International projectionist (Jan-Dec 1954)

than an uncooled crater, it is true, but crater brightness in the cooled carbon is approximately 30% under that of an uncooled carbon when the amperage lemains the same. Waler-cooled carbons burn somewhat more slowly than uncooled carbons, and this means just one thing — less light at the same power-consumption. Charles A. Hahn of J. E. McAuley Mfg. Co., stressed this fact in the July 1953 issue of IP (page 21) . "As Simple as That" In the case of carbon arcs," he wrote, "the main product we seek through their consumption is light, and its quality or volume is determined by the amount of carbon that is consumed. . . . All things being equal, we doubt if anybody can reduce the consumption-rate of a fuel and still come up with a gain in the output of the product you burn it for. It's as simple as that." Purely fanciful claims won't increase your light. If your screen were smaller, and if you could feed 100 amps to your arcs, then carbon-coolers would be advantageous to you. Your lamphouses would be less hot, your screen would be more evenly lighted, and color changes would not be so apt to occur during changeovers. Under these conditions you would use carbon cooling. But under the actual existing conditions in your theatre, carbon coolers won't do you a bit of good, and, in fact, will rob you of light you can't afford to lose. For comprehensive technical studies of the effects of water-cooling on arc operation, we refer you to The Effect of Carbon Cooling on High Current Arcs by Wolfgang Finkelnburg, Research and Development Laboratories, Fort Belvoi'-, Virginia (IP for June 1949, p. 14), and to High-Brightness Carbon Arcs by M. T. Jones and F. T. Bowditch, National Carbon Co. (IP for July 1949, p. 12). Now let's take a look at your projection setup and see if anything can be changed to give you more light. Your metal arclamp reflectors are doing your business as much harm as anything. This is something else you can get away with if you have plenty of light to begin with. The type of metal mirrors you are using are made of steel plated with rhodium, a shiny metal of the platinum family; These mirrors reflect only about 75% as much visible light as silvered glass mirrors, but just as much heat (infrared). So gst glass mirrors for your lamps, and you will gain 25% more light. Two new mirrors will be sufficient, as you can save your metal mirrors for spares in case of breakage. Optical Set-up Vital When changing mirrors, don't fail to line up your lamps for maximum light production. The focal lengths of commercial mirrors often differ slighdy, so if it's possible to readjust the mirror-to-aperture distance, try different distances until you get the brightest light. And, of course, all the optical components — mirror, carbons, aperture, and projection lens — must be in a straight line to get good light. Even a slight deviation can ruin your screen illumination. The writer has often used metal mirrors, and while he appreciates mirrors that can't pit or break, he is not too happy about a 25% loss of light. The light from metal mirrors is yellower, or "softer," than that from silvered-glass mirrors, so you can expect color films to look better on your screen after you have made the change to glass mirrors. Protect your glass mirrors from sudden drafts and deposits of soot to insure against breakage. Sooty spots, which may be Screen used for »he first CinemaScope drive-in presentation at the Motor Vu DriveIn Theatre, Salt Lake City. Enormity of screen, 102 by 48 feet, may be gauged by comparing its range with human figures shown at lower left. formed when the arc is struck too slowly, often crack mirrors because they absorb a great deal of heat. Carbon Trim, Power Supply You are burning the proper amperage in your lamps, but the voltage supplied by your rectifiers is a trifle too low for the best light. The rectifiers you are using are better suited to 8/7 mm trims. Your present 9/8 mm trim requires at least 45 volts to produce a crater of proper diameter and depth. You will probably find that the craters in your positive carbons are too small in diameter and too deep. Just as an experiment, you might try the smaller trim (8 mm positive with 7 mm negative, both copper-coated) just to see what happens to the light. Better still, change to rectifiers supplying 50 volts. Travel-ghost isn't a good thing; and if you were getting plenty of light, you wouldn't want a trace of it in your pictures. But it would seem that you were justified in trimming your shutters a little beyond the limit to get every bit of light posible. Every lumen helps when the light is inadequate. And this brings up the subject of your screen. The white asbestos siding with which your screen is surfaced has a reflective power of approximately 65%. A regular white movie screen reflects from 75% to 80% even when perforated. We realize that asbestos siding is a common screen surface in drive-ins, but by coating this siding with a weatherproof flat white paint of good quality the reflectivity can be stepped up to at least 80%, and may even go up to 90%. It is best to obtain a paint made for this purpose and, at any rate, paints containing white lead should be avoided. White lead turns yellowish in the course of time due to the formation of brownish-black lead sulfide. Proper Screen Surface It may be best to use such a white surface on your new CinemaScope screen. The use of an aluminized surface is ruled out if your projectors tilt upwards toward the screen. With an upward projection angle, an aluminum screen would merely throw most of the light into the sky where it would do no good. It is unfortunate that no one has yet devised a weatherproof beaded INTERNATIONAL PROJECTIONIST OCTOBER 1954