148 RADIATION BIOLOGY 



2. SOURCES OF RADIANT ENERGY 



Sources for the visible and adjacent spectral regions may be divided 

 arbitrarily into three general classes: the thermal radiator, the electrical 

 discharge or electron-excited source, and the fluorescent lamp. The ther- 

 mal, or complete, radiators include the sun and the incandescent lamps. 

 The spectral energy distribution is continuous, with a single maximum 

 that is in the near infrared for most artificial sources. The electrical dis- 

 charge sources include arcs and discharges in metalUc vapors and inert 

 gases. The spectrum usually consists of lines characteristic of the ele- 

 ments present in the discharge, although the line spectrum may be 

 superposed on a continuous background of thermal radiation from hot 

 arc gases and incandescent electrodes. In the fluorescent lamp, ultra- 

 violet energy from a low-pressure discharge is absorbed by a phosphor 

 coating on the inside of the lamp and then, by fluorescence, reemitted at 

 longer wave lengths. 



Although in biological research one is seldom interested directly in the 

 design or construction of lamp sources, basic data on fundamental ele- 

 ments of design and electrical and radiation characteristics can be helpful 

 in the selection of the proper lamp for a research application. It is 

 toward this objective that the following physical and engineering data 

 are given. For discussions of the characteristics and application of 

 sources of all types, reference is made to the general articles by Aldington 

 (1945), American Society for Testing Materials (1946), Barnes et al. 

 (1939), Dushman (1937), Greider (1931); to the books by Bourne (1948), 

 Forsythe (1937), Harrison et al. (1948), Illuminating Engineering Society 

 (1952), Roller (1952), Macbeth and Nickerson (1949), Strong (1943); 

 and to the manufacturer's pamphlet by Weitz (1950). 



THERMAL SOURCES 



GENERAL CHARACTERISTICS 



Black-body Radiator. The spectral energy distribution of most incan- 

 descent thermal sources can be approximated closely by the complete 

 radiator (variously referred to as "perfect," "black-body," "ideal," or 

 " Planckian ") at a hypothetical temperature. The radiation laws of such 

 a complete thermal radiator are completely derived from thermodynamic 

 and quantum theories. The "complete," or "black-body," radiator is 

 so called because it is assumed to be a complete absorber of radiant 

 energy in all spectral regions and neither transmits nor reflects radiant 

 energy. Such a hypothetical body radiates more power (flux) at any 

 given temperature than any other body at the same temperature, pro- 

 vided that the radiant flux is due solely to thermal collision. The spectral 

 energy distribution and total power radiated by a complete radiator are a 

 function only of its temperature. 



