International projectionist (Jan 1963-June 1965)

Aluminum-painted screens require a difficult and expensive installation of the screen surface. This is also true when corrugated or lenticulated aluminum plate is used for optimum light distribution. It is absolutely necessary to tilt aluminum-surfaced screens so that the light will be thrown down into the viewing area. A perfectly vertical aluminum screen is verv unsatisfactory in a drive-in. and is worse than useless if the upward projection angle is much greater than 5 degrees. Aluminum-painted surfaces reflect light mirrorwise; and failure to tilt the screen downward will result in the light being directed over the tops of the cars and up into the sky . Only the owls and nighthawks will see a bright picture. The downward tilt of an aluminum drive-in screen should be 2 or 3 degrees le?s than the upward projection angle. That is. if the projectors tilt up at a 7-degree angle, the screen should tilt down by about 5 degrees. I Too great a tilt will rob the ramps behind the projection building of light! i \fter some months of weathering, an outdoor aluminum screen loses both light-reflecting power and directional "gain." An aluminum-painted screen which initiall\ had an overall "integrated" reflectivitj of ">()'< dropped to 65', after one winter. This is a change in color from a bright silver) white to light gray. Then. too. the initial light gain of 3 of thi> screen la center-screen reOectivit] of 300^5 measured on the optical axis 1 dropped to l1^ l center-screen reflectivity of 15095 in the same length of time. Even though the extreme ends of the ramps received a hit more light than they did when the aluminum paint was fresh, the overall reflectance was down, and the middle of the viewing area suffered excessive dimming of the picture. Aluminum paint manufacturers mav claim that the particular brand of paint used on this screen was of poor quality, but this severe loss of reflecting power and gain in a 12-month period is entirely in line with our experience with several brands of aluminum screen paints used out of doors. Indeed, we are reluctant to assign a center-screen reflectance greater than 150% • to any weathered aluminum-painted drive-in screen even when the screen is properly tilted so as to give the audience the benefit of this admittedly moderate lightgain. An aluminum-screen reflectivity of 150%, conservative though it may be. is a very safe one to guide us in the choice of a suitable screen size and a set of arc lamps to insure the standard minimum brightness of 41 L. FL. Therefore, if the aluminum screen has an axial reflectivity of 150%. it should be no wider than 90 feet when the lamps burn regular 13.6-mm carbons at about 160 amps., or no wider than 105 feet when the lamps are of the blown-arc type. Of course, a higher gain than 1 j L> will permit the projection of still larger pictures having a brightness of at least 4VL> FL, but we refuse to count on a gain much exceeding 1% for outdoor aluminum screens. The foregoing maximum screen-width recommendations are for 35 mm non anamorphic projection, and are based on the assumption that the light transmission of the projector shutter is at least 50% ( it is usually a little more in drive-in projectors I. that there are no physical obstructions in the path of the light beam a it comes from the arc lam)) I onlv modern mechanisms fill this requirement), that the projection lens is antireflection coated and has an optical speed of at least f 1.''. and preferably f 1.7. If projector-port glasses are used in the drive-in projection building to prevent urilt\ du-t from blowing in and settling upon the lenses, LUMENS FOR H F00TLAMBERTS LUMENS FOR 10 F00TLAMBERTS SCREEN WIDTH Q5% CARBON 150^ CARBON 85% CARBOl 150^ CARBON (Feet) MATTE AND ALUMINUM AND MATTE ANI » ALUMINUM AND SCREEN AMPERAGE SCREEN AMPERAGE SCREEN AMPERAGE SCREEN AMPERAGE 20 1 ,600 7mm 40A 900 1,400 7mm 40A 3,500 7mm 42A 2,000 7mm 40A 25 2,500 7mm 40A 7mm 40A 5,500 7mm 50A 3,100 4,500 7mm 40A 30 3,600 4,900 6,400 7mm 42A 2,000 7mm 40A 8,000 9mm 80A 7mm 46A 8mm 60A 3 7mm 46A 2,800 7mm 40A 11 ,000 10mm 110A 6,100 8mm 60A 3,600 7mm 42A 1 4,000 13.6 145A 8,000 9mm 80A ±5 8,100 9mm 80A If, 600 7mm 46A 18,000 13.6 165A 10,000 10mm 100A 50 10,000 10mm 100A 5,600 7mm 50A 22,000 Blowr i arc 13,000 11mm 115A 55 12,000 10mm 105A 6,800 8mm 65A (27,000) 15,000 13.6 150A 60 14,000 11mm 120A 8,000 9mm 80A (32,000) 18,000 13.6 165A 65 17,000 13.6 160A 9,500 9mm 85A (37,000) 21 ,000 Blown arc 70 20,000 Blown arc 11,000 10mm 110A(43,0O0) 25,000 Blown arc V 23,000 Blown arc 13,000 1 1f, 000 11mm 115A(50,000) (28,000) 80 (26,000) 11mm 120A(57,000) 13.6 155A(64,000) (32,000) 85 (29,000) 16,000 (36,000) (41 ,000) 90 (32,000) 18,000 13.6 165A(72,000) 95 (36,000) (40,000) 20,000 Blown arc (80,000) (45,000) 100 23,000 Blown arc (88, 000) (50,000) 105 (44,000) 25,000 Blown arc (97, 000) (55,000) 110 (48,000) (27,000) (110,000) (61,000) 115 (53,ooo) (30,000) (120,000) (66,000) TABLE 1 — Required screen lumens (projector running without film) and suggested sizes of positive carbons and arc currents for screen widths of 20 to 115 feet for either 4J/2 or 10 footlamberts brightness on white and International Projectionist April 1963 aluminum screens. (Lumen values in parenthesis exceed the capacities of the most powerful arc lamps.) MULTIPLY ALL LUMEN VALUES IN THIS TABLE BY 2 TO EQUATE WITH MANUFACTURERS' RATINGS. 5