International projectionist (Jan-Dec 1952)

did not help either the theatres or the projectionists. High-acetyl base contains from 42.5% to 44.0% acetyl, and is thus not quite so highly acetylated as the "normal" triacetate (44.8% acetyl). And herein lies one of the secrets of the success of the new safety film. Because it is slightly less acetylated than the highest degree of acetylation possible, it is very much more readily attacked by suitable solvent liquids and accordingly is easily cast into the form of film by the manufacturer and is easily spliced by projectionists and exchanges when properly compounded film cements are used! The physical characteristics of highacetyl safety film are now very well known. They resemble those of nitrate film very closely, except that its tearstrength and rigidity are slightly lower. But safety film doesn't buckle as badly as nitrate film: it retains its original flexibility and shrinks comparatively little. It doesn't become brittle rapidly like nitrate, hence is superior for superhigh-intensity projection. And old safety print gives a better picture on the screen than an equally old nitrate print because safety film retains most of its original qualities. Safety film, unlike nitrate film, is permanent in storage and doesn't burn any better than wet paper. This is a far different story than could be written about the earlier types of safety film. Characteristics of High Acetyl Stock Now to offer a few more words about the alleged shrinkage and brittleness of high-acetyl safety film about which projectionists have been complaining. Nitrate film of good quality may lose up to about 0.7% of its length through shrinkage. Older types of nitrate film could lose up to nearly 2.0%, and actually tear on the sprockets. The shrinkage of nitrate film is seldom uniform: it varies from foot to foot. The pitch of the sprocket-holes varies accordingly, causing a vertical weaving of the picture and, in moderately severe cases, a tearing sound at the intermittent sprocket. If a vertically-weaving picture is obtained with a safety-film print, it is only because the picture was printed from shrunken nitrate negatives. This is frequently the case with reissues of old pictures printed on fresh safety stock. Under average conditions, a highacetyl safety print seldom shrinks more than 0.15% after about 300 runs; while a nitrate print shrinks about 0.35% — more than twice as much — after the same number of runs. After approximately 300 runs, safety and nitrate films have about the same degree of brittleness. Does this mean that safety film is no better than nitrate film in regard to brittleness? Not at all. If both types of film were given even a fewer number of The second and concluding article on the performance of safety film stock will be published in the next issue of IP. It would be particularly helpful to the craft at large if there could be appended thereto a summary of projectionist opinion based on actual operating experience with this film stock. Such opinion is earnestly solicited. runs on equipment employing the highest-powered arcs, it would undoubtedly be found that safety would be appreciably less brittle than nitrate. Nitrate has a slightly higher degree of flexibility when both types of film are fresh, but nitrate loses its flexibility, rapidly becoming brittle with both use and age, while safety film retains most of its original flexibility. (In certain cases the writer has observed that comparatively fresh nitrate stock is unduly brittle, fracturing when creased.) Some Projectionist Complaints The matter of film-buckling has elicited complaints aimed at safety film. In order to convince the writer that modern safety stock buckled more badly than nitrate stock, several of the projectionists interviewed directed attention to the "waviness" often seen in the turns of film in large rolls of safety film. One projectionist emphasized this point by removing two reels of film from the film-cabinet, one safety and the other nitrate. The convolutions of the 2,000foot roll of safety film were wavy, and the outside turn was definitely buckled in appearance. The layers in the 2,000foot roll of nitrate film were perfectly circular to the eye, and lay flat and smooth on the reel. Unwinding a few feet from each reel, the writer noticed that the safety film was pliant and tough, and possessed a curliness that looked very much like severe buckling. Gentle stretching of the safety film, however, instantly brought it out reasonably flat with only a slight negative curl (emulsion side convex, base side concave) and without any trace of actual buckle. The nitrate film, on the other hand, had a stubborn curl that mild tension could not remove without a slight fluting or waviness of the edges of the film — even though the film looked perfect wound tightly on the reel. The curliness in this case was due in some measure to slight buckling, the defect which was suspected in the safety film! Any projectionist can make tests like these for himself. Also noticeable in the nitrate print was pronounced frame-embossing, due, of course, to the large number of projections it had undergone. The nitrate print was the older of the two films. It is of interest to note, however, that even though frame-embossing is not deleterious to satisfactory projection providing that the embossing is uniform in degree throughout the entire reel of film, safety film is markedly less subject to embossing than is nitrate film. Returning to the subject of the wavy appearance of some rolls of safety film, even when the film is quite new, Mr. W. I. Kisner of Eastman Kodak comments: "Of the complaints of this nature on high-acetyl safety film which we have investigated, we have found no case which was so serious as to cause projection difficulties." This comment is significant to the writer because it lends the strongest support to the conclusions reached by personal experience, namely, that the waviness of the safety film does not affect focus on the screen. Both Mr. Kisner's statement and the writer's experience are further substantiated by an opinion, based upon extensive research, expressed earlier by Dr. F. J. Kolb, Jr., of the Eastman research staff:* "It has been shown that film in the aperture is almost never flat, and that its position bears no relationship to the curl of the film-loops or other shapes the film may assume either entering the top of the projector gate or leaving the bottom of the gate. "Film in the aperture under the influence of the light-beam," added Dr. Kolb, "behaves as though the emulsion surface were expanding with reference to the base dimension, so that each frame is distorted into a pincushion shape with the emulsion surface on the convex side: since the edges are held, the center of the frame is displaced toward the arc. This is a perfectly normal phenomenon occurring in all cine projection." Dr. Kolb points out additionally, that if the film is threaded up so that the emulsion faces the projection lens, the same expansion of the emulsion-side occurs, in this case with the center of the frame displaced toward the screen. The conditions prevailing in standard projection, with the emulsion-side facing the lamp, are, however, more satisfactory optically, permitting a clearer over-all focus on the screen. The writer's most recent experience in theatres, operating on different makes and models of projectors under diverse conditions, provided convincing evidence (Continued on page 29) * "Air Cooling of Motion Picture Film for Higher Screen Illumination" by F. J. Kolb, Jr., IP for January 1950, p. 10, col. 2. A liberty has been taken with the first quotation in order to make clearer Dr. Kolb's meaning. 8 INTERNATIONAL PROJECTIONIST • October 1952