Phase-Sensitive Passive Terahertz Imaging at 5-m Stand-Off Distance

A novel imaging modality for passive THz scanners is described. The method relies on the phase-sensitive detection of THz radiation which is emitted or reflected from the object under investigation. A blackbody which is implemented in the imaging system provides a reference source. By alternate viewing of the scene and the reference source, phase-sensitive detection with a lock-in amplifier is realized. This enables simultaneous intensity and phase detection. Experiments which demonstrate the capabilities of the detection scheme have been performed with a passive THz scanner for stand-off detection at 5-m distance. The system operates at 345 GHz. When choosing the effective temperature of the reference source as the average of the object and its surroundings, only the edges of the object under investigation were detected in the intensity domain. This is hardware-based edge detection, in contrast to software-based edge detection where edges in an image are determined by a software algorithm. An additional advantage is the phase detection which effectively acts as a threshold detector and can improve the signal-to-noise ratio. Because only edges are detected in this scheme, it seems feasible that the privacy problem is relaxed or the privacy problem might be overcome.