23 Bidimensional, polarization and spectral acquisitions
The POLDER instrument includes a matrix array CCD of 288x384 detectors with dimensions
23x23 pm, in the focal plane of a wide field of view optics with 3.565 mm focal length. This
provides an angular coverage with viewing angles 6y of ±42° in the along track direction and
±51° in the crosstrack direction (figure 1).
Figure 1 : Imaging principle of the POLDER instrument.
The filter wheel of the airborne simulator has 10 filter positions. One blind position is used for
measurement of the darkness current. The system allows for measurements in 2 polarized and 3
unpolarized bands. For cloud studies in polarized light, we measured polarization in POLDER
wavelength bands, centered at 450 and 850 nm.
A measurement sequence thus generates 9 spectral polarized images. By processing the 9
images, the spectral and polarization properties of the observed scene may be retrieved.
2.4 Data processing
First, to smooth the residual registration errors, the results were averaged within a 5x5 pixel
window. Then, the entrance reflectance p and polarized reflectance pp were deduced from the
averaged data.
p = n L/(cos0 s E s ) 0)
pp = 7t Lp/(cos8 s ) (2)
L is the radiance, Lp the polarized radiance, 6s the solar zenith angle, and E s the extraterrestrial
solar irradiance at the top of the atmosphere. Calibration of the POLDER instrument was
performed in the laboratory. Accuracy on the absolute reflectance, p, is about 5%; inaccuracy on
the degree of polarization P, is about ±0.005, for the case of entrance unpolarized light.
3 CLOUD POLARIZATION FEATURES
3.1 Multiband polarization observation
Reflectances measured above the cloud field in the 450 nm are presented on figure 2.
We look at the polarized reflectance rather than at the polarization ratio, because the polarized
reflectance is known to be formed in the upper cloud layer (Hansen and Travis, 1974) and to be
nearly insensitive to the cloud optical thickness or to the properties of the underlying ground.
Polarized reflectances measured at 450 nm are presented in figure 3.
The most significant feature in this image is the highly polarized arch (see figure 4).
