International projectionist (July-Dec 1934)

12 INTERNATIONAL PROJECTIONIST October 1934 FIGURE 5 Spectral energy distribution from white-flame carbon arc 4000 5000 WAVE LENGTH IN ANGSTROM UNITS light pick-up from a solid angle of 45 deg. to one of approximately 100 deg. In this type of lamp, diagrammatically illustrated in Figs. 3 and 4, the carbon trim is horizontal with the crater of the positive carbon directly facing the mirror, thus exposing its full area to the optical system and at the same time simplifying the problem of the automatic control. At arc currents of 28 to 42 amps, with positive carbons 12 or 13 mm. in diameter, the D. C. low-intensity reflector arc provides a sufficient intensity of screen illumination for theatres of considerable size. More lamps of this type than of any other are used in motion picture theatres today. They usually are operated from the standard voltage line through a transformer and rectifier or a motor generator set. The arc voltage is usually about 55 volts in this type of lamp. The intrinsic brilliancy of the crater in the low-intensity neutral-cored carbon arc is limited by the vaporizing temperature of carbon, which is 4,126 deg. K. When this temperature has been reached, further increase in arc current will increase the crater area and the rate of carbon consumption, but cannot increase the intrinsic brilliancy of the crater any more than the boiling point of water in open air can be raised by applying more heat. Light from this arc approaches pure white in color, but retains a definite tinge of yellow. The Flame Arc Introduction of flame-supporting minerals in the cores of carbon electrodes transfers the principal source of light from the tips of the carbons, as in the neutral cored carbon arc, to the flamelike arc stream, and provides a much longer arc than that between neutral cored carbons. There is little difference in the appearance, behavior, or output of the flame arc on D. C. or A. C. The intrinsic brilliancy of the A. C. flame arc is relatively low, but, because of its size, it gives off a large volume of light and is a very efficient source of radiation. For 7000 like reason certain types of flame arcs are highly efficient sources of ultra-violet radiation. An important characteristic of the flame arc is its flexibility, that is, the possibility of modifying the quality of the light by the choice of flame-supporting materials in the core. In Fig. 5 is shown the energy distribution for the whiteflame carbon arc, the core of which contains minerals of the rare earth group, particularly cerium. The resultant light is bluish white, closely resembling daylight. Carbons having polymetallic cores containing several metals including iron, nickel, and aluminum (known commercially as the "C" type) gives less visible light than the white-flame carbon, but FIG. 4. Low-intensity D. C. reflector arc lamp with elliptical mirror much greater ultra-violet output. Strontium increases the red and infra-red emission from the arc with slightly lower ultra-violet output than that from the cerium cored carbon. Wide Industrial Application The white-flame carbon arc finds extensive application in photography, where its light is considered photographically equivalent to daylight. It is used more largely in photo-engraving than any other light source. In light therapy a similar carbon known as the "Sunshine" carbon is used extensively where it is desired to duplicate artificially the physiological effects of natural sunlight. The "C" type and the strontium-cored "E" type carbons likewise find extensive therapeutic use where emphasis on the ultra-violet or the infra-red bands of radiation is desired. There is increasing use of the flame arc in industrial and photochemical processes such as accelerated testing of paints and dyes, the processing of certain materials, such as linoleum, patent leather, and tobacco, and the rapidly extending irradiation of milk for the purpose of increasing the vitamin D content. The low-intensity A. C. white flame arc has found limited application in small motion picture theatres where D. C. is not available; but the superiority of the D. C. low-intensity reflector arc, operated from a suitable rectifier or motor generator set, has given it preference over the former type for projection purposes. Reference will be made later to a recent development that promises to modify greatly projection practice in theatres of small and intermediate sizes. The various types of lamps using flame type carbons embrace a wide range of arc conditions. Arc currents from 6 to 100 amps, or more are used, and arc voltages from 25 to 60. The large lamps most popular in industrial applications and light therapy at the present time are operated at 60 amps, and 50 volts on A. C. and at 50 amps, and 60 volts on D. C. D. C High-Intensity Arc Development of the high-intensity D. C. arc overcame the limitation of intrinsic brilliancy referred to in the discussion of the low-intensity arc. The positive electrodes of the D. C. high-intensity arc are operated at current densities much higher than those of the low-intensity arc — 450 to 860 amps, per square inch (70 to 133 amps, per square centimeter) for the former in comparison with 120 to 200 amps, per square inch (18.5 to 31 amps, per square centimeter) for the latter. Cored positive carbons 9 to 16 mm. in diameter without metallic coatings are operated with copper-coated negative carbons of smaller diameter at arc currents ranging from 60 to 190 amps., the arc voltage ranging from 45 to 90 volts. In the condenser type high-intensity arc lamp as indicated in Fig. 6, the positive carbon is held in a horizontal position with its crater directly facing the condenser assembly. Since the current density in the carbon is very high and the high efficiency of this type of arc is dependent on the maintenance of a wellformed cup-like crater, the positive carbon is allowed to project but a short distance from the holder and is rotated FIGURE 7 (below) . H. lamp I. reflector type FIG. 6. H. I. condenser type lamp APERTURE PLATE APERTURE PLATE