International projectionist (Jan-Dec 1947)

THE requirement mentioned previously, that it must be possible to place the filament as close as possible to the condenser, leads to the fact that the envelope, usually of a tubular form, must be given as small a diameter as possible. A limit is set by the fact that the wall of the envelope would become too hot if it were too close to the incandescent filament. This can also be enhanced by the blackening which normally appears after the course of time on the wall due to the evaporated tungsten, which is deposited on the wall and which absorbs part of the radiation and thus makes the envelope still hotter. Three methods are available for arriving at a small diameter of the envelope. In the first place, the envelope is made of a special hard glass which has a very high softening point, namely 750° K. Second, especially in projectors for 8 and 16-mm film, a forced cooling is employed by means of a fan. Third, attempts are made to prevent the deposition of the evaporated tungsten on the wall in the neighborhood of the filament. This is indeed desirable in any case because the blackening is accompanied by a gradually increasing reduction of the light during the lifetime of the lamp. The gas with which the lamp is filled plays an important part in these phenomena. While on the one hand it has the disadvantage that it becomes warm itself by heat conduction from the filament and therefore heats the envelope more strongly, on the other hand it has a favorable effect as far as the blackening is concerned, because of the fact that currents occur in the heated gas FIGURE 9. Philips 16-mm film lamp, 110 V, 10G0 W. Above the filament are two nickel plates along which the rising gas flows and upon which the particles of evaporated tungsten have an opportunity to be deposited. The upper part of the envelope is sprayed black in order to prevent light escaping through the ventilation holes of the projector into the auditorium. Incandescent LAMPS for Film PROJECTION By J. J. A. MANDERS Research Laboratory, Philips Incandescent Lamp Company, Eindhoven, Holland The third and final article in a series which covered comprehensively the requirements which have to be met in order to attain maximum efficiency with incandescent film projection lamps. Invaluable data for every professional projectionist. which carry the evaporated tungsten along, so that most of it is not deposited close to the filament but at some other spot. This effect can be reinforced by introducing nickel gauzes or plates above the filament in the rising gas current, upon which the tungsten particles are deposited (see Fig. 9 . In this way it has been possible to limit the diameter of the envelope of lamps lor 8 an! 16-mm film for 1000 and 1200 watts to the same as that for a 750-watt lamp, namely only 38 mm. Still more effective is the employment of the so-called hanging construction (Fig. 10). The lead wires, in this case above the filament, which become very hot close to the filament, function as a sort of chimney, which causes the rising current of gas to pass first along the axis of the lamp. In the spherical enlargement at the top of the envelope the gas takes on a whirling motion in which practically all of the tungsten carried with it has the opportunity of being deposited on the wall at that spot. The descending, cooled gas, in which there are only few tungsten particles left, deviates from the wall again slightly above the filament and flows toward the filament (Fig. 11). Therefore, at the height of the filament the wall remains almost entirely free of blackening, while also there is no heating of the wall here by passing hot gases. In this way in the case of the 750-watt lamp shown in Fig. 10, which had to operate without artificial cooling, the diameter could be reduced to almost the same value (44 mm) as in the case of the aforementioned lamps with forced cooling. [The gas currents described can be made visible in a very interesting way by constructing such a lamp with a tube fused on, through which, while the lamp is burning,, a small amount of air can be admitted to the lamp. The tungsten particles coming from the filament are then imnlediately oxidized to a white powder whose motion can be followed with the eye. [The well-known method of Toepler can also be followed, in which the current phenomena are made visible by means ot the accompanying very slight local variations in the index of refraction. These methods are very useful in the investigation of the shape of envelope which is most suitable for obtaining the desired course of the currents of gas.] In still another way the gas helps inprotecting the envelope. If. due to an incipient blackening or possible insufficient cooling, the envelope should become soft at a certain spot, then at that spot it would be slightly compressed by the external pressure if there was a vacuum or diminished pressure inside the lamp. It would thus come nearer the filament and become still hotter and still softer, and this accelerated process would quickly lead to the destruction of the lamp. Now, by filling the lamp with a gas at such a pressure that when hot there isa slight excess pressure in the lamp, the compression of the envelope is made impossible. The excess pressure can be employed without any danger because of the small size and thus the strength of the envelope. The relatively high pressure gives the additional advantage of a still greater increase in the permissible working temperature and an in FIGURE 10. Two Philips cinema lamps 15 V, 750 W, with the so-called hanging construction. The left-hand lamp has an auxiliary mirror of evaporated aluminum on the inside of the envelope. 14 INTERNATIONAL PROJECTIONIST January 1947 ,: