1e 
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an 
b) 
c) 
- 581 - 
Parameters which characterize the conditions of 
measurement: 
- zenith angle (9) and azimuth Cfi) of the sun, 
zenith angle (44) and azimuth (y,) of the instrument 
- field of view ahd spectral resolution of the instru- 
ment 
- object distance 
- relation of direct sun to global radiation 
- degree of cloudiness 
- atmospheric turbidity 
- wind speed and direction 
- gurface wetness 
Parameters which depend on the momentaneous positions of 
the sun and the instrument: 
- repartition between directly illuminated and shadowed 
parts within the field of view : 
- relative composition and texture (f.e. specific 
structures or leaf positions) of the dominant compo- 
nents of the object 
The realization of in situ measurements of the spectral 
reflection properties should be oriented to the following 
points: 
a. 
The interpretation and the comparison of the results 
make it necessary to determine and to document the above 
mentioned parameters as far as possible. 
The measurements of the target reflection and of the 
reference standard reflection should be carried out with 
the same irradiation conditions, i.e. simultaneously and 
at the same level. 
The diameter of the field of view should be several times 
larger than the period length of dominant structural 
features. 
The aperture of the instruments should be rather small 
because of the angular dependence of the reflected 
radiation. 
For the determination of the angular dependence of the 
reflection properties a large number of combinations of 
sun's position, observation direction and principal 
structure orientations of the object should be investi- 
gated. 
For the determination of seasonal variations of the 
reflection properties the measurements should be carried 
out at different typical stages of the vegetation cycle.