134 



BIOPHYSICALLY ACTIVE LIGHT 



Spectral distribution 



of 



radiation from the sun 



at 



Washington, D.C. 



appearing as depressions in the curve. Their depths indicate to what 

 degree the atmosphere is opaque to these wavelengths. Those in the 

 infra-red are due chiefly to water vapor which was slightly above average 

 when these data were obtained. There is a rapid decrease between 



5000 and 4000 A, indicating a 

 rapid increase in opacity of the 

 atmosphere in this violet region. 

 Although the short-wavelength 

 limit of the solar spectrum as 

 recorded on earth is set close to 



o 



2885 A, for all practical purposes, 

 owing to smoke, it is usually ac- 

 cepted as 2950 A in summer and 



o 



3100 A in winter. In order to 

 appreciate the absorption effect 

 of the atmosphere, curve B in 

 Fig. IV-1 has been introduced to 

 show the relatively greater inten- 

 sity of the energy at very high 

 altitudes where absorption is a 

 negligible factor. 



4000 



H 



8000 12,000 16.000 



o 



Wavelengths in A 



20,000 



Fig. IV-1. The spectral distribution of 

 radiation from the sun. A, as measured at 

 Washington, D. C, with 1.37 cm water 

 vapor in the atmosphere, for the sun at 

 zenith. B, similar measurements made at 

 altitudes so great that absorbing atmosphere 

 is negligible. Shaded area shows visible 

 spectral region with maximum visibility at 

 5560 A. (By courtesy of J. H. Clark 

 [1931].) 



Figure IV-2 shows the fluctuations of 

 the ultraviolet content of sunlight during a 

 typical clear day at different times of the 

 year. It was found that the ultraviolet 

 content of sunlight rises more sharply to a 



11 12 1 

 Hour 



Fig. IV-2. Ultraviolet con- 

 tent of sunlight on a typical 



clear day in the months indi- 

 maximum and then decreases more rapidly cated (By courtesy of j H 



than the total radiation. This difference is ciark [1931].) 



due to the relatively greater atmospheric 



absorption for the ultraviolet radiation. The predominance of the 



ultraviolet content between 11 a.m. and 

 summer, is apparent from these curves. 



1 p.m., especially during the 



