 et seq.), tests and experiments have continued. The behavior of the film in the projector gate, under the critical conditions imposed by large-screen operation, has been further examined. Today's projection problems are more severe than ever before, because of the continuing trend toward larger screens, greater magnification, and higher arc-light levels. The use of larger screens, whether flat or curved, makes good overall focus a very critical requirement. The increase in picture size, alone, creates demands for better picture steadiness, vertically and horizontally, but even this important factor toward screen image quality, although readily appreciated, is not necessarily the most significant. Another less obvious factor, with which we are here concerned, is the tendency for the image to move in and out of sharp focus, at a rate too fast for the eye to follow. This results in a general impression of fuzziness which cannot be corrected merely by refocusing the projection lens. The article referred to above explained how movement of the film along the optical axis produces this effect. Now let us further examine the consequences of film motion in the light of today's projection requirements. The high speed of modern projection lenses, and correspondingly small depth of focus, places particular emphasis on position stability of the film in the gate. The high magnification of short focal length lenses further emphasizes any out-of-focus effect due to film motion in the gate and high arc -light levels provide an environment which favors film motion. The aforementioned article stated: "Each single picture frame goes through a cycle which starts with pulldown into the aperture, proceeds through the first exposure, the flicker blade cut-off and the second exposure, By WILLY BORBERG General Precision Laboratory, Inc. and ends with the pulldown of the next frame. During the two exposure intervals the film arrests some of the radiant energy from the light source and transforms it into heat. This causes the film to buckle (or bulge) in a manner very similar to that observed in the operation of a bimetallic element. The emulsion being more opaque than the base, absorbs energy, expands, and becomes the outer or convex surface of the bulge. The magnitude of the deformation produced varies continuously during the frame cycle and by an amount which is more than sufficient to affect sharpness of image focus. "The emulsion side of 35-mm film is toward the light source; and hence the film tends to move toward the light, away from the lens, while it is in the aperture. "In accordance with the accepted terminology, the deformation is called negative when the emulsion side is convex and, conversely, positive when the emulsion side is concave. Flat film is considered to have zero deformation." Early Equipment for Studying Film Deformation Figure 1 shows a typical curve of film displacement at the center of a frame in a two-bladed shutter mech anism. To obtain such a curve, the film position along the optical axis is measured directly in terms of lens displacement, and a dial indicator, calibrated in thousandths of an inch, is affixed to the lens-mounl for this purpose. Initial calibration for zero position on the dial indicator is made by focusing the lens to produce a critically sharp image of a conical hole in a flat steel plate, the small end of the hole being in the same plane as the emulsion contacting surfaces of the film trap. The addition of a viewing shutter and screen scope to the equipment enables observation of successive phases of the cyclicly varying film frame motion. The viewing shutter's drive-motor stator can be rotated so that the shutter opening of about 9 degrees can be phased in relation to the synchronously running projector. With this stroboscopic arrangement, it is possible to view the screen image in time increments of about one milli-second through all exposure phases of successive frame cycles. The film-emulsion position during any specific phase of the exposure period can thus be established without regard to possible out-of-focus conditions during the remaining unobserved portions of the cycle. Dial indicator readings are then recorded in relation to the phase settings. Studies made with this equipment led to the use of a modulated air blast, in which jets of pulsed air countered t SMPTE Journal, October, 1957 FIG. 1. Film displacement due to buckling at center of frame. No air. Straight gate. -30 NO AIR INTERNATIONAL PROJECTIONIST • JUNE 1958 13