The Identification of Explosives in Millimeter-Wave Imaging Systems

Commercial Advanced Imaging Technology (AIT) systems use arrays or synthetic arrays of millimeter wave antennas to generate holographic images and identify anomalies in those images that may present threats. In order to provide additional information for the AIT assessment, we are developing a technique that enables the identification of threat composition based on dielectric constant. The dielectric constant is extracted from the spectral content of the broadband holographic imaging data. The technique is demonstrated from images collected in a prototype personnel-screening system. The dielectric constant is obtained by numerically fitting the reflection coefficient as a function of frequency to an optical model. The reflection coefficient is a function of frequency because of propagation effects, such as multiple reflections and energy loss, that are associated with the material's complex dielectric constant. In order to accomplish the analysis using an imaging system, the spectrum is obtained from an integration of the reflectivity image spectrum, which is an intermediate result in the image reconstruction algorithm. The present use of an imaging array demonstrates the ability to detect dielectric constant in small areas on a complex target. In principle, the implementation of this technique in standoff imaging systems would allow threat assessment to be accomplished within the scope of millimeter-wave screening.