661 
Figure 2 - A photo of the experimental arrangment. 
2-3 Atmospheric measurements 
We first set up the ground-based of LOA near by La Crau. This station is routinely used for 
example during the SPOT calibration (see Santer et al, 1992) measuring multispectral solar extinction, 
aureole in the almulcantar at 850 nm and measurements at 850 nm and 1650 nm of the radiance and of the 
degree of polarization in the principal plane. Aerosol optical thicknesses and aureole measurements are 
inverted to derive the particle abundance for 24 size classes. This inversion is not very sensitive to the 
particle refractive index. The determination of this parameter is conducted on the polarization 
measurements. On June 21th, the size distribution was quite close from a Junge size distribution 
n(r) = r~4 with a particle refractive index of m = 1.45. This determination was done at a distance of 30 km 
from the agriculture site. To confirm the model, we also have extinction measurements in Camargue and 
the Angstrom coefficients are identical between Crau and Camargue which make suitable the aerosol 
model for the two sites. The main feature of the day was a high visibility with an aerosol optical 
thickness of 0.1 at 450 nm. 
3. ANALYSIS OF THE MEASUREMENTS. 
3.1 The POLDER polarized radiances. 
On the image of figure 1, we selected large field with homogeneous areas of 3 x 3 pixels (60 m 
x 60 m) and used the polarized reflectance at 650 nm. As, an example, we reported in figure 3 the 
polarized reflectance for three closed fields. This parameter is plotted versus the view angles which varies 
along the image series while the solar angle was quite constant (SZA = 44°). As we can see, the 
sunflower presented a polarized reflectance twice higher than the other field and appeared quite bright on 
the image. These measurements are quite consistent with our ground based measurements for which the 
sunflower seems to produce twice polarized reflectance compared to grass or wheat. This result also 
qualitatively corresponds to the structure of the sunflower canopy which was planiform at the time of the 
measurements while the grass has a more isotropic structure and the wheat was yellow.