576 
measure the directionality and polarization of the sunlight scattered by the ground-atmosphere system (Deschampset 
al. 1994). The instrument images the ground-atmosphere scene with the aid of a 2 dimensional CCD detector anav 
coupled to wide field of view telecentric optics. During an aircraft data collection overflight, POLDER acqiresan 
image of the experimental site every few seconds. Any given ground pixel will appear in multiple images, each 
showing the site viewed from a slightly different direction. A filter wheel provides the spectral analysis of the radiance 
of the scene. Scene polarization is determined in each of several spectral bands by combining three images collected 
through three polarization analyzers rotated by 60°. 
IV.B. Cloud targets 
The interest in polarization for cloud studies was outlined long ago (Coffeen and Hansen 19721. First, polarization 
should be able to provide cloud altimetry by the way of molecular scattering. Because cloud droplets are quite large, 
the polarized light scattered by clouds varies little with wavelength in the visible range; moreover, cloud polarization is 
especially small for scattering angles near 90-100°. Therefore, the polarized light measured over ciouds in these 
directions and at short wavelengths should be indicative of the optical thickness of the atmosphere above the cloud, i.e. 
of the cloud lop altitude. On the other hand, the angular polarization signature of clouds as measured in near infrared 
wavelengths could help for discriminating between liquid and solid phases of the cloud particles. Light scattered from 
liquid cloud droplets (e.g. spherical particles) forms a characteristic rainbow polarization feature observable at scattenng 
angles near 140°. while the lack of this feature should be indicative of the presence of ice crystals. Calculations of the 
light polarized by prisms, cylinders, hexagonal crystals and spheroids show effectively that the rainbow polarization 
feature tends to disappear when the scattering particles depart from a spherical geometry (see e.g. Cai and Liou 1982; 
Takano and Javaweera 1985; De Haan 1987; Brogmez 1992: Masuda and Takashima 1992. Mishchenko and Travis 
1993). 
POLAlnZtO REFLECTANCE 
O.OOt-i * 11 km CLOUD OBSERVA HONS 
100.00 ISO.00 
Fig. 5. Polarized reflectance Rp, measured about 7 km. above a homogeneous cloud, in channels 
centered at 850 nm (lower points) and 450 nm (upper points). For each image pixel, P has been reported 
as a function of the corresponding scattering angle. 
During airborne campaigns, POLDER images of cloud fields, acquired in polarized bands centered at 450 and 850 nm. 
were used to test these applications of polarization data. Observations performed over rather homogeneous clouds 
allowed the entire Bidirectional Polarization Distribution Function to be examined from analysis of one single picture 
(Goloub et al. 1994). When reporting, for each image pixel, the measured polarized reflectance as a function of the 
corresponding scattering angle, the resulting diagram is typically as shown in Fig. 5. The BPDF highlights the liquid 
phase of the cloud droplets by the large characteristic polarization detected in the rainbow directions, ranging from 
about 135-150°. On the other hand, for scattering angles ranging from 80-110°, while the cloud drop polarization 
measured in the 850 nm channel is negligible, the polarized light in the 450 nm band is large as a result of the 
increased efficiency of the underlying molecular atmosphere. The cloud top pressure altitude derived from these 
polarization measurements according to the simple scheme proposed in Goloub et al. compares well with the cloud top 
altitude observed during these flights. 
IV.C. Terrestrial targets 
POLDER measurements were conducted over land surfaces on different occasions, specially during two (lights 
conducted over the area of La Crau in June 1990 and July 1991. The results may be used to examine the ability to 
discriminate, in the polarization observed over terrestrial targets, the signature of the surface from that of the 
atmosphere. 
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