International projectionist (Jan 1963-June 1965)

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Light rays from the aperture spread out as they pass through a conventional projection lens, represented here by a simple biconvex element; and the shorter the focal length of the lens, the more rapidly the rays diverge. This may result in shading of the sides and corners of the picture by the lens holder and the hole in the front of the mechanism. A reverse-Galilean lens, on the other hand, employs a strong positive element close to the aperture to "pipe" the rays through the lens holder with very little divergence. The negative element at the front of the barrel diverges the nearparallel rays and establishes the desired focal conditions. Many drive-in theatres, unfortunately, are operating with lamps, lenses, projectors, and sound systems; that are wholly inadequate for their exacting job and totally incompatible with the high picture and sound quality today's moviegoers insist upon regardless of the type and size of the theatre. It is clearly the duty of the projectionist, after he has evaluated the drive-in's equipment unfavorably, to point out its shortcomings to the management and strongly urge the purchase of more suitable units. Such projection troubles as jumpy pictures, sidesway, travel ghosts. "wows" in the sound, high-frequency attenuation, frequent film breaks, etc. are common to all theatres and not peculiar to the drive-in. Actually, the severe light requirements of giant outdoor screens with attendant heat and focus troubles constitute the drive-in's Number One problem, as suggested by the foregoing. And the logical place to begin an attack on the screen-light problem is at the drive-in screen, itself! Most drive-in screens are constructed of large, flat, weather-proof panels with the joints between them sealed with a cement compound. Cement-plaster screen surfaces laid over a wire-grid or steel-rod backing are also commonly used. But whichever material is used, the screen surface, if perpendicular to ground level, should be painted with at least three coats of a matte (non-gloss, or "flat" ) durable white paint of high reflective power. The screen must be repainted whenever the surface becomes dull, discolored, streaked, or damaged in any way. Adequate picture illumination is too difficult and too costly to achieve in a drive-in theatre to be wasted by a dull or darkened screen surface! Reflectivity Varies You must choose your drive-in screen paint with a full knowledge of its pigment composition. Don't be misled by "secret formulas"! There are only a few suitable white pigments of high reflectance — titanium white (titanium dioxide, also called permanent white I , white lead ( basic lead carbonate, also known as flake white and Cremnitz white ) . zinc white (zinc oxide, also called Chinese white), and lithopone (mixture of barium sulfate and zinc sulfide, sometimes called silver white ) . Lithopone has the highest initial reflectivity (97%), but is unsuitable for painting drive-in screens because it is slightly darkened by exposure to daylight. Zinc white has the lowest reflectivity of the four (93%), and also the least effective opacity, or covering power. However, zinc white is completely permanent, and is not discolored by time, light, or gases. White lead has excellent covering power and a fairly high reflectivity when fresh (95%), but is quickly turned yellowish brown by sulfurcontaining fumes such as coal gases, auto exhaust, etc. For this reason white lead should be avoided for painting screens. Titanium white has good covering power and a very high reflectivity (96%), and is the most permanent and stable of all white pigments. It does not discolor with age or upon exposure to light or gases. Titanium white is definitely the choice for use as a screen paint. Pigment Critical Screen paints for -drive-in screens should have a pigment composition of pure titanium dioxide alone, and not adulterated with any other white pigment or with such commercial "extenders" as asbestine, talc, silica, whiting, china clay, or barytes. Neither should they contain magnesium oxide, a substance which has low covering power despite its extremely high reflectivity in thick layers (98%). Freshly powdered titanium white has, as we said, a reflectivity of 96%. In the form of paint containing linseed oil, it has a reflectivity of about 90% when fresh, and about 85% after the oil has aged. "Chalking" of the surface caused by the effects of weather may restore reflectivity to a value close to 90%, but it is safer to reckon on 85% as the reflective power of aged titanium white screen paint. This is very bright for a white paint, and is the value adopted in Table I, given below. Now a word of warning. Never use aluminum paint for surfacing a perpendicular drive-in screen. Aluminum paint is directional; and used on a perpendicular screen lighted by projectors pointed up at it, most of the light will be reflected mirror TABLE I ARC CURRENTS AND CARBON TRIMS NEEDED FOR 4y2 TO 5 FOOTLAMBERTS ON 85%-REFLECTING WHITE SCREENS SCREEN CARBON TRIM WIDTH Pos., Neg., and Type (Feet) 25 7 mm 6 mm, Surrex 30 7 mm 6 mm, Suprex 35 7 mm 6 mm Suprex 40 8 mm 7 mm, Suprex 45 9 mm 8 mm, Suprex 50 10 mm 11/32 in, Reg. HI 55 10 mm 11/32 in, Reg. HI 60 11 mm 3/8 in, Reg. HI 65 13.6 mm 7/16 in, Reg. HI 70 Blown Arc, 10 mm Hitex pos. 75 Blown Arc, 10 mm Hitex pos. 80 Blown Arc, 100 mm Ultrex pos. REQUIRED ARC AMPERES 40 42 46 60 80 100 110 120 160 130 138 155 TABLE II ARC CURRENTS AND CARBON TRIMS NEEDED FOR 4J/2 TO 5 FOOTLAMBERTS ON DIRECTIONAL SCREENS (150% Refl.) SCREEN CARBON TRIM WIDTH Pos., Neg., and Type (Feet) 50 7 mm 6 mm, Suprex 55 8 mm 7 mm, Suprex 60 9 mm 8 mm, Suprex 65 9 mm 8 mm, Suprex 70 10 mm 11/32 in, Reg. HI 75 11 mm 3/8 in, Reg. HI 80 11 mm 3/8 in, Reg. HI 85 13.6 mm 7/16 in, Reg. HI 90 13.6 mm 7/16 in, Reg. HI 95 Blown Arc, 10 mm Hitex pos. 100 Blown Arc, 10 mm Hitex pos. 105 Blown Arc, 10 mm Ultrex pos REQUIRED ARC AMPERES 50 65 80 85 110 115 120 155 165 130 138 155 International Projectionist March, 1964