International projectionist (Jan 1963-June 1965)

Volume 38 April, 1963 Number 4 Proper Drive-ln Projection by Robert a. Mitcheii A Picture that is consistently bright and clear on the screen bespeaks a high degree of competence on the part of the "unseen showman" in the projection room. A brilliant, uniformly illuminated screen, free from brownish or bluish discolorations, reveals that the projectionist takes extra care to line up his lamp equipment and to keep the arc burning properly at all times. Likewise, a knife-sharp focus of the picture (barring "fuzzy" prints^ is another hallmark of the conscientious projectionist, inasmuch as different prints, and even different portions of the selfsame reel, ordinarily require slightly different lens settings for the highest resolution of the image. It's a safe bet that the projectionist who gives frequent attention to the arcs and to the focus presides just as diligently over all the multifarious details of the complex projection process. Now. when we go from an adequately equipped indoor theatre to the usual drive-in having an excessively large screen (perhaps badly weathered and in need of repainting ) , the aforementioned indications of expert work in the projection room come in for a rude shakingup! How can the picture possibly be well lighted when the tremendous area of the drive-in screen overtaxes the capacity of the arc lamps? How can a knife-sharp focus be obtained when the radiant heat blazing upon the aperture is so intense that the film flutters like a rag in the breeze? The projectionist in the average drive-in labors under a disadvantage even when he does the best he can with the unsuitable equipment given him to operate. Lowpowered lamps forced beyond their capacity, the absence of heat filters, cold mirrors, or water-cooled film gates, and the use of "slow" uncoated projection lenses all contribute to the shockingly low quality of much of the drive-in projection presently on view. Minimum Brightness Raised to 41/2 FL The dim, shadowy images which wander like halfmaterialized ghosts on most drive-in screens today are wholly unnecessary. Up to a certain screen size for each type of screen surface, it is possible to achieve an acceptable screen brightness level in drive-in projection. Modern arc lamps of high power are capable of adequately lighting white-painted drive-in screens up to width of 75 feet, and aluminum-painted screens of only moderate "gain" up to a width of 105 feet. The lamps we have in mind are of the "blown-arc" type; but the maximum allowable screen sizes when the most powerful regular rotating-positive mirror lamps are used are only 10 or 15 feet smaller in width. So gratifying have been the results obtained on drivein screens with the powerful lamps designed expressly for large-screen projection, that the old unofficial standard of 4 footlamberts as the minimum center-screen brightness for drive-ins has been supplanted by SMPTE INTERNATIONAL PROJECTIONIST, published monthly by the International Projectionist Publishing Co. division of The Northern Publishing Co., Post Office Box 6174, Minneapolis 24, Minnesota. Editorial offices, 1645 Hennepin Avenue, Minneapolis 3, Minn. Second-class postage paid at Minneapolis, Minn. 4 Recommended Practice RP12, which requires a minimum of 4.5 footlamberts measureed on the viewing axis at the center of the screen illuminated by one projector running without film. A screen brightness of 4% FL is 45% of the indoor minimum standard of 10 FL (the brightness difference between a rather pale light gray and white ) , or 28% of the indoor median standard of 16 FL. The maximum center-screen brightness for both indoor and drive-in theatres, specified by American Standard PH22. 124-1961, as 20 FL.* Unfortunately, more than half of the drive-in theatres recently surveyed by professional groups have screen brightness of less than 3 FL, substantially below the official recommended minimum. (About 40% of all driveins have screens from 80 to 100 feet wide, while another 40% have screens wider than 100 feet. ) Reflectivity of White Screens Most drive-in screens are surfaced with matte (nongloss white paint presumably formulated for high reflectance and resistance to sun, wind, rain, and the severe weather conditions of winter. If the white screen paint has a pigment composed principally of titanium dioxide, and if two coats are applied by spraying over a bright aluminum undercoat, the screen should reflect somewhat more than 90% of the light. A "diffuse reflectivity" in excess of 90% cannot be surpassed by any other white paint having acceptable covering power and resistance to time, weather, and chemical fumes. Screen paints formulated with zinc oxide and/or barium sulfate have diffuse reflectivities of about 85%, which must also be considered good. Paints containing white lead ( basic lead carbonate ) have excellent covering power and are initially very white, but should be avoided because lead pigments darken and turn, first yellow, then brown, when exposed to the sulfurous fumes usually present in the air in the vicinity of towns and highways. The disastrous color change is due to the formation of traces of black lead sulfide. A very cautious consideration of all available arclamps, lens, and screen data enables us to state that matte drive-in screens of 85% reflectivity be no wider than 65 feet for 4% footlamberts or more of brightness with 18inch mirror lamps burning 13.6-mm carbons at 160 amps., or no wider than 75 feet for the same light levels when blown-arc lamps — the most powerful of all — are used. Screens wider than 75 feet (most of them are!) require "directional" aluminum-type paints to obtain at least the minimum recommended 4% footlamberts with even the most powerful arc lamps presently available. *The SMPTE-American Standards specification of 10, 16, and 20 footlamberts for the minimum, median, and maximum indoor screen brightness levels is somewhat mystifying, inasmuch as visual brightness differences are Weber-Fechner functions of physical luminance. If 16 FL be accepted as the median, and 10 FL the minimum, we should expect a maximum allowable brightness very close to 23 FL. International Projectionist April 1963