Seeing Around Obstacles Using Active Terahertz Imaging

In this article, we show how active terahertz (THz) imaging systems can exploit the unique propagation properties of THz waves to reconstruct images of nonline-of-sight (NLoS) scenes. Most building surfaces' material properties and roughness allow for a unique combination of diffuse and strong specular scattering. As a result, most surfaces behave as lossy mirrors that enable propagation paths between a THz camera and the NLoS scenes. We propose a mirror folding algorithm that tracks the multireflection propagation of THz waves to correct the image from cluttering and see around occlusions without prior knowledge of the scene geometry and material properties. To validate the feasibility of the proposed NLoS imaging approach, we carried out a numerical analysis and developed two THz imaging systems to demonstrate real-world NLoS imaging experiments in sub-THz bands (270–300 GHz). The results show the capability of THz radar imaging systems to recover both the geometry and pose of LoS and NLoS objects with centimeter-scale resolution in various multipath propagation scenarios. THz NLoS imaging can operate in low visibility conditions (e.g., night, strong ambient light, and smoke) and uses computationally inexpensive image reconstruction algorithms.