International projectionist (Jan-Dec 1955)

Prevention of Damage To Prints By ROBERT A. MITCHELL The third and final article of a series relating to conservation measures in the projection room. BLISTERING of film emulsion is likely to occur when high-intensity (H-I) arc amperages exceed 80 amperes, no heat filters or air-cooling of the film at the aperture being used. A great deal depends upon the optical efficiency of the lamp, of course, and also on the transmission efficiency of the rotating rear shutter of the projector. While certain makes of arclamp can be operated at 100 amperes without blistering the emulsion, more efficient lamps may damage the film in this way at arc currents as low as 70 amperes ! Tests prove that rotating barreltype shutters (Motiograph, DeVry) are the most efficient on account of their rapid double action in cutting the light beam. The conical shutter as used in the Simplex X-L has a similar high degree of efficiency because it is positioned so close to the film plane. The old single-rotor, fan-type rear shutters are the least efficient of all, passing the least amount of light even when the blades are trimmed to the point where travel-ghost just appears. 'Blistering' of Prints Lightly-blistered film shows a fuzzy, grainy spot near the middle of each frame on the emulsion side. The center of each blistered spot has a charred appearance where the bubbles of burnt gelatine have broken through. The color of the burnt spots is brownish-gray. Examined on the base side of the film, the fuzzy, blistered areas have a whitish, milky appearance. The burnt areas are larger in cases of heavy blistering, sometimes covering the greater part of each frame. •ORIGINAL EDGE' /-SHEARED i n n n n n n n r FIG. 1. Edges of film sheared by improperly adjusted pad roller. Blistered prints are wholly beyond all possibility of restoration, and are totally unfit for projection again. Fortunately, there are two effective measures for preventing blistering, namely, infrared filters interposed between the lamp and the aperture, and air-cooling of the film in the aperture. Water-cooling of the gate, so necessary for the prevention of buckling and physical deterioration of the film due to the effects of heat absorbed by the film base, fails to reduce the chances of blistering. Worn, Maladjusted Equipment Heat-buckling of the film and blistering of the emulsion, while more common than in the days of smaller screens, nevertheless comprise a small fraction of the damage inflicted on prints during projection. Aside from normal wear, most of the film damage specifically attributable to the projection process is due to worn and improperly adjusted equipment. The scratching of prints is becoming an increasingly serious problem. We know definitely that much of this trouble is caused by CinemaScope magnetic soundheads. CinemaScope magnetic-track prints are frequently scratched by contact with the magnetic pickup and by impedance rollers which are scratched or burred, or fail to revolve freely. Regular optical prints threaded to by-pass the magnetic reproducer are sometimes severely scratched by contact with the edges of the holes in fire-valve box and mecnanism castings when the installation engineers have failed to provide the guiding rollers designed to keep the film from touching these areas. Valve Rollers Prime Cause Next to CinemaScope magnetic soundheads, the chief cause of scratched film today seems to be worn and jammed valve rollers in the upperand lower-magazine fire chutes. Rollers which fail to revolve are rapidly worn down so that the middle part of the roller contacts the picture and soundtrack areas of the film and in flicts multitudinous scratches which have the appearance of "rain" on the screen and produce extraneous noise in the sound. Periodic removal of accumulated dirt and film chips from the valve boxes is the best preventive of jammed rollers; but considerable criticism has been directed toward the design of these rollers. There is an ever-present danger of film-scratching when the middle portion of the fire rollers is not lathed down to a sufficient depth. Only the perforation margins of the film should be contacted. Scratches and "rain" effects that weave from side to side on the screen can be blamed on the valve rollers in the upper magazine in nearly every case. The valve rollers of most projectors require no lubrication beyond a drop or two of light oil at long intervals. As every projectionist knows, there are two types of valve-roller assemblies in wide use, the 3-roller and the 4-roller types The latter type is the most troublesome and difficult to service. Required by law in Massachusetts and a few other localities, 4-roller fire valves are much more likely to clog and cause film breaks than the more popular 3-roller type. The 3-roller valve assembly has two fixed-bearing rollers and a free roller which drops against them by gravity. The free roller accordingly contacts the film snugly, and yet is able to move away from the fixed-bearing rollers whenever a thick or buckled film splice passes through. Size of Loops Important Excessively large upper and lower film loops are another cause of scratched footage While there is danger in loops which are too small (they may pull tight and break if the picture is framed during projection) no loop should be so large that it rubs against any part of the projector FIG. 2. Sprocket and pad roller. The arrows point to the places where edge-shearing of the film may occur when the pad roller is set too closely to the end of the sprocket. INTERNATIONAL PROJECTIONIST AUGUST 1955 15