GENERATION, CONTROL, AND MEASUREMENT 185 



num or magnesium in an atmosphere of oxygen. Such sources produce 

 a continuous spectrum that is rich in visible and ultraviolet, and the flash 

 lasts for 50-100 msec. As these lamps can produce only one intense flash, 

 their usefulness for biological work is very limited. 



EXPLODED WIRES 



One of the most intense sources known is that of wires exploded by a 

 high-voltage capacitor discharge. Such a discharge can produce bright- 

 ness values of over 150,000 c mm~^ and a spectral energy distribution 

 corresponding to a black body at 20,000°K (Conn, 1951). The duration 

 is of the order of microseconds per flash. Such wires can be arranged in 

 a magazine and fired in quick succession to produce repetitive flashes. 



SPARKS 



Repetitive flashes can be obtained by gaseous discharge sources, either 

 as open arcs, as sparks, or as enclosed discharge lamps in various gases 

 and metallic vapors. The open spark has the advantage of relative sim- 

 plicity, easy replacement, and almost unlimited power input. This type 

 of source is used extensively for ballistic studies of projectiles, explod- 

 ing objects, and rotating machinery. Such discharges from capacitors 

 charged to high voltages have color temperatures in the range 6000°- 

 7000°K. The spectral characteristics of the discharge are determined in 

 large part by the nature of the electrodes, for which aluminum, tungsten, 

 and various iron alloys have been used extensively (Frungel, 1948; 

 Hollaender and Foerst, 1933; Hoyt and McCormick, 1950; Quinn and 

 Bourque, 1951). 



STROBOSCOPIC DISCHARGE LAMP 



Practically any discharge lamp can be operated at very high intensities 

 of short duration by discharging high-voltage capacitors through it. 

 When used as a stroboscopic source, the tube is flashed at the frequency 

 or a subfrequency of the speed of rotation of a mechanical system. The 

 mechanism then appears to be stationary, and many detailed operations 

 can be observed. The spectral characteristics of flash lamps are deter- 

 mined by the filling gas. Krypton, xenon, neon, and mercury vapor are 

 frequently employed. High-voltage discharges produce a spectrum char- 

 acteristic of that of a high-pressure type of arc; the spectrum is a broad 

 continuum, usually concentrated in the visible and near ultraviolet. 



Flash tubes and sparks are triggered by various methods, including a 

 high-frequency discharge to start ionization, a third electrode placed close 

 to a pool of mercury or other electrode for initiating ionization, or the 

 use of grid-controlled gas- type tubes for controlling the main discharge 

 (Bourne, 1948; Forsythe and Adams, 1948). 



