International projectionist (Jan 1963-June 1965)

FIG. 3 — A xenon lamp of European manufacture (Zeiss Ikon). The square metal box seen in the rear of the lamphouse contains the high-voltage igniter unit which starts the xenon bulb burning. XENON from page 6 pending upon the size and rating of the particular xenon bulb used. The xenon arc, itself, is a blue-white, intensely bright flame-like arc which "fans out" from a brilliant gas ball close to the tip of the lower (negative) electrode — the cathode. The lamp is provided with an auxiliary spherical mirror which superimposes a reversed image of the fanshaped flame upon the actual flame, thus effecting great uniformity of the screen illumination. Power Requirements for Xenon Xenon bulbs normally have an extremely long useful life, and are seldom discarded before blackening of the quartz bulb, caused by slow evaporation of the tungsten electrodes, requires too great an increase in arc current to maintain the initial high light output. It should especially be remembered that AC ripple pulsations in the DC supplied to a xenon bulb shorten its life by "atomizing" the tungsten cathode and increasing the length of the arc. A longer arc increases the operating voltage which, in turn, increases the thermal stress and hastens breakdown. It is generally believed that an operating voltage much in excess of 30 volts is unsafe, and indicates that the bulb should be replaced with a new one. The "ripple content" of the direct current supplied to a xenon bulb should never exceed 17% of the maximum DC voltage, otherwise bulb life will be materially shortened. Certain old-style carbon-arc rectifiers are therefore unsuitable for xenon operation. Then, too, a current source for xenon lamps should have voltage characteristics which impress the required 60 75 volts upon the bulb after the initial high-voltage discharge, then automatically dropping to the normal operating voltage of 20 30 volts when normal current is being consumed. A low ripple content and the proper voltage characteristics are the two imperative reasons why rectifiers for xenon lamps should be specifically designed for the job. The Strong Electric Corporation of Toledo manufacturers excellent silicon-diode rectifiers for xenon operation. However, motor-generator sets having a terminal voltage of 60 to 80 or so volts may also be used after certain 16 changes have been made in the ballast rheostats. Consult the manufacturer of the xenon lamps before making these changes. Two Mirrors in Lamphouse The xenon lamphouse, itself, is very similar to the lamphouses designed for reflector-type carbon arcs, except that it is mechanically simpler. There are no motors, crater-positioning devices, or carbon-feeding mechanisms. But unlike all carbon-arc lamps except the "blown arc," which has a cylindrical flame, the xenon lamp employs a small sperical-surfaced "backing mirror" to intercept and utilize the light which would otherwise be wasted. By forming an inverted image of the xenon arc upon the actual arc inside the bulb, the auxiliary mirror practically doubles the screen illumination and gives it greater uniformity. Correct optical adjustment of this mirror is very important. The large main reflector functions exactly like the elliptical mirror in a carbon-arc lamphouse. It intercepts the light from the xenon arc ( plus the light from the image superposed upon it by the auxiliary mirror) and focuses it as an intensely bright "spot" upon the film aperture, Certain European manufacturers of xenon lamp equipment place a "honeycomb" relay condenser, consisting of two reticulated pressed-glass lenses, in the cone of the lamphouse. The intention is to insure a high degree of screen-light uniformity. We know that honeycomb condensers cause a loss of light amounting to nearly 20% : and reports have come to us that they are hard to clean and are liable to cracking. American-made xenon lamps attain a uniformly distributed maximum screen light without the use of troublesome honeycomb condensers. Xenon-Lamp Safety Precautions The high pressure of the gas in xenon bulbs demands a strict observance of safety precautions. If carelessly or accidentally broken, xenon bulbs explode and throw sharp fragments of quartz glass in all directions. This is why xenon bulbs are encased in a removable protective shell of tough plastic. The shell must be left on while the bulb is being installed in the lamphouse, and not removed until the cables are connected to the two bulb terminals (upper one positive, lower one negative). In addition to the plastic shell for the bulb, a protective face mask is supplied by the lamp manufacturer ( obtainable as Part No. 76181 from the Strong Electric Corp. ) . The projectionist shall not fail to wear the protective mask when handling the bulb and when opening the lamphouse! Even though accidental breakage of xenon bulbs rarely occurs, the possibility of losing an eye through an explosion should provide sufficient inceptive to guard against carelessness and wear the face mask when handling, installing, adjusting, inspecting, and removing xenon bulbs. The face mask shall be worn while cleaning the xenon bulb and the lamphouse mirrors. Never touch the quartz envelope, as fingerprints will burn in and dull the bulb. If fingerprints are made, the bulb should be cleaned with chemically pure methyl ("wood") alcohol. Wear the face mask and replace the plastic protective shell on the xenon bulb when cleaning the lamphouse or using tools which might conceivably break the bulb. When removing the bulb from the lamphouse. put the face mask on before opening the door. Encase the bulb in its protective shell before disconnecting the terminal cables. Discarded xenon bulbs should be wrapped in International Projectionist July, 1963