26 LIGHT, VEGETATION AND CHLOROPHYLL 



experimentation on the properties of these radiations in 

 relation to vegetation or any other kind of phenomenon, is 

 undertaken by a few speciahzed laboratories possessing the 

 necessary equipment and experienced personnel because it is 

 extremely delicate work. 



It is, however, desirable that manufacturers of sources of 

 illumination, or those who use them for their experiments, 

 should be able to determine easily and with reasonable 

 accuracy the curve of spectral emission, from the ultra- 

 violet to the infra-red, of their lamps or Hghting units under 

 the same conditions as those in which they will be used. The 

 experiments would thus be much more informative. 



0-2 0-4 0-6 0-8 lO 1-2 14 16^ 



Fig. I, 7. Upper curve: composition of solar radiation 



beyond the earth's atmosphere. Lower curve: composition 



of solar radiation at ground level, with the sun at the zenith 



and an atmosphere of average humidity 



The Hght from the sun is the most important to human 

 beings just as it is to plants. At ground level its power and 

 its composition vary according to several factors; a special 

 chapter of this book will be devoted to it. But, in order that 

 it can be compared with other sources of light, we shall give 

 the composition of solar radiation as it reaches ground level 



