International projectionist (Jan 1963-June 1965)

Drive-in's No. 1 Problem 35 and 70mm Neither "cold" mirrors nor heat filters can remove all of the heat, however, for the needed light rays are just as hot as the unwanted infrared ra\s. The use of a heat filter with a "cold" mirror is wasteful because the mirror, itself has already removed most of the heat-producing infrared. Any further reduction of heat can come onh through reduction of the light — and light is something we are trying to get more of, not reduce! So don't use heat filters with your "cold" mirrors. It would be cheaper to diminish your arc current to reduce the heat I and the light I b\ about 15% ! This should not be necessary — not even if you are using the most powerful arc lamps at the highest currents possible. Arc Current You Need! The minimum recommended screen brightness for drive-in theatres is 41 o footlamberts at the center of the screen when the projector is run without film and the brightness is measured from the center line of the viewing area. If you can get a brighter screen than this, all the better: but the 1 ' -j-footlambert level actually represents a higher light level than the majorit) of drive-in theatres are getting. The all-important question is: What arc current should be burned in order to achieve this minimum level of screen illumination? Assuming that the projection lenses are optically fast and antireflection coated, and that no obstructions are present in the optical trains of the projectors, the required arc current depends entirelv upon the light reflecting power and size of the screen, provided that the arc lamps are efficient. Now. the average white drive-in screen several years old has a reflectivity of about 85' c . By calculating the lumens of light needed for 41/-? footlamberts of brightness on 85%reflectivity screens of different widths, and equating the results with the luminous outputs obtained with various trims of carbons burned at suitable currents in efficient reflector arc lamps, we arrive at the information listed in Table I. These data assume a 50% shutter transmission and hold good regardless of the aspect ratio of the 35-mm projection. International Projectionist March (Continued from Page 7) Nor does it matter whether the projection is regular or anamorphic — the extra light passed by the CinemaScope aperture is wasted by the anamorphic lens. Never forget that high-intensitv arc-lamp carbons burn most efficientlv with the brightest, whitest light at or very close to their maximum rated currents. It is a serious mistake to ''underburn" carbons just to slow down consumption and save on carbon costs. If burned well below their maximum ratings, carbons give an unpleasantly dim. \ellowish light. For quality projection in drive-in theatres you need snow-white light of at least 1 ' ■_> footlamberts of brightness! Of course, if you can go beyond this minimum level and come up to the indoor-theatre minimum level of 10 footlamberts. all the better, but this is generalh impossible in the larger drive-ins. The actual "make-ready" and ''lining up" of projection and sound equipment which has lain idle all winter, or partially dismantled, is a most important part of the drive-in projectionist's job. These procedures will be outlined in next month's IP article with a particular view to sparing the projectionist complicated knock-down and inspection jobs that seem to us unnecessary and timewasting. It shouldn't be necessary, for example, to rebuild the intermittent units or to inspect hundreds of in-car speakers every day. First Consideration The very first matter to be considered when a drive-in opens for the season, however, is the matter which has been discussed in this article — adequate picture brightness. The screen surface, the projection lenses, the lamps, mirrors, and heatreducing means, and the correct carbon trims and currents for quality projection are the things which must be checked at the outset. The owner of the drive-in may have to spend money for the replacement of inadequate ecjuipment. but he really has no choice if his object is to stay in business and make a profit. The moviegoing public insists upon professional quality in the exhibition of motion pictures, and will no longer tolerate substandard projection in drive-in theatres. i m ' * MMCL To Be Concluded 1964 13