# Full text: Mesures physiques et signatures en télédétection

```3.2. Geometrical and Illumination Parameters
One parameter in this group has been changed: direct to total irradiance ratio
S'^/Q^ is replaced by the Angstrom turbidity factor 3.
3.3. Optical Parameters
solar zenith angle;
relative view azimuth
Angstrom turbidity factor.
The optical parameters of the FCR model are calculated in submodels, and now the
input parameters of these models serve as the MSCRM parameters.
The specular reflection of direct radiation on leaves should be discussed in
more detail. The magnitude of specular reflection is determined by the refractive
index of leaf cuticular wax n w and by the structural characteristics of the wax
layer. Roughness of the wax layer surface, deposition of dust and the presence of
hair on the leaf surface all decrease specular reflection from the leaf. In the
Nilson-Kuusk (1989) model this decrease of specular reflection has been accounted
for by decreasing the refractive index and by introducing a supplementary parameter
- the leaf hair index k. Unfortunately, it is difficult to give any reasonable
values for the leaf hair index k. In the inverse problem the behaviour of this
parameter was of rather casual nature (Kuusk, 1991a). Thus in the FCR model its
value has been fixed to k = 0.1, and the independent parameter, which determines
the specular component, is the refractive index n.
In a multispectral model it is tempting to take advantage of the tabulated
refractive index of the PROSPECT model. To consider the diminishing of specular
reflection due to the leaf hair or destruction of the wax layer, an additional
parameter - the factor c - is introduced to describe the specular reflection from
leaves,
%U) = c n n(A), (5)
where n(A) is the refractive index of the leaf material tabulated in the PROSPECT
model. The value of the leaf hair index k is fixed as in the FCR model, k = 0.1.
Now we can list all the parameters of this group. These are the parameters
that determine the optical properties of leaves and soil:
2
C AB = leaf pigment concentration, pg/cm ;
CT = leaf water equivalent thickness, cm;
N = the effective number of elementary layers inside a leaf;
c n = the ratio of refractive indices of the leaf surface wax and
internal material;
s. = the weights of the Price functions i = 1 ... 4 or i = '
depending on the approximation in use.
4. MODEL VALIDATION
Despite the small number of input parameters the validation of the model is rather
complicated. Measurements of the canopy reflectance spectra concurrent to versatile
phytometrical measurements and measurements of leaf optical properties are rather
rare. All the parameters cannot be measured directly. Some of them must be
estimated indirectly by performing some kind of inversion. Such parameters are the
effective number of leaf elementary layers N, the ratio of refractive indices c »
the leaf inclination distribution parameter e, the Angstrom turbidity factor 3, the
weights of soil basis functions Sj ... s 4 .
Here, the canopy reflectance data, and accompanying phytometrical and optical
data provided by Ranson et al. (1985) will be used for the validation of the model.
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