296 
One can immediately notice that QN, SP, and GQ provide similar results in terms of retrieved parameters and 
the lowest RMSE's indicate the best reconstruction of the measured reflectances; MQ systematically finds other 
solutions and seems to be less efficient. QN and GQ have the best RMSE but QG requires much more 
computation time. In the absence of precise information on the measured biophysical characteristics of these 
canopies, it is somewhat difficult to interpret this difference we did not note when running inversions with 
synthetic data. 
Two parameters out of five (N and Si) keep the value of the lower or upper bounds, whatever the 
method. This may mean that the measured reflectances do not incorporate enough variability due to these 
parameters. Jacquemoud (1993) already pointed out that the leaf mesophyll structure parameter N had little 
influence on canopy reflectance because the effect of a varying leaf reflectance was partly compensated by the 
varying leaf transmittance. As for the hot spot size parameter Si, measured reflectances are too few and far from 
the hot spot region to permit its good estimation. To illustrate that point, we used the parameters estimated by 
the QN method to calculate the directional 
of the viewing zenith angle 0o (Figure 2). 
Zenith viewing angle 
of three surfaces (peas, wheat, and potato) as a function 
Figure 2. Comparison between the directional 
reflectances measured over three different patches (a —> 
peas, b wheat, c -» potato) at the three CAESAR 
bands (green —t circle, red plus, near infrared -» 
star) and those modeled with the PROSPECT+SAIL 
model using optimally fitted parameters. The 
measurement configurations are 03=36.1° and <po=7.4°. 
Therefore, in future inversions in such experimental conditions it may be better to fix the value of these 
variables in order to avoid conflict with other variables and to save time in the inversion procedure. With 
regard to the other 3 parameters, retrieved values are generally consistent from one optimization method to 
another. Unfortunately, the ground truth was only available for the leaf area index and we had no available 
measured values of Cab and 01 with which to compare the inversion findings. One can see that fitted LATs 
globally agree with measured values (first column of Table 3). Fitted chlorophyll a+b concentrations are those 
of plants in good health and seem quite reasonable for the studied crops. It is also significant to note that the 
leaf orientation estimated foe the wheat crop indicates vertical leaves, and more horizontal leaves for the other