178 RADIATION BIOLOGY 



is used principally as a laboratory source of monochromatic energy. The 

 lamp contains both a filamentous cathode and an anode at each end. 

 Since neon is present for initiating the discharge, the radiation is charac- 

 teristic of that of neon during the first few minutes of operation, after 

 which sodium atoms take over the discharge. The characteristic line 

 spectra of cesium, potassium, and rubidium may be obtained with lamps 

 containing these elements separately (Beese, 1946). 



MISCELLANEOUS DISCHARGE LAMPS 



There are a number of discharge lamps of low power designed for 

 specialized applications. When the lamps are of the high-voltage low- 

 pressure type, they are known as "Geisler tubes." The so-called "glow 

 lamps" are of the low-voltage low-pressure type. The radiation of the 

 glow lamps can be modulated readily wdth alternating current. They 

 are used as voltage-stabilizing elements in electrical circuits, since the 

 voltage drop across them tends to be constant and characteristic of the 

 ionization potential of the gas. 



The helium tube is especially useful for the calibration of spectropho- 

 tometers, since there is only one strong line in the visible at 587.6 m^u 

 (Table 3-12). It is an excellent monochromatic source for interferometry 

 and other optical applications. The argon lamp emits radiation rich in 

 ultraviolet, wdth relatively little visible, and consequently has been used 

 without filtering as an ultraviolet source for exciting fluorescence. The 

 neon lamp is high in red-orange energy and is available both as a low- 

 pressure discharge in the conventional low-wattage glow lamp and as a 

 moderately high-pressure discharge with oxide-coated cathodes. Lamps 

 of this latter type have been manufactured in sizes of several hundred 



watts. 



FLUORESCENT LAMPS 



Ultraviolet resonance radiant energy at 253.7 m^u, produced by a low- 

 pressure mercury discharge, is absorbed by a phosphor coating on the 

 inner side of the fluorescent lamp tube and reemitted at longer wave 

 lengths. The thin layer of phosphor crystals serves as an efficient radi- 

 ation transformer, absorbing quanta of one frequency and reemitting 

 quanta of a lower frequency in accordance with Stokes' law. A compre- 

 hensive treatment of the fluorescent lamp is presented by Forsythe and 

 Adams (1948). 



PHOSPHORS 



The ideal phosphor coating absorbs all the 253.7-m)u energy and reemits 

 that energy with a quantum efficiency of 1. Actual phosphors very 

 nearly attain this ideal with quantum eflftciencies of 0.76-0.90. Even 

 with a quantum efficiency of 1, there is a large loss in radiant energy. 

 As an example, the conversion of a high-energy ultraviolet quantum at 



