693 
2.3. Sky Radiation 
Several papers focus on the spectral distribution of direct and diffuse solar 
radiation at ground level. Here, to describe the wavelength dependence of diffuse 
to total irradiation ratio, results from (McCartney, 1978) and (McCartney and 
Unsworth, 1978) will be used. According to these results the relationship between 
the ratio of diffuse to total spectral irradiation D /a for solar zenith angles 
30-60* and the vertical attenuation coefficient t(X) is almost linear with the 
slope rU) and the intercept a Q (X) varying slowly with wavelength, 
D X/QX s ♦ r(X) x(X) . 
(3) 
Functions a Q (X) and y(X) have been given by McCartney (1978). They can be tabulated 
similar to pigment and water attenuation coefficients and the refractive index in 
the PROSPECT model of leaf optical properties. Vertical attenuation coefficients 
have been calculated for various aerosol size distributions. Many common size 
distributions yield relationships 
t(X) : 3X' x , < 4 > 
where x = 1.3 ± 0.6 in the visible spectral region (McCartney, 1977) with a modal 
value x » 1.4 (McCartney, 1978), and 8 is the Angstrom turbidity factor. This 
approach describes the spectral distribution of the relative sky flux by a single 
parameter - the Angstrom turbidity factor. The approximation works better in the 
visible spectral region. However, the rapid decrease of the skylight ratio with 
wavelength reduces the influence of increasing relative errors in the skylight 
ratio on canopy reflectance in the NIR spectral region. 
3. A MULTI SPECTRAL CANOPY REFLECTANCE MODEL 
By combining the leaf optical model, the soil reflectance model and the skylight 
ratio model described in previous paragraphs in Kuusk's (1993) FCR model, we get 
a multispectral canopy reflectance model (MSCRM), describing the directional 
reflectance of a canopy with high spectral resolution over 400 to 2500 nm. The 
number of input parameters of the MSCRM does not depend on the number of spectral 
channels under consideration. The set of input parameters includes (cf. Kuusk, 
1993) a) structural parameters, b) geometrical and illumination parameters, and 
c) optical parameters. 
3.1. Structural Parameters 
The set of the structural parameters of the new model coincides with that of the 
FCR model (cf. Kuusk, 1993): 
leaf area index, where u L is the leaf area density and 
H is the canopy height; 
relative linear size of leaves, where 1 is the mean chord 
length of leaves; 
modal leaf inclination; 
eccentricity of the elliptical distribution of leaf 
normals.