Measurement results from stand-off sensing of material characteristics by polarimetric MMW radiometry

Abstract

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. In the recent past an approach has been established using polarimetric millimeter-wave radiometry for extracting the real part of the permittivity of surfaces. Hereby an imaging radiometer scanner has been used to measure the first three components of the Stokes vector of a scene containing various targets of interest. Those are for instance plane surfaces being tilted such that cold sky radiation is reflected towards the radiometer. It was shown experimentally that reasonable values for the permittivity of such surfaces could be extracted. However, more measurements are necessary to validate the method and to estimate possible error sources. Hence the paper will illustrate and discuss new measurement results.