88 RADIATION HIOLOGY 



LIP'E AM) l)i;i'Hi:('IATION 



Life ratings of all electrit'-dist'harge lamps involve complicated factors 

 of economy as \vi>ll as physical mortality. The hurn-out life is usually 

 that at which one-half the lamps of a test fiiroup reach a physical or 

 economic end of life. The latter is very dependent on variable factors of 

 lamp and electric costs and the nature of the ai^plication. For oj)erating 

 intervals of less than 5 hr the life decreases rapidly with the length of the 

 operating interval. For longer operating intervals the total life may 

 increase to several times the 5-hr interval rating. The general form of 

 the depreciation curve is that of Fig. 2-11. 



An important variable in the depreciation of all higher pressure mercury 

 arcs is the "solarization" effect of the ultraviolet and heat on the trans- 

 mission of the fused (juartz generally used. The effect increases as the 

 wave length decreases and may reduce the output of 2200-2500 A energy 

 to one-third the initial and of 2500-2800 A to one-half the initial in a 

 few hours of operation. This accounts for some of the discrepancies in 

 published data on the shorter ultraviolet output of commercial sources. 

 The effect is relatively small at wave lengths longer than 2800 A. 



RESEARCH DETERMINATION OF OUTPUT AND INTENSITY 



As was emphasized for low-pressure sources, although their commercial 

 radiation ratings should be fully specified in all research reports, these 

 ratings can be considered only a first approximation to the energy output 

 and intensity actually effective in any specific research. Whenever 

 direct measurements of energy at the point of application are impractical, 

 calibrated lamps should be used. The intensities that they provide can 

 be calculated for various distances by the methods outlined for low- 

 pressure sources as long as there are no intervening optical systems or 

 filters. 



In work at wave lengths less than 2800 A, and especially with the higher 

 pressure sources, the source itself should be frequently calibrated for out- 

 put where direct measurements of the irradiated surfaces of material are 

 in practice. 



MERCURY-AMALGAM AND OTHER MET.\L ARCS 



Laboratory workers generally fabricate their own electric-discharge 

 metal- vapor arcs, other than mercury, although such lamps have been 

 occasionally imported for sale in the United States. Various commercial 

 types availal)l(' in the i)ast are described by Meyer and Seitz (1949). 

 Their availability is too uncertain and thcii- radiation characteristics are 

 too unstandardized for inclusion in this chapter. It should be noted 

 here, however, that cadmium-amalgam lamps proxiding low-intensity 

 cadmium lines of wa\-e lengths greater than .SOOO A arc commercially 



