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est possible differentiation between stands of different tree 
Species in ihe aerial photographs. 
Already in the first paper from 1947 the general course of 
the reflection curves for leaves of the different tree Species 
was made clear (fig. 1). On the whole these curves may be char- 
acterized thus, that the remission power is relatively small, for 
example a few percent, for wavelengths belonging to the visible 
Spectral range, but with a comparatively marked maximum within 
the green spectral range. At the boundary between red and infra- 
red the remission power increases very quickly and stops at last 
after about 8000 À at a remission value perhaps 10 times greater, 
which thereupon remains rather constant for still Ereater wave- 
lengths as far as our measurements reach. 
From this it is evident that chiefly two different spect- 
ral ranges are suited for aerial photography for forestry purpo- 
Ses, which promise a good differentiation between different tree 
species. One of those lies in the infrared and comprises the wave: 
lengths beyond 8000 À. The other lies in green and ought to be 
limited rather narrowly 5200 to 5800 A. Within these two ranges 
the curves are more or less parallel relative to each other and 
the distance between them ought to cause a more or less distinct 
difference in density between areas of different tree species in 
the aerial photograph. For the infrared range this is rather ob- 
vious already from fig. 1. For the green range, however, it be- 
comes more evident if the curves are drawn in a somewhat larger 
scale. In fig. 2 this has been done for 6 tree Species, and these 
curves refer to the same season of the year, the latter part of 
May. AS can be seen there, the different curves, which refer to 
different tree species, separate well from each other at the max- 
imum point in green (5500 A), but the curves come more and more 
near to each other and can even intersect when we leave this 
  
    
   
   
    
   
   
   
    
  
   
   
   
   
   
   
   
    
   
   
   
    
    
   
   
   
   
   
   
  
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