It is composed of 15 separate collimators associated to 15 distinct (lens-filter-polarizer-detector) blocks (fig. 1). The principle of polarization measurement is the same as for POLDER-1/2 (fig. 2).

Figure 2 : Schematic principle of polarization measurements

 

Three polarizer orientations (shifted by 60°) at each wavelength give three independant measurements which then are combined to derive total radiance L, polarized radiance Lp as well as polarisation angle for a given viewing geometry.
The absolute calibration accuracy is about 3% for radiance and polarized radiance at wavelength smaller than 0.9 µm and about 5-7 % for the longer wavelengths. In the 1.6 and 2.2 µm channels detectors are stabilized at a temperature of -10°C (InGaAs detectors). It has been shown that the effect of temperature variation on calibration is smaller than 2% at maximum. This instrument has a narrow IFOV about of 2° (fig. 3).

Figure 3 : MicroPOL fied of view

 

When this instrument is setup in aircraft, we also have the possibility to select several viewing angles (0°, 15°, 30° and 45°) which allows the selection of the more pertinent scattering angle with respect to the experiment objectives. Acquisition time for one detector is 0.5 ms, so that the set of total and polarized radiances at the 5 wavelengths needs no more than 7.5 ms. The radiometric accuracy (NeΔL) is 2 10-5.