International projectionist (Jan-Dec 1955)

a permanent deformation that produces a troublesome in-and-out-offocus nutter on the screen. "Buckle" is the opposite of twist. The edges of buckled film are shorter than the center section, giving the middle area of the film a characteristic humpy appearance. Buckle is due to temporarily shrunken side margins. Less severe buckling is caused by contact of the perforation margins of fresh film with excessively hot gaterunners and tension pads during projection. The darker areas of the photo-emulsion do absorb heat. However, the interval of exposure to the arc beam is so brief that only the outer surface of the emulsion is raised to a high temperature, the base remaining relatively cool. In fact, the emulsion may even become blistered by irradiation without appreciably heating the base. Contact of the perforation margins with the hot gate rails and pads, on the other hand, heats both the emulsion and base in these areas by direct conduction, (cooled gates exempted). The effectiveness of water-cooled gates is proven by the fact that the film winds up "cold" in such projectors even when the arc beam is strong enough to blister the emulsion. Buckled film assumes a pin-cushioned form over the projector aperture. "Buckle" is called positive when the emulsion side is concave (the film bulging toward the lens) and negative when the emulsion side is convex (the film bulging toward the light source). "Normal" buckle is negative, the film bulging toward the lamp. Severely buckled film flops in-and-out of focus. Increasing the gate tension does NOT materially reduce the fluttery effect of buckled film! Causes of Embossing Embossing is a deformation produced by the effect of heat absorbed by the emulsion of the picture areas during exposure to the hot arc beam at the aperture. Each frame is exposed for two 1/96-second intervals when projected, with a 1/96-second blanking-out interval separating the exposures. Even this brief "flashing" is enough to expand the picture area so that the frames are raised above the general level of the film. Repeated projections increase the effect, until the frames can easily be seen to stand out like small cushions. Projection with extremely high-powered arcs may even emboss the photographic images, giving them the appearance of basrelief carvings when examined by reflected light. Embossing does not seriously impair the quality of the screen image, although when embossed film is spliced to unembossed film, the slight shift in the distance of the image from the lens when the splice passes by the aperture may require refocusing. "Focus drift" and "flutter" are special effects common in 35-mm film projection. They may be caused by arc heat or other factors. Focus drift is generally regarded as a normal phenomenon which must be accepted by the projectionist, especially if lenses of short focal-length are used, in order to maintain a crisp focus on the screen at all times. Because the convolutions, or individual turns, of film are smaller near the center of a roll than those near the outside, the amount of curl in the film is usually greater near the end of any reel. As the showing of a reel progresses, therefore, the slight degree of concavity of the film over the aperture gradually increases, moving the photographic image toward or away from the lens. Unless the focus be sharpened once or twice during the projection of a reel of film, the picture on the screen becomes increasingly blurred. Flutter of the film over the aper ture is often plainly visible on the screen, and is usually due to buckled film. There is no remedy for this condition. The Arc Power Factor Projection via powerful arcs causes a different kind of film flutter which becomes most pronounced during the second or third showing. The emulsion side of the film begins to expand when the intense light beam strikes it, and, accordingly, the film image moves away from the lens. The film begins to return to its normal position during the blanking period when the balancing blade of the shutter cuts off the light. The expansion resumes during the next exposure. Now, this type of flutter is so rapid that it gives the effect of poor focus. It is impossible, however, to correct this condition with the focusing knob because the film moves back and forth twice during each "dwell" period when the intermittent sprocket is at rest. At the very highest intensities, the film buckles and flops in and out of focus visibly. No projectionist can follow these irregular movements of the film to obtain a good focus. A light beam powerful enough to produce this very severe buckling may also destroy the print by blistering the emulsion. This phase will be considered subsequently. I TO BE CONTINUED 1 How Many? Was this copy dog-eared when it came to you? How many men read it ahead of you? You would receive a clean, fresh copy if you had a personal subscription— and you wouldn't have to wait— you would be first to read it. Use coupon below. INTERNATIONAL PROJECTIONIST, 19 WEST 44 St., New York 36, N. Y. . . □ 1 year-12 issues-$2.50 Enter my subscription for _ _ ni . & . nn 7 K □ 2 years— 24 issues— $4.00 Foreign and Canada: Add 50c per year. Name Address City. Zone State 30 INTERNATIONAL PROJECTIONIST • JULY 1955