Stand-off sensing of material characteristics by polarimetric MMW radiometry

The characterization of dielectric materials is of great importance for many applications, being for instance quality control during product fabrication or status control of outside constructions over time. In many outside situations the objects of interest have limited accessibility, and the investigation has to be done without destruction of any part of the object and without any health risks for an operator. Hence remote sensing from stand-off position is desirable, and the use of microwaves, millimeter-waves or THz waves offers some penetration capability into matter, depending on its chemical and physical decomposition and of course frequency. Many objects of interest consist of a dielectric coating or enclosure, which can electromagnetically be treated as a dielectric layered structure or a dielectric slab surrounded by air. Radar as an active remote sensing technology has great potential with respect to precise range measurements and spatial resolution. However, its mostly mono-static implementation suffers from low or no back reflection of signals in case of plane and tilted surfaces, delivering hence no information. In contrast radiometric imaging as passive remote sensing technology uses naturally generated noise-like radiation of thermal origin, being available everywhere and from all directions. Quite often the sky can be used as large illumination source, providing rather low radiation power compared to objects on Earth, using frequencies up to few hundred GHz. Furthermore, thermal radiation originally is un-polarized, but becomes polarized when reflected partly on a dielectric surface. Hence, the use of polarimetric measurements at various incidence angles can provide information on the type and structure of the reflecting object. The approach and the measurement setup are described. Experimental results of polarimetric measurements are shown and discussed.