International projectionist (Jan 1963-June 1965)

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luminous powers of lamps, the lumen values indicate :he light flux from the projector without the shutter. (Divide by 2 to find the approximate value with the shutter running.) Table 2 supplies similar data for the three models of xenon projection lamp equipment produced by the Strong Electric Corporation of Toledo, Ohio. TABLE 1 SIMPLIFIED HI CARBON-ARC LAMPS Carbon Trim Amps Lumens (No shutter) Screen Width for 10 Footlamberts 85% Matte 150% Directional 7 mm-6 mm 40 7,000 20' 26' 7 mm-6 mm 42 8,600 22' 29' 7 mm-6 mm 46 10,000 24' 32' 7 mm-6 mm 50 11,700 26' 34' 7 V2 mm-6 Vi mm 55 12,500 27' 35' 8 mm-7 mm 60 13,100 28' 36' TABLE 2 XENON LAMPS Xenon Bulb Wattac ie Lumens (No shutter) Screen Width for 10 Footlamberts 85% Matte 150% Directiona 900 W 4,500 16' 21' 1600 W 9,000 23' 30' 2500 W 12,500 27' 35' LACK -A A + WIRING DIAGRAM PLATE 1799 NO. 14 STRANDED WIRE NO. 4 STRANDED WIRE ±i± LINE 000 SAFETY SWITCH FIG. 2 — Wiring schematic for a xenon projection lamp powered by a 3-phase rectifier. A comparison of these two tables serves to show that the highest-powered xenon projection lamp (the 2500-W model) is equivalent to a Superex-type lamp burning a hypothetical 7% mm 6V2 mm carbon trim at 55 amps, to give 12,500 screen lumens without the projector shutter running. This is why we say that theatres needing more than 55 amps, of arc current for adequate screen illumination must make use of high-intensity carbon arcs. Xenon lamps have already proved popular in hundreds of the smaller theatres in Europe and in dozens of preview rooms in the United States. Their use in American theatres has only just begun, and with extremely gratifying results from the point of view of all concerned — the moviegoer, the projectionist, and the theatre owner. And once again we must point out the fact that color films are seen at their best when projected by pure daylight-white xenon light. The xenon lamp is undoubtedly very desirable for the critical visual evaluation of color prints in studio and laboratory projection rooms because of the freedom from color-temperature changes of this light source. Anatomy of Xenon Bulb The structure of the xenon short-arc bulb is simplicity, itself. The transparent "envelope" is made of fused quartz, a material which is more resistant to the softening effects of heat than ordinary glass, and much less likely to crack from sudden temperature changes. Even so, it is plain commonsense to protect the bulb by preventing down-drafts and rain from entering the lamphouse through ventilating pipes. The two cone-shaped electrodes inside the strong quartz envelope are made of pure tungsten, a silvery-gray metal which has a very high melting point. The larger electrode is the positive one (anode) , and is positioned above the smaller negative electrode {cathode) when the bulb is installed in the lamphouse. The bulb is filled with the chemically inert gas xenon under a pressure of from 8 to 10 times that of the atmosphere. The pressure increases to 20 or 30 atmospheres when the bulb is in operation. Any increase in the current at which the bulb is burned increases the temperature, and hence the pressure of the gas inside it. Xenon, itself is a very rare constituent of the air ( 1 cubic foot of xenon in 15,000,000 cu. ft. of air), and is obtained by the fractional distillation of liquefied air. This is the only way to get it because xenon, like its sister elements of the helium family, forms no chemical compounds whatever. Igniting the Xenon Bulb The flamelike arc in a xenon bulb is started by a high-voltage discharge which breaks down the electrical resistance of the gas and establishes a conducting path between the two tungsten electrodes. The source of momentary high voltage (20,000 30,000 volts) is an "igniter" device consisting of an induction coil and capacitors, and is usually installed as a compact unit inside the lamphouse to keep the high-voltage leads as short as possible. Once a conducting path has been established by the high-voltage spark (applied for only a fraction of a second), the regular current supply for the lamp takes over. This is ordinarily a rectifier designed to furnish an initial voltage of 60 to 75 volts to establish a directcurrent discharge. The arc thus established, the voltage spontaneously drops to the normal operating value of 20 to 30 volts in a current range of 30 to 70 amperes, de (Please turn to page 16) International Projectionist July, 1963