> ima- 
le re- 
2 tree- 
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the 
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reflec- 
ur 
ase 
Previous investigations show that there may be a decreased 
or increased reflection of the incident light in relation to 
the nature of the "stress" or of the "damage", the duration 
of the affecting factors, the age of the trees and the diffe- 
rent tree-species. There is still no general agreement about 
the physiological changes within the leaves responsible for 
the change of the reflectance characteristics. The interac- 
tion of the many factors affecting the leaf reflectance makes 
the understanding of the final colour-image very difficult. 
Moreover there is a difference between the results based 
upon laboratory measurments and the air-borne sensing of the 
spectral reflectance characteristics. 
2.2. The photographic and geometric aspects 
Often the photographic and geometric aspects of the informa- 
  
tion transfer are neglected. 
This point will be illustrated with one of the most 
significant factors and perhaps the one which is mostly igno- 
red by the interpreters. It is the combined effect of the 
radial image displacement and the direction of the sun rays. 
The field of an aerial photograph can be divided into zones 
with high contrast and with low contrast. In the high contrast 
zone the shadowed side of the tree canopy is seen as well as 
its complete shadow image. On the contrary, in the low con- 
trast zone the illuminated side of the canopy is seen while 
it covers partially the shadow on the background. 
Thus similar trees growing in the same conditions will show 
a different colour according to their position on the aerial 
photograph. Also their image to background relation is comple- 
tely different, as for example the influence of the transmit- 
tance of background radiation. Therefore it is of utmost 
importance that the interpreter always recognises these con- 
trast zones upon the aerial photographs. 
To illustrate these thoughts densities in each layer of the 
CIR emulsion have been measured for tree-canopies in two