parameters, (2) structural parameters, and (3) geometrical and illumination 
parameters. 
The structure of a vegetation canopy, and the view and solar angles do not 
depend on the spectral channel used. At the same time, leaf optical properties, 
soil reflectance and the ratio of direct to total irradiance vary significantly 
with wavelength. Using the Nilson-Kuusk model or the FCR model for canopy 
reflectance calculations we need a common set of structural parameters for all 
spectral channels, and a set of optical parameters for every spectral channel. The 
same is true for the inversion problem: given the measured canopy reflectance in 
several spectral regions we have to determine a set of structural parameters, and 
a set of optical parameters corresponding to each spectral channel. To overcome 
this complication the leaf optical model PROSPECT by Jacquemoud and Baret (1990), 
the soil reflectance presentation with basis functions by Price (1990), and the 
wavelength dependence of sky radiation given by McCartney (1978) will be integrated 
into Kuusk's fast canopy reflectance model. 
2. MODEL COMPONENTS 
2.1. Leaf Optical Properties 
Jacquemoud and Baret (1990) proposed a model of leaf optical properties spectra 
(the PROSPECT model), which has only three input parameters and describes optical 
properties of green leaves in the 400 to 2500 nm spectral range. The spectral 
resolution of the model is 4 nm in the visible and 17 nm in the NIR spectral 
region. The input parameters are a) the pigment concentration C representing 
chlorophyll A and B, and carotenoids, b) the water equivalent thickness C^, and c) 
a structure parameter N - the effective number of elementary layers inside a leaf. 
As an output the PROSPECT model gives leaf diffuse reflectance and transmittance 
spectra. 
2.2. Soil Spectral Reflectance 
The spectral distribution of soil reflectance depends on a very large number of 
soil parameters, see (Biehl et al., 1982). However, Price (1990) succeeded in 
describing the whole variety of soil spectra using only four basis functions. These 
basis functions permit the reduction of the number of soil parameters to four in 
a multispectral canopy reflectance model. Even more, in (Price, 1990) we see that 
the weights of first two basis functions are significantly correlated, and two 
first functions describe 94% of the soil spectrum variance. In the case of dense 
homogeneous canopies (ground cover over 70 - 80% ) the soil is screened by above 
vegetation and its influence on the canopy reflectance is small. Then we can 
approximate the soil reflectance as 
Psoil^) s S1<P-|U)+ S2<P2(M, 
where 
S 2 = a + b s-| , (2) 
q> l (A) and <p, (M are the two first basis functions by Price. 
The angular distribution of soil reflectance is nonlambertian, similar to 
Nilson and Kuusk (1989) and Kuusk (1993). 
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