International projectionist (Jan-Dec 1950)

ring during the projection of each successive frame. Thus, the departures from ideal flatness in the gate that are indicated by measurements of film position with this technique are the maximum values for each frame. Such measurements of film position provide one of the best methods of following those high-intensity projection effects which depend on previous projection history. If one plots the best visual focus for succeedig projections of the same film, relatively minor trends will be found if no image trouble is encountered. A typical curve is that for condition A of Fig. 3. This represents normal negative drift, decreasing slightly with time; it may be accompanied by the appearance of embossing and change in reflected image tone. If intensities are increased to a level that produces in-and-out of focus, however, a curve like that for condition B of Fig. 3 results. With successive projections, the focus position changes from negative to positive; as the zero position is approached, image flutter appears, and around zero, in-and-out of focus will be found. With successive projections, negative drift is completely replaced by positive drift, and the image quality improves. Number-of-Runs Factor During an accelerated laboratory test, in-and-out of focus may be encountered after 2 to 15 or more projections, and the subsequent improvement in image quality may come in from 2 to 10 projections additional. In a theater where generally longer times elapse between projections, in-and-out of focus may begin in several days to several weeks, and last correspondingly longer. It is particularly convenient when comparing various projection conditions to study the changes in focal position during accelerated laboratory tests. A rapid focus drift from negative toward positive indicates early trouble in the theater and unsatisfactory projection conditions; a slow or negligible change indicates tint no serious image difficulties should be encountered. Mechanical Displacement of Film It will be apparent from what has already been said about the negative drift of film in the aperture under normal intensities, and the in-and-out of focus combination of negative and positive drift at higher intensities, that film in the aperture under certain conditions can assume an equilibrium position anywhere over a range of approximately 0.100 inch. It might be assumed from this that the mechanical force of an air jet impinging on the film could contribute to positioning the film somewhere in this range, and the experimental work showed indeed that film can be moved in the aper ture solely by the mechanical force of the impinging air jets. Figure 4 presents a generalized picture of what the forces of air jets can contribute to displacement of film in normal projection. The data for Fig. 4 were obtained during the projection of a roll of fresh film at normal intensities, where, if left to its own devices, without air impingement, the film would have taken an equilibrium position at approximately minus 0.026 inch. With a series of air flows directed both from the emulsion side and the base side of the film, it will be seen that this equilibrium position can be displaced to a maximum negative displacement of minus 0.035 inch and a maximum positive displacement of greater than +0.060 inch ! Balancing Drift Directions With no air on the base side and air impinging only from the emulsion side, it will be seen that near the mid-point of the air-flow range the film was restrained from its normal negative drift to the ex llDPHRT REAL SPARKLE to your presentations with this more brilliant spot THE STRONG TROUPER HIGH INTENSITY SPOTLIGHT A portable A.C. Carbon arc that produces a steady, sharp, uniformly illuminated snow-white spot. Silvered glass reflector and twoelement variable focal length lens system. Draws only 10 amperes from 110-volt A.C. convenience outlet. Adjustable, self-regulating transformer in base. Automatic arc control. Carbon trim burns 80 minutes. Horizontal masking control. Can be angled at 45 degrees in each direction. Color boomerang. Six slides. Ultra-violet filter holder. Mounted on casters. Easily disassembled for shipping. Listed by Underwriters' Laboratories. SEE ANY OF THE FOLLOWING DEALERS OR USE COUPON FOR OBTAINING LITERATURE ALBANY Nat'l. 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Montreal, Quebec Perkins Elec. Co. Ltd. Toronto, Ontario General Theatre Sup. Co. Winnipeg THE STRONG ELECTRIC CORP. "The World's Largest Mann facturer of Projection Arc Lamps" Please send free literature, prices and name of the nearest dealer in Strong Spotlights. NAME. THEATRE. STREET. CITY & STATE. INTERNATIONAL PROJECTIONIST • February 1950 27