The Cine Technician (1939)

84 THE CINE TiE C H N I C I A N Aug. -Sept., 1937 to a current of air. Since too much colour in the glass would have been objectionable, it was necessary to use a density only just sufficient to reduce the heat to a safe value. The filter finally adopted was in the form of several f-inch strips of Corning extra light Aklo, 2 mm. thick, held loosely side by side in a rectangular metal frame, and cooled by a gentle current of air from a small furnace blower. The location of the filter in the optical system must be such that the edges of the glass strips are not visible on the screen. In the present instance, the glass was mounted on the front of the shutter housing at a distance of approximately 3| inches from the film gate. No trace of the edges of these strips has ever been noticed on the screen. The air was directed on to both sides of the glass by appropriate baffles. With this filter, which transmits only 25 or 30 per cent, of the total heat energy, the heat of the aperture is actually less than that obtained with some of the better projection lamps now in use. The familiar "biscuit" appearance on projected prints is entirely lacking. Part of the air from the blower is directed on to the film gate. This gives a slight additional cooling to the film and to the metal parts around the aperture. A Relay Condenser System To see what could be done with the 120 ampere high intensity arc used with a condenser system, a Hall and Connelly lamp was set up with a set of 7-inch condensers and a relay system. In a relay system full advantage can be taken of the entire crater surface because this is not imaged at the aperture. Furthermore, advantage can be taken of the fact that the entire crater area emits red light with practically uniform intensity. Since in colour work the limiting colour seems to be red, the entire crater surface can be made use of. The measurements of screen brightness made with this set-up show that it is possible to get equally as bright a screen with the "Magnarc" system, and it becomes somewhat easier to maintain the screen uniformity. Therefore, where there is sufficient room in the projection booth to accommodate the increased length of a relay system, this type of lamp would very well serve. The remarks about to be made about adjustment and operation of the optical system apply equally well to condenser systems and reflector systems. Adjustment and Operation A great number of observations were made with the best types of black and white illuminating systems at present in use in order to determine, if possible, what effect the operating conditions and the adjustment and alignment of the optical system had upon screen brightness. Based upon these observations, it is believed probable that the average theatre's projection machine often does not deliver much over half the screen illumination of which it is capable. Losses occur in many ways — accumulation of dirt on the screen lowers its reflecting power ; the reflector or condenser surface facing the arc becomes clouded with smoke, pitted with flying particles, and has to be constantly cleaned in order to preserve the light transmission. Because of the imperfections in the commercial mirrors and condensers, the screen uniformity is not at its best when the system is adjusted to give the maximum screen brightness.* The projectionist, therefore, has to sacrifice a considerable amount of screen brightness in order to improve the uniformity. Errors in the centering of condenser systems can be responsible for appreciable losses in illumination. Some projection lenses are in use which have a lower transmission than is desirable. Carbon arcs are somewhat erratic in behaviour. The crater sometimes burns unevenly, and the crater brightness varies from time to time. A substantial improvement could be made in all of these operating conditions. Possibly new equipment would have to be designed in order to free the projectionist from the necessity of constantly attending to the adjustments of the various manual controls found on the present lamps. If the arc operation could be sufficiently stabilised, and the arc crater accurately held on the optical axis, the entire system could be set up and adjusted once for all, and the projectionist would then be required to make only the single adjustment of keeping the arc crater in the correct position along the optical axis. There is no reason, furthermore, why an arrangement using photo-electric cells could not be devised that would make even this adjustment automatically. Reduction of the Shutter Loss Since 50 per cent, or more of the incident light is lost at the shutter, it seemed worth while to attempt to recover some of this loss by speeding up the pull down movement, and using shutter blades of the narrowest possible angle. No originality is claimed for the method used. Inside the housing of the Geneva pull-down mechanism used on all Simplex machines there is a pair of small spur gears through * Cook, A. A. — "A Review of Projector and Screen Characteristics and their Effects upon Screen Brightness," /. Soc. Mot. Pict. Eng., XXVI. (May, 1936), p. 522. Axial View. View from Bottom of Screen. View from Top of Screen. View from Side of Screen. FIGURE v