International projectionist (Jan-Dec 1955)

temperatures in order to achieve full brilliance, consequently their life is considerably shorter than that of ordinary household lamps. In spite of these "high efficiency" requirements. Sylvania projection lamps last longer — from 10 hours for 1000watt lamps up to 100 hours for the 110watt lamps. Why Do Some Lamps Have Black Tops? Tremendous amounts of light are developed by the higher wattage lamps to achieve full brilliance for large size screens. The black caps on these lamps absorb excess light that would other CONDENSERS '¥P¥ FILM GATE OBJECTIVE LENS LIGHT LOSS LIGHT LOSS WITH OVERSIZED LAMP wise radiate throughout darkened rooms from the machine's ventilating system, interferring with viewer's vision. Shutter Blade Width & Image Flicker The question of flicker in the projected screen image and its direct relationship to shutter blade width and illumination level has been discussed frequently in these columns — yet it constantly keeps cropping up. Here it is again. By ALLAN P. MULCAHY Rossland, B. C, Canada A brother projectionist who has been in the craft ever so many years longer than I abides by a cardinal rule that as one increases the illumination one should increase the width of the master (and flicker) shutter blade in order to eliminate flicker. Originally he had a set-up of Simplex Regular heads with 90-degree ar shutters and Peerless lowintensity (L-I) lamps. He asserts that when he replaced his lamps with Peerless Magnarcs he had to make the blades about 109 degrees in order to eliminate flicker. His theory would be that since the amount of light is greater with H-I light than with L-I, then the period of darkness during the pull-down should be longer to suppress flicker. Shutter Cut-off Period It seems to me that the period of darkness should be as short as possible irrespective of any illumination level so that "persistence of vision" would more readily lend itself to the impression of illumination during the time the shutter is closed. I can see, however, that with larger lamps and larger mirrors the shutter has to cut a greater "disc" of light, thus making necessary increased shutter blades, or else moving the shutter as close to the aperture as possible, as on the Simplex X-L head. Editor's Reply: Even though the increase in light transmission is too small to be perceptible, many projectionists trim their shutter blades down beyond the theoretical limit. A shutter excessively trimmed reveals travel-ghost on the screen. Travel-ghost, however, is much fainter than the brightest areas in the picture, hence it is not seen by the average theatre patron if screen illumination be kept below a certain limital value. When a change is made in lamp equipment to increase illumination, the travelghost which previously was practically invisible suddenly shows up. As a result, the shutter blades have to be widened. This is exactly what has been done in this case. Several projector manufacturers furnish two types of shutter: one with blades of normal width for high-level screen brightness, and another having excessively trimmed blades for drive-ins and other theatres where adequate screen illumination is difficult to obtain. Still other manufacturers make their shutters with adjustable-width blades. Screen Brightness Factor If intermittent pull-down and cut-off of the light by the shutter were truly instantaneous, shutters for use with standard 3/1 Geneva intermittents should have 90-degree blades to mask all travel-ghost. Although the action of shutter and intermittent sprocket is not instantaneous, no harm is done by 90degree shutter blades if screen light be not too bright. The width of the blades is determined by such factors as size and rotational speed of the shutter, its distance from the aperture, and the diameter of the light beam where cut off by the shutter. Since the last-named factor is governed INTERNATIONAL PROJECTIONIST • APRIL 1955 by the lamp equipment, no definite rules anent blade width can be given. It is best to have the blades a trifle wider than is absolutely necessary in order to "kill" travel-ghost, rather than a bit too narrow. The amount of light gained or lost in this case is insignificant. A 3% increase of light is much too small to be seen by anyone; yet many propectionists trim their blades by 5 degrees to get this slight amount of extra light and run the risk of spoiling the clarity and steadiness of their pictures. 'Reasonable' Trimming Even when the blades aren't trimmed down enough to cause a definite "ghost," an annoying trembling of bright objects in the picture may be introduced. This is why we advocate shutter blades that are wide enough to conceal all motion of the film. If one blade of the shutter be a little wider or narrower than the other, a terrible flicker will be produced. Otherwise, the width of the blades, within reason, has no perceptible effect on flicker. BOOK REVIEW Kinoteknik, by Max Scharnberg. Third revised edition, 580 pages. Published by Jul. Forlag, Copenhagen, Denmark. This edition is an important contribution to the technical literature; it is regrettable that it is written in Danish and thus is available to only a limited number of projectionists. The history of projection is accorded a remarkably complete treatment in Kinoteknik, as are also the optical, electrical, and acoustical principles involved in the projectionist's art. The bulk of this comprehensive textbook, however, is devoted to practical instruction in 35 and 16-mm sound projection. Equipment commonly used in Denmark is described in detail. Famous Projector Names Missing Among the theatre projectors illustrated we find the Kalee, Aga-Baltic, Askania, Bauer, Bofa, Gaumont-Kalee, Philips, Westar ( Century -Westrex), and Zeiss-Ikon (Ernemann). But we looked in vain for our old reliable friends, the Simplex, Motiograph, and Brenkert of world-wide fame and favor! On the other hand, American textbooks of projection make no mention of well-known European machines. Among the many technical credits in the book, we note with a glow of pride, the inclusion of IP in many instances. Herr Scharnberg's treatment of projection (including color movies, theatre-Tv, 3-D, wide-screen, and CinemaScope) is comprehensive, concise, systematic, and downto-earth. Only the language-barrier is to be regretted in this valuable source book. 21