Absorption, Transmission, And Scattering Of Expanded Polystyrene At Millimeter-Wave And Terahertz Frequencies

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.