Passive Terahertz Non-Line-of-Sight Imaging

We demonstrate the capability of the THz frequency (submillimeter wavelength) band to perform nonline-of-sight imaging by reflection from common rough surfaces. Active bidirectional reflectance distribution function measurements at 336 GHz show that surface roughness affects the strength, but not the width, of a specularly reflected beam of submillimeter-wavelength light. Uncooled passive direct-detection receivers built using high-frequency InP transistor technology achieve noise-equivalent temperature differences (NETD) <= 33 mK. One and two-dimensional, scanned single-pixel images made using such a passive receiver show that determination of the location, orientation, and pose of human targets can easily be made on surfaces whose specular component of reflectance is as low as 8%, without any algorithmic assistance. With such low instrumental NETD levels, the radiometric component of image quality is dominated by real fluctuations in the radiometric temperature of ordinary indoor backgrounds.