646 
233. LAD. Since the LAD is the main parameters influencing the relationship between WDVI and LAI, inforautioe 
on this parameter is very important. In Looyen et al. (1991) the possibilities of acquiring information on both 
LAI and LAD by means of a so-called dual look (two viewing angles) concept are illustrated. Figure 2 shows 
a nomogram of the simulated WDVI (SAIL model) at an oblique viewing angle (52°) plotted against the simu late 
WDVI at nadir viewing for several leaf angles and LAI values. By plotting measured WDVI values into this 
nomogram, an estimate of both LAI and leaf angle is obtained. 
2.3.3 Leaf Optical Properties. In practice it will be very difficult to ascertain leaf optical properties unless leaves 
are analysed in the laboratory. A more practical measure may be offered by the red edge index. However, Clevers 
& Buker (1991) have shown that this index is determined by both LAI and leaf optical properties (related to leaf 
chlorophyll content). Two (independent) measurements are therefore needed, one more related to LAI (like WDVI) 
and one more related to chlorophyll content (like red edge index). Figure 3 illustrates the simulated influence 
of LAI and leaf chlorophyll content on the position of the red edge and the WDVI (using a combined PROSPECT-SAIL 
model). In this study, the method of Guyot&Baret (1988) for determining the position of the red edge was applied, 
using only four wavelength bands. First, they estimated the reflectance value at the inflexion point halfway minimum 
(at 670 nm) and maximum (at 780 nm) reflectance. Secondly, they applied a linear interpolation procedure between 
the measurements at 700 nm and 740 nm for estimating the wavelength corresponding to the estimated reflectance 
at the inflexion point. Measurements of the WDVI and the position of the red edge may be combined for ascertaining 
the leaf chlorophyll content. 
Figure 2. Nomogram of the influence of LAI and leaf angle on the directional WDVI. The WDVI is simulated 
for nadir viewing and at an oblique viewing angle of 52° using SAIL. Solar zenith angle is 36° and azimuth angle 
between plane of observation and sun is 7 (conditions are of MAC Europe 1991, CAESAR overflight July 4th - 
calendar day 185 13.30 GMT). 
Figure 3. Nomogram of the influence of LAI and leaf chlorophyll content on the WDVI (from nadir) and the red 
edge index. Simulations are with a combined PROSPECT-SAIL model (measurements are of MAC Europe 1991, 
AVIR1S overflight July 5th - calendar day 186 - 13.08 GMT).