Chapter IV 



EMISSION AND ABSORPTION OF BIOPHYSICALLY 



ACTIVE LIGHT 



Statistics yield much evidence that sunlight is of direct benefit to 

 human beings. During the winter the sun is at lower altitudes and the 

 maximum possible duration of sunshine is much less than in summer. 

 Furthermore, the solar radiation must pass through a greater air mass 

 since it reaches the earth more obliquely. As a consequence, the photo- 

 biologic radiations in sunlight are greatly weakened in winter. 



There is some evidence that sunlight increases the resistance to 

 infection. Sunlight tends continually to sterilize earth and water 

 because of the resulting photochemical activity. Perhaps the most out- 

 standing importance of sunlight, or its artificial equivalent, is in its rela- 

 tion to the prevention and cure of rickets. Rickets occurs with marked 

 frequency during the winter and spring and almost disappears in mid- 

 summer. Sunlight or its equivalent is also known to promote calcium 

 anabolism, and the most important function of all is that chlorophyll, 

 the green coloring matter in the leaves of the plants, makes use of cer- 

 tain wavelengths in fixing the carbon from carbon dioxide gas in plant 

 structure. 



Spectral Transmission of the Atmosphere 



The atmosphere serves as a gigantic filter for the sun's radiant energy. 

 The short-wave ultraviolet energy is absorbed by the ozone in the upper 

 strata, and smoke, water vapor, and the dense gases near the earth's 

 surface all act as scattering agents of light. The visual proof is the 

 clear blue color and the brightness of the sky. Without this scattering, 

 the sky would be as dark as it is at night. 



As a result of absorption of solar radiation by the atmosphere, the 

 spectral nature and the intensity of the sunlight vary with the altitude 

 of the sun. 



In order to give an idea of the spectral distribution of direct solar 

 radiation, curve A, Fig. IV- 1, has been plotted from Abbot's data, 

 which clearly shows the prominent atmospheric absorption bands 



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