International projectionist (Jan-Dec 1946)

Record Details:

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rv FIGURE 4. Filament of doubly spiralized wire (HO V, 250 W). Note the connections of the two spiral sections: the current flows through both in the same direction, so that there is only half the lamp voltage between each corresponding pair of points in the two sections. Because of this the two sections may be situaated closer together than when the two extremities at which the current is applied and taken off are situated side by side at the same height. pecially when an auxiliary mirror is used in addition. These two-plane filaments are far superior to the others. In the case of one such lamp, by means of a combination of methods — two-plane assembly, low voltage and auxiliary mirror — a brightness of 4150 cp/sc has been obtained. This is certainly the highest value that has yet been attained with incandescent lamps. For the sake of comparison it may be mentioned that the spiral of an ordinary 220-volt lamp for the home has a brightness of 612 cp/sc a fluorescent tubular lamp 0.3. the carbon arc 18,000, the highintensity arc 72,000. the water-cooled super high-pressure mercury lamp in the axis of the discharge 41,000 cp/sc. The measures described all have the effect of promoting a greater concentration of the filament and a higher average brightness. Their effect is, however, partly cancelled by a more irregular distribution of temperature, of which they themselves are the cause. Since the temperature of the hottest part of the filament determines the lifetime, the more irregular the distribution of the temperature the lower the average temperature must be chosen for a given lifetime. Because of this, part of the gain in brightness must be sacrificed. This effect— the unfavorable ratio between highest and average temperature — is already appreciable upon passing over from single to double spiral; it is stronger in the case of the transition from single-plane to two-plane filament; and it is further accentuated when an auxiliary mirror is added because of the fact that part of the reflected radiation falls upon the filament again and raises its temperature locally. If the decreased lifetime be accepted, it would actually have been possible to make some gain in brightness without the mirror by raising the temperature of the filament. Independent of this controversy, however, remains the gain obtained in the projector due to the fact that with more uniform brightness the image of the filament can be focussed closer to the film aperture. In speaking of the non-uniformi,ty of the brightness of the filament we have FIGURE 5. Two-plane filament, seen obliquely from above. The system of hooks which protect the spiral sections on the upper side from lateral deviations is fastened rigidlv to the leads; the corresponding hooks on the lower side, on the contrary, can move up and down in a vertical direction thanks to an arrangement of two tubes which slide over the leads (so-called sag arrangement). The insulators should be noted between the hooks and the leads. had in mind until now the more or less periodic variations which are encountered as one passes across the spiral sections and the intermediate spaces of the filament, or even along the length of a spiral over the windings. Superimposed on this is a gradual decrease in brightness from the middle to the edges of the filament, caused by the fact that the FIGURE 6. Distribution of brightness in the filament image obtained with a two-plane filament with auxiliary mirror. Above, the distribution measured along the line a-b; to the right, that along c-d. The brightness is measured with a photocell of about half the width of the spiral image. It is plotted in arbitrary units. a— FIGURE 7. The instrument for adjusting the filament with respect to the centering socket. To the right on the screen may be seen the filament images, one of which is projected directly on the screen, and the other, seen' perpendicular to it, is projected via a prism. By turning and shifting the lamp the images must be made to coincide with a drawing on the screen. The correct position of this drawing with respect to the lamp holder is checked from time to time with a normal lamp. edges cool more rapidly because of the unhindered lateral radiation, [Figi 6j. Of itself this not too rapid decrease toward the edges [which is reproduced in the illumination of the film picture and consequently also in the illumination on the screen] is not serious difficulty. It necessitates, however, a certain correction in the statement made previously about the required size of the filament: it is indeed an advantage to make the filament somewhat larger than the minimum size. When this is done only the middle, hottest part of the filament is used, whose average brightnes may be corrsiderably higher than the rest. ■ And in practice this advantage is still further increased by the fact that at a given voltage a larger filament corresponds to a higher power and consequently a higher current, a thicker wire and thus a higher permissible temperature for the same lifetime. Gas-Filling Procedure Now a few words about the gas filling. The voltage at which, for a given distance between sections of the Spiral, breakdown may happen depends upon the gas. For normal lamps a mixture of nitrogen and argon is used. The argon causes less loss of energy i>y the conduction of heat and therefore improves the light yield per watt, but it decreases the voltage for breakdown. Because for film projection the specific light yield is less important than a maximum concentration, these lamps are filled with pure (Continued on page 36) 20 INTERNATIONAL PROJECTIONIST • December 1946