International projectionist (Jan 1961-Dec 1962)

Volume 36 February, 1961 Number 2 "tow-Powered" Projection Ruled Out By "High-Powered" American Standards cROM THE TECHNOLOGICAL I point of view, two European projection lighting developments of the last decade are of outstanding interest. These are the xenon short-arc bulb and the "pulsed" mercury-vapor tube, the latter permitting simplified, more compact projector design, cooler operation, the complete elimination of shutterflicker, and the elimination of the shutter, itself. The revolving shutter of the conventional projector wastes a large proportion of the light output of illuminants which burn in a continuous manner. The xenon lamp, developed in Germany, has a high color temperature, a smooth spectrum in the visible region, and has the advantage of clean, reasonably trouble-free operation throughout its long useful life (8001200 hours). There is no mirror pitting with xenon lamps, though theoretically the possibility of dangerous explosion exists. The xenon lamp emits no gases or dust, and hence requires no exhaust ventilation. Because the quartz-glass bulb of the xenon gas-discharge lamp is gradually blackened by tungsten vaporized from the intensely heated electrodes, the luminous efficiency of the lamp decreases 25-40 per cent during its useful life. These lamps accordingly have to be replaced in pairs of equal light output from the two projectors. Special high-voltage ignition devices are necessary for the operation of xenon lamps, and only low-ripple rectifiers are suitable. Xenon lamps emit strongly in the near-infrared, and hence must be regarded as "hot' light sources — much more heat-producing, lumen for lumen, than the high-intensity carbon arc. This drawback is met by the use of heat filters or "cold'' mirrors. Xenon lamps are less expensive to operate (for equal light outputs) than tungstenlilament bulbs, but are not necessarily By ROBERT A. MITCHELL New European developments in projection technology have rightly evoked the admiration of projection' ists everywhere. "Why aren't xenon lamps and shutterless projectors used in the United States?" The unvarnished reasons, explained in this article, are fundamentally the same reasons why tungsten-filament bulbs and low-intensity arcs are no longer suitable for theatre projection on this side of the Atlantic. more economical than high-intensity carbon arcs. Xenon Lamp Optically Inferior The size and diffuse character of the glowing xenon gas discharge precludes the high optical efficiency which has always made the carbon arc attractive to projection engineers; and the fanlike shape of the xenon light source precludes optimum screen-illumination uniformity except when a special optical system, such as the "honeycomb" relay-condenser, is used. The interposition of the two raster plates of the honeycomb condenser in the light beam wastes considerable light by surface reflections and adhering dust particles. The prime disadvantage of the xenon lamp from the American point of view (explained in detail anon) is its low luminous output. This, measured with a projector employing a standard aperture, revolving shutter of 50 per cent transmission, and f / 1.7 coated Gauss-type projection lens, is only 2400 lumens for the 1000-watt xenon bulb and 4000 lumens for the 1800-watt bulb, each in optimum optical adjustment and fitted with a spherical aux iliary reflector. Soviet illumination engineers are presently engaged in research aimed at increasing the luminous output of the xenon short arc to make this type of illuminant suitable for theatres of at least moderate size, but in view of the inevitable troublesome sublimation of the tungsten electrodes, we fail to see how this can be accomplished without increasing the area of the light source and thereby decreasing its optical efficiency still more. Pulsed Lamp More Promising The "pulsed" high-pressure mercury lamp developed in Holland appears to hold greater promise than the xenon lamp in spite of the somewhat unsatisfactory spectral-energy distribution of its radiation. This modern light source gives a blue-white light of a slightly greenish tinge, but actually lacks the blue and deep red wavelengths which would make it less color-distorting when used for the projection of dyeimage color films. Aside from the spectral characteristics of its radiation, the pulsed source is extremely efficient from the optical point of view and eliminates shutter losses completely. The small size of the International Projectionist February 1961