International projectionist (Jan-Dec 1936)

DEFINITE TREND TOWARD H.-I. ARCS IN THEATRES AND STUDIOS By W. C. KALB ENGINEERING DEPARTMENT, NATIONAL CARBON COMPANY Distinct improvement in the carbon arc light source has resulted in a higher quality of motion picture lighting. The present trend toward a more extensive use of the h.-i. arc is pronounced, no less in studios than in theatres — a development of prime interest to projectionists because of the interdependence of both branches for overall best results. The appended paper, presented before the S.M.P.E., draws some interesting comparisons between the various carbon arc light sources. — Editor. THE necessity for using, in projection, a light-source of very high intrinsic brilliancy is illustrated by the following example: A screen image 20 feet wide is 90,000 times the area of the 0.800-inch aperture through which the light is projected, and with a magnification of 6:1 from the crater to the aperture, is 3,240,000 times the area of that portion of the light-source focused within the aperture limits. Disregarding all losses in pick-up and transmission, an intensity of 10-foot candles incident upon the screen calls for a brightness of 111 candles/mm. at the source. A 120-degree mirror picks up only about 75 per cent of the total light emitted by the source. Losses through the film and the lens further reduce the intensity of illumination of the screen. It is therefore obvious that screen sizes now in use have gone well beyond the limit at which the low-intensity arc with a maximum intrinsic brilliancy of 175 candles/mm. can provide a satisfactory level of screen illumination. Low-Intensity Arcs. — There is a definite limit to both the intrinsic brilliancy and to the whiteness of the light available from low-intensity reflecting arcs, fixed by the subliming temperature of carbon. It has been determined that the maximum brightness temperature of the positive crater of the carbon arc is approximately 3810CK. Fig. 1 shows the energy distribution curve of the low-intensity carbon arc operated with 12-mm. positive and 8-mm. negative carbons at 30 amperes, 55 volts, d.c. ; and, for comparison, the theoretical curve of black body radiation at 3810°K. It is apparent from these curves that, with the exception of the peaks at approximately 2500 and 3900 A, which are characteristic of all carbon arcs, the energy distribution from this arc is a close approximation to the theoretical limit. The maximum intrinsic brilliancy under the conditions defined is slightly less than 175 candles/mm. High-Intensity Arcs. — The high-intensity arc is not subject to the same limitations as the low-intensity arc. Introduced to the motion picture industry more than 15 years ago, it has been described and its characteristics discussed in numerous papers. The high-intensity arc is operated at much higher currentdensities in the electrodes than is the low-intensity arc. The positive carbon burns with a deep crater and is provided with a central core containing rare-earth minerals. The vapors from this core, confined by the arc stream to the cup-like crater, attain a brilliancy far greater than that associated with the temperature at which carbon sublimes. The result is a snow-white light and a crater brilliancy much higher than that attainable in the low-intensity arc. In the larger high-intensity arcs, the brilliancy of the crater exceeds 800 candles/mm. From Fig. 2, which shows the energy distribution of a typical highintensity arc, it is evident that the colors are more evenly balanced and the light consequently whiter than that of the lowintensity arc. The larger theatres were prompt to make use of the greater volume and improved quality of light that the highintensity arc provided. However, its use for projection in small theatres was not practicable, from an economic standpoint, up to 3 or 4 years ago. The first important extension of the high-intensity principle of arc operation came with the , "•--., ,' I / / ,' Jf 2000 4000 6000 AN8STROM UNITS 8000 \tsooo ULTRA-VIOLET | V a G Y O R IHFRA-RrC FIGURE 1 Energy distribution front low-intensity arc: (solid line) 30-ampere, 55-volt, d-c. arc, 12-mm. pos.; (dotted line) radiation curve of theoretical black body at 3810°K. 2000 3000 4000 5000 6000 7000 ANGSTROM UNITS ULTRA-VIOLET | VIOLET BLUE GREEN YELLOW ORANGE RED FIGURE 2 Energy distribution from high-intensity, 125-amperey 63-volt, d-c. arc; 13.6-mm. positive carbon [15]