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c)
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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.