International projectionist (Jan 1961-Dec 1962)

needle-like light source enables it to be placed very close to the film aperture. As a matter of fact, the entire optical assembly consisting of lamp, cylindrical mirror, condensing lenses, and ultraviolet filter is so compact that two complete assemblies are placed together, the non-operating one ready to drop into place automatically when the first short-lived tube burns out. The pulsed mercury lamp emits only a few weak lines in the infrared, and thus more closely approaches the ideal of a "cold" illuminant than any other light source. Even so, the tiny quartz tube is surrounded by cool running water to prevent melting and bursting under the operating pressure of about 1500 pounds per square inch. A complicated vacuum-tube "pulsator" apparatus having a thyratron tube synchronized to the intermittent flywheel of the projector is a necessary part of the pulsed-light system. Pulsed Light Flickerless The elimination of the revolving shutter is, as we said, a prime advantage of the pulsed mercury lamp. Moreover, the use of three 2/4 -millisecond "flashes" per frame results in a screen-light frequency so high that the human eye can detect no flicker even in the brightest highlights of the picture, or when the projector is run without film. This is the same frequency as that produced by a 3-blade shutter, viz. 72 per second at normal soundfilm speed. Perfectly flickerless projection has long been the dream of motion picture technicians; and even though American audiences briefly enjoyed the absence of shutter-flicker in a few early Todd-AO productions, the propensity of film producers to pinch pennies in the wrong places has reduced the frame-rate of 70-mm productions from 30 to 24 per second, and hence the screen-light frequency from a flickerless 60 to a flickering 48 per second. The pulsed-light shutterless projector employs a standard 3-to-l geneva intermittent movement. This is made possible by the short duration of the light pulses (2'/2 milliseconds or 1/400 of a second). Although this allows an extremely small latitude of safety when 3 pulses per frame are employed (only 93/100,000 sec. on the leading edge of the first pulse and on the trailing edge of the third), there is no danger of travel ghost only so long as the pulsator functions properly and does not broaden the duration of the pulses by more than 93/200,000 sec. (less than Yi millisecond). Luminous Output The luminous output of the pulsedlight projector was rated as high as 6000 lumens with CinemaScope aperture, Petzval-type coated f / 1.6 lens, and anamorphic attachment. A more usual estimate is 5000 lumens under these conditions at the maximum load of 800 watts. We know that a CinemaScope aperture passes 1.21 times more light than a standard non-anamorphic aperture because it has 1.21 the area, but we also know that a CinemaScope anamorphic attachment wastes most, and sometimes all, of this extra light. In addition to this consideration, Petzval lenses are no longer approved or used in American theatres except in cases where extremely long focal lengths are required for long, narrow auditoriums. Modern Gaussian lenses are necessary for a wide field of shap focus in modern widescreen projection; and this type of lens contains one or two elements more than the simple Petzval lens widely used a decade or more ago. Our estimate of 4500 lumens for the average total light output of the pulsed-light projector at maximum current load and employing a Gaussian lens or f / 1.7 speed is therefore most generous and realistic, and places this novel machine 500 lumens ahead of a conventional projector employing the most powerful xenon short-arc lamp available for motion-picture projection. The details of the relative merits, advantages, and disadvantages of these "futuristic" European projection systems nevertheles do not constitute the burden of this article. What we wish to make clear are the more demanding requirements of theatre projection in the United States — requirements which can be met ONLY by the use of highpowered American high-intensity car FIG. 3 — The time sequence of a pulsed-light source in a projector having a standard 3-to-l geneva intermittent movement. The lamp is timed to emit 3 brief flashes per frame, giving 72 flashes per second, a frequency so high as to be completely flickerless. sec. i 24 sec K-^; sec.->Ki Pulldown i Ligkt L_L 400 sec»"*| i i 96 sec 93 100000 sec. 72 sec. >i International Projectionist February 1961