2. Factors influencing the colour of the tree canopy on C.I.R. 
    
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
     
  
  
   
  
  
  
  
  
   
  
   
  
  
  
  
  
Damaged trees are detected by colour differences in the ima- 
ge of the tree canopies. These colour variations are the re- 
sult of the changing reflectance characteristics of the tree- 
crown itself, but they are also strongly influenced by the 
environmental conditions. 
2.1. Environmental conditions 
Studies published on reflectance characteristics are mainly 
based upon an analysis of hemispherical leaf reflectance. 
After a critical overlook of this literature there seems to 
be no general agreement and even contradictory results may 
be found. The main reason for this is that the hemispherical 
spectral reflectance measurements of the leaves are not suffi- 
cient to understand the photographic picture. To allow a full 
understanding of the image the Multi-directional Spectral re- 
flectance of the whole tree canopy must be analysed. 
As J.E.COLWELL (1974) already pointed out, that in addition 
to hemispherical leaf reflectance,several other parameters, 
can be important in order to understand the colour of the 
tree canopy as for instance 
- transmittance of leaves ; 
- amount and arrangement of leaves ; 
- characteristics of other components of the tree canopy 
(stalks, trunks, limbs) ; 
- characteristics of the background (soil-reflectance, 
amount of leaf litter) ; 
- view angle ; 
- solar elevation and azimuth angle. 
All these parameters are influencing the total canopy reflec- 
tion and thus partially responsible for the final colour 
image. 
The spectral reflectance response upon stress and disease 
conditions is very complicated.