164 RADIATION BIOLOGY 



GASEOUS DISCHARGE LAMPS 

 GENERAL CHARACTERISTICS 



The gaseous discharge lamps fall into three general classes, depending 

 on the operating pressure of the conducting and radiating gas (Bourne, 

 1948; Forsythe et al., 1941; Roller, 1952). The low-pressure lamps have 

 a gas or vapor pressure of between 0.001 and 10 mm Hg; the medium- 

 pressure lamps, from 0.5 to 30 atm; and the high-pressure lamps, from 

 30 to several hundred atmospheres. The low-pressure discharge is char- 

 acterized by a diffuse luminous glow of low intensity; the spectrum con- 

 sists of sharp lines with Uttle or no continuous energy between the lines, 

 and at the lowest pressures most of the energy may be in the resonance 

 hues of the element. The medium- and high-pressure discharge lamps 

 usually have a sharply defined arc stream of high intensity. With 

 increasing pressure the lines become complex and broadened, and a 

 continuum appears bet^veen the lines owing to the high temperature of 

 the ionized gas, and at extreme pressure and current density the line 

 spectrum may be masked completely by the continuum. 



The medium-pressure arc may be operated as an open arc in air at 

 1 atm or as an enclosed arc. The open carbon arc has almost unlimited 

 power input. However, the air gases cause the rapid erosion of the 

 electrodes, and comphcated electrode feed mechanisms are required for 

 stable operation. 



Cathodes. Most gaseous discharge lamps can be designed to operate 

 on either direct or alternating current. On direct current one electrode 

 serves as the cathode or negative terminal, and the other as the anode or 

 positive terminal. The cathode supplies the electrons, which are acceler- 

 ated by the potential gradient. On alternating current each electrode is 

 a cathode for half the cycle and an anode for the other half. Therefore 

 the electrodes are usually asymmetrical in d-c lamps, with only one elec- 

 trode as the electron emitter, but in a-c lamps both electrodes are emitters. 



The discharge may be excited between either cold cathodes of large 

 area or hot cathodes of relatively small area. The cold cathode is less 

 efficient but relatively indestructible. It is used principally in lamps 

 that are operated intermittently, as in low-pressure high-voltage sign 

 lighting and flash tubes. The cold cathode is less efficient than the hot- 

 cathode emitter because of the large losses resulting from the high cathode 

 potential drop. 



The emission of electrons from a cathode surface is dependent upon 

 the work function of the surface and the temperature. To be an efficient 

 emitter, pure tungsten must operate close to its melting point, whereas 

 the addition of a small amount of thorium greatly increases the electron 

 emission and makes possible a lower operating temperature. Oxides of 

 the rare earths, especially those of barium and strontium, are the most 



