T [RI 330 325 320 315 310 305 300 295 290 L Spectral Decomposition of Ultrawideband Terahertz Imagery * E.N. Grossman 1 , C. R. Dietlein 1,2 , J. Chisum 1,2 , and A. Luukanen 3 J.E. Bjarnason 4 , and E.R. Brown 4 1 Optoelectronics Division, National Institute of Standards and Technology, Boulder, CO 2 Electrical and Computer Engineering Dept., Univ. of Colorado, Boulder, CO 3 Millimetre-wave Laboratory of Finland, Tietotie 3, Espoo, Finland 4 ECE Department, Univ. of California, Santa Barbara, CA ABSTRACT We investigate the spectral response of a THz imaging system based on ultrawideband cryogenic microbolometers. The bandwidth if this system, nominally 0.2 – 1.8 THz, is broad enough to span large variations (>10 dB) in clothing transmittance and diffraction-limited spatial resolution (factor of x8), factors that are presumably partly responsible for the unusually high quality of the images taken with it. The chief tools we have used for this are a simple THz monochromator based on a specially designed frequency selective surface, and a specially designed blackbody source that provides an accurately known power spectral density over the full bandwidth of the imager. Two completely independent measurements of the microbolometer’s spectral response, in the first case using a filtered blackbody and in the second using an ultrabroadband, THz photomixer, referred to a Golay cell, agree within 5%. Evidence of frequency-dependent scattering from ordinary clothing material, distinct from simple linear attenuation, is presented from an idealized laboratory experiment. However, the scattering is relatively weak, and unlikely to have a significant effect in practical THz imaging scenarios, particularly with ultrawide bandwidths. Keywords: Bolometer, concealed weapons, imaging, millimeter-wave, terahertz, ultrabroadband 1. INTRODUCTION As described elsewhere, we are currently developing a terahertz camera based on a large, mechanically scanned linear array of antenna-coupled, superconducting microbolometers[1, 2]. Early imagery taken with a single-pixel version of this camera (Fig. 1) shows a remarkably high capability for detecting and identifying threat items concealed beneath clothing. However, despite ongoing analysis being done on this imagery[3], it is still an open question as to what accounts for the (subjective) high quality of these images. Clearly a combination of good spatial resolution, high sensitivity (i.e. low NETD), and adequate clothing transmittance[4] is essential, but the relative importance of these factors is unclear. Moreover, these factors have opposite dependences on frequency, lower frequencies providing better penetration, but higher frequencies offering better spatial resolution. Therefore, it is not yet possible to identify the optimum frequency band for THz personnel screening, despite the question’s obvious practical importance. Spectral decomposition of this type of imagery is needed in order to resolve that. * Contribution of the U.S. Government. Not subject to copyright. Fig. 1. (left) Ultrawideband image obtained with mechanically scanned 0.1 – 1.2 THz microbolometer system. (right) corresponding visible image of same scene. Passive Millimeter-Wave Imaging Technology X, edited by Roger Appleby, David A. Wikner, Proc. of SPIE Vol. 6548, 654807, (2007) · 0277-786X/07/$18 · doi: 10.1117/12.719632 Proc. of SPIE Vol. 6548 654807-1