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; 
view zenith (nadir) 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.