Absorption, transmission, and scattering of expanded polystyrene at millimeter-wave and terahertz frequencies † Charles R. Dietlein a,b‡ , Jon E. Bjarnason b , Erich N. Grossman b , and Zoya Popović a a Department of Electrical and Computer Engineering, 425 UCB, University of Colorado at Boulder, Boulder, CO, 80309 b Optoelectronics Division, National Institute of Standards and Technology, 325 Broadway, MC 815.04, Boulder, CO, 80305 ABSTRACT Conventional material measurements of transmission and reflection in the millimeter-wave and terahertz frequency range do not differentiate between scattering and absorption, grouping effects from both mechanisms together into “loss”. Accurate knowledge of the balance between scattering and absorption is critical in applications such as radiometric scene modeling for concealed object detection, where evaluation of object detectability depends strongly on the amount of scattering due to concealers such as clothing. We describe an experimental setup for the measurement of spatial bidirectional reflectance distribution function (BRDF). Previous measurements have shown extremely low-level grating lobes from periodic clothing materials such as corduroy, around 30 dB below the transmitted beam. To adequately address this issue of high dynamic range, we utilize a cryogenic antenna-coupled microbolometer for detection. We present data on several types of expanded polystyrene, a common structural material for systems and experiments in this frequency range. In these measurements of BRDF, transmission agrees with previous measurements, and the balance between low and high angle scattering, specular reflectance, and absorption is examined. Keywords: absorption, BRDF, expanded polystyrene, material measurement, millimeter-wave, scattering, terahertz 1. INTRODUCTION Conventional measurements of millimeter-wave and terahertz material loss simply measure the decrease in power coupled from source to detector when a sample is introduced. They therefore do not distinguish between absorption and scattering; effects such as grating lobes due to periodic structure are neglected, and beam broadening in reflectivity measurements due to diffuse reflectance from, e.g., surface roughness, is not measured. The distinction between scattering and true absorption is an issue in both active and passive millimeter- wave/terahertz imaging. Figure 1 pictorially demonstrates the extreme cases of concealers’ material properties being strongly scattering, reflective, and absorbing, for both imaging modalities. In the active modality, thermal emission from the body, concealer, and concealed object are assumed negligible; the transmitted radiation is orders of magnitude greater than the blackbody radiation, and the detector does not have the dynamic range required to receive both thermal emission and reflected incident radiation simultaneously. In the passive modality, thermal emission is assumed dominant; a sensitive detector (cryogenic or heterodyne) is typically used. Previous work has measured transmission through clothing samples in the millimeter-wave and infrared regimes 1 , but did not specifically consider scattering. The precise distinction between scattering and absorption plays an important role in radiometric scene simulation 2,3 , as well as the scattering/reflectance balance of building materials. A reflectometer has been built to measure broadening of specularly-reflected radiation for the latter purpose 4 , and is essentially a predecessor to the system described in this paper. Besides imaging applications, engineering of systems in this frequency range often require extremely low-loss materials. One example of a system requiring a low-loss structural material is the Aqueous Blackbody Calibration source 5 . The ABC source is a water-based broadband blackbody emitter for the millimeter-wave/terahertz frequency regime. Its upper cutoff frequency is currently determined by the balance between scattering and absorption in the _____________________________________ † Contribution of the U.S. government. Not subject to copyright. ‡ e-mail: dietlein@boulder.nist.gov , phone: +1 303 497 4843 Passive Millimeter-Wave Imaging Technology XI, edited by Roger Appleby, David A. Wikner Proc. of SPIE Vol. 6948, 69480E, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.784598 Proc. of SPIE Vol. 6948 69480E-1