Terahertz standoff imaging testbed design and performance for concealed weapon and device identification model development

Abstract

This paper describes the design and performance of the U.S. Army RDECOM CERDEC Night Vision and Electronic Sensors Directorate's (NVESD), active 0.640-THz imaging testbed, developed in support of the Defense Advanced Research Project Agency's (DARPA) Terahertz Imaging Focal-Plane Technology (TIFT) program. The laboratory measurements and standoff images were acquired during the development of a NVESD and Army Research Laboratory terahertz imaging performance model. The imaging testbed is based on a 12-inch-diameter Off-Axis Elliptical (OAE) mirror designed with one focal length at 1 m and the other at 10 m. This paper will describe the design considerations of the OAE-mirror, dual-capability, active imaging testbed, as well as measurement/imaging results used to further develop the model. INTRODUCTIONIn recent years, research has advanced terahertz technology to the point of being a potential solution for identifying weapons through clothing 1-2 . While millimeter-wave imaging has demonstrated very effective imaging in fog and dust, THz technology offers higher resolution for a given aperture size, and also provides penetration of a variety of nonconducting materials at significant standoff distances. This paper will discuss the rationale behind the design of a 12-inch-diameter, OAE-mirror-based testbed, the data acquisition methodology and set-up of the Virginia Diodes, Inc. (VDI) 0.640 THz source and heterodyne receiver components, and will present a preliminary implementation of the latest DARPA TIFT THz Imaging System Performance Analysis Model (Alpha Plus) that is currently in development. BACKGROUNDThe U.S. Army NVESD has over fifty years of experience in developing sensor-based models. NVESD and the U.S. Army Research Laboratory (ARL-SEDD and ARL-CISD) have partnered with Ohio State University (OSU), Wright State University (WSU), and University of Memphis (U of M), under the DARPA TIFT program, in the development of a terahertz-band imaging system performance analysis model. Jacobs et al., has outlined the methodology, as well as, radiometric and human-visual-system-response considerations for extending the U.S. Army target acquisition models to THz imaging for the task of concealed weapon identification 1 . In 2005 Petkie et al., successfully developed millimeter/submillimeter/terahertz systems that could be configured to study both active (0.640-THz source and heterodyne receiver) and passive imaging (bolometric detector), transmission and scattering measurements for common clothing, and associated phenomenology 2 . The imaging studies were based on a 12-inch, raster-scanned mirror system designed and built at OSU.Murrill et al., has adapted the latest U.S. Army NVESD sensor model to accommodate THz imaging 3 . A defined target set consisting of small handheld weapons was blurred in a known fashion and then used in a perception experiment to