1_>() RADIATION lUOLOGY 



.sunlight as a function of season, altitude, or some other parameter. A 

 second investigator, using the same biological test object, might wish to 

 determine the action sp(M'trum for the lethal effect of ultraviolet radiation 

 in groat detail over a wide range of wave lengths. Each of these workers 

 would draw from the same reservoir of available tools but would combine 

 them in a different fashion for his own particular problem. The function 

 of this chapter is to serve as a guide. 



SOURCES 



The source of radiation is of great importance among materials for a 

 study of effects of ultraviolet radiation on biological systems. .V number 

 of excellent chapters have been written containing detailed considera- 

 tions of light sources in general and of tlu; ultraviolet light sources in par- 

 ticular (see, e.g., Ellis d at., 1941; Forsythe, 1937; Harrison etal, 1948; 

 Sawyer and Vincent, 1939; and Roller, 1952). No effort will be made 

 to reproduce these detailed discussions in this chapter but rather to dis- 

 cuss types of light sources and their characteristics, factors to be con- 

 sidered in the choice of a light source, and finally to present a tabular 

 compilation of noncommercial and commercial laboratory light sources 

 which have characteristics making them particularly useful in a study 

 of radiation effects. 



CLASSIFICATION OF LIGHT SOURCES 



Sources of radiant energy have been classified in a number of ways 

 among which are: (1) the spectral range of radiation of useful intensity, 

 (2) the method used for exciting the radiation, and (3) the distribution of 

 energy within the spectral range. The ultraviolet spectrum, which is 

 discussed in this volume, covers the range 4000-10 A. This broad range 

 has been subdivided primarily on technical grounds into the near, the far, 

 and the extreme ultraviolet. Sources differ considerably in the fraction 

 of the total energy emitted in each of these ranges. The near ultraviolet 

 extends to 3000 A which is near the short wave-length limit for the sunlight 

 at the earth's surface. The far ultraviolet in biological work extends to 

 about 1900 A. In this vicinity quartz begins to absorb strongly as does 

 atmospheric oxygen (Schneider, 1940; Ladenburg et al., 1932). Because 

 of this increasing atmospheric absorption below 1900 A, the extreme 

 ultraviolet is also known as the vacuum ultraviolet. The lower limit 

 of the extreme ultraviolet is arbitrary. There is considerable overlap in 

 this region between the longer wave lengths of radiation produced by 

 the techniciues employed in the excitation of X radiation and those found 

 by excitation of ultraviolet radiation. 



Method of Exciting the Radiation. Ultraviolet radiation of any portion 

 of the ultraviolet spectrum may be produced by any one or more of the 

 following means of excitation: (1) incandescent or thermal, (2) spark, 



