Journal of the Society of Motion Picture Engineers (1930-1949)

302 BOURNE AND BEESON September RELATIVE ENERGY UNITS — PO OJ -k — D 0 3 0 0 w° 0 LA^ p / kT KW r n CO ^JTI MUC US 1 1 II n p— i n ooc 40 00 WF 50 F l>i 00 HT . 1 60 00 70( " — — . Lfi MP AT 0 K W FL ASH HO * IS tc. __ ^ \ 3000 6000 4000 5000 WAVELENGTH X Fig. 2. Spectral distribution of mercury cadmium lamp. 7000 are required, together with still higher lighting intensities. In this field, too, the new lighting unit described in this paper should be more effective than other sources hitherto available. THE COMPACT-SOURCE LAMP The compact-source mercury vapor lamp which is available in sizes from 250 to 10,000 w in England is now also becoming known in the United States.7 A typical lamp, which is shown in Fig. 1, consists of a spherical transparent quartz bulb containing mercury and a rare gas filling. Two tungsten electrodes between which an arc operates are sealed into the bulb and the current is led in by molybdenum foil seals diametrically opposite to one another. Brass caps are fitted at tne end of the seals. Connections are made through a flexible lead to the anode and through the special base to the cathode of the lamp. The light source itself is small in size and has a brightness of approximately 35,000 candles/sq cm with a luminous efficiency of 45 to 50 lumens per watt. At the normal rated power the life of the lamp averages 500 hr. An important feature of the compactsource lamp is that it may be operated for short periods at ratings greatly in excess of its normal power, and under these pulsed conditions it produces a correspondingly high light output.8 The radiation from a compact-source mercury vapor lamp is dis