International projectionist (Jan-Dec 1958)

: on the screen. The blue-reflecting coating is as hard, or even harder, than glass, but on account of its extreme thinness it must be cleaned with x delicate touch. Remove dust from lens surfaces by blowing or brushing, whenever possihle, reserving the use of moisture and cotton cloth (or lens tissue) for stubhorn spots of oil, grease, or stains left by fingers which have accidentally touched the lens surfaces. As a rule, the lens surface facing the film aperture gets soiled rather <|uickly because the heat of the arc beam volatilizes the oil on old prints to produce a mist of oil. Image quality deteriorates rather seriously when this oil fog condenses upon the lens. Use a mild soap solution followed by pure water, and avoid the use of organic solvents (acetone, alcohol, etc.) which often contain waxy impurities, tarry denaturants, and even compounds dissolved from bottle caps. Anamorphic attachments require the same care as your fine projection lenses. They should never be taken apart, and, in addition, never readjusted for focus without the use of a test film having the highest possible photographic resolution. Each matched pair of projection lenses is best stored in a clean velvetlined box together with the required aperture plates. Required Sets of Lenses Most theatres today have three sets of lenses and apertures on hand in the projection room: short-focus lenses and undersized apertures for nonanamorphic widescreen projection, long-focus lenses, anamorphic attachments, and large apertures for CinemaScope, and the old regular lenses with standard 0.825-inch x 0.600-inch apertures. Standard projection is unfortunately not much used nowadays in spite of the better light and focus it makes possible. Moreover, the conventional 1.375/1 aspect ratio provides a more versatile frame for cinematographic techniques, and is easier on the eyes because it corresponds exactly to the shape of the natural field of direct vision. (See the accompanying diagram.) Don't forget that clean port-hole glasses are every bit as important as clean lenses! And if a steep projection angle prevails in your theatre, read the item on projector ports in this month's Projection Clinic! The sound reproducers and associated equipment do not normally need checkups as frequently as the lamps and projector mechanisms. Then too, many theatres have RCA or Altec sound service to insure perfect functioning of the sound system at all times. The busy projectionist, thus freed from the highly specialized task of checking amplifiers and making the finer adjustments in soundheads, may then concentrate upon the mechanical components of the sound reproducers, and readjust optical-tube focus or replace magnetic pickup clusters only in cases of real emergency. Care of Soundheads The big difference in the maintenance of optical and magnetic soundheads is a clearly defined one. Mechanical wear is not a factor in the output quality of optical reproducers, but magnetic heads, like phonograph needles, progressively deteriorate with use. Optical heads need no replacement parts aside from exciters, photocells, and sprockets, and these at infrequent intervals, while magnetic reproducers require new pickup clusters (or pole caps in certain models) every year or two. Bad sound from magnetic heads cannot be corrected without the installation of new clusters, but all an optical reproducer needs for top-quality performance is readjustment of the optical tube or, infrequently, a new sound sprocket. The pickup cluster of a magnetic reproducer requires correct lateral and axial alignment. If not lined up axially one edge of the film may contact the pole piece with less pressure than the other edge; and this may cause weak or distorted sound in one or two of the three stereophonic channels. A weak left or right channel has the effect of shifting the sound to one side of the screen, a common and disturbing effect in many theatres having CinemaScope stereophonic sound. Demagnetizing Projector Components CinemaScope magnetic sound requires demagnetization of the entire projector film path. An AC electromagnet enables the projectionist to "degauss" steel sprockets, idlers, film runners, tension shoes, fire rollers, etc. Once a projector has been thoroughly demagnetized, it is not necessary to repeat the process until such time as screwdrivers and other steel tools are used for working on the film side of the projector. Screwdrivers which have been used in the vicinity of arc lamps, motors, generators, rectifiers, transformers, amplifiers, etc. usually have more or less magnetism which is readily transferred to projector parts. Contact of magnetic tracks with magnetized objects may result in partial erasure of the sound record and the introduction of troublesome clicking, thumping, humming, or whirring noises. A preliminary optical line-up of the projection equipment includes adjustment of both arc lamps for optimum output and lamp or mirror-distance corrections, when necessary, to minimize hot-spot effects. Unless adequate screen illumination is a problem, the lamps should be adjusted for a light distribution more uniform than that (Continued on page 20) Perimeter chart showing the angular extent of the binocular visual field (clear area) wher the eyes are fixed upon a median point in the horizontal plane. Superimposed over this field of direct vision is a rectangle corresponding to the standard soundfilm projector aperture of 1.375/1 aspect ratio. This shape of screen is obviously better suited for continuous viewing, and hence less tiring to the eyes, than the "mailslot" screens of excessive aspect ratio. *°° '90 ,ao 170 *° INTERNATIONAL PROJECTIONIST • JANUARY 1958