Passive Millimeter-Wave Imaging for Civil Engineering Inspection Using a Thermal Blackbody Source

Terahertz and millimeter-wave imaging have attracted considerable interest for non-destructive testing (NDT) because of their ability to penetrate many low-loss dielectric materials while providing contrast related to their electromagnetic properties. These techniques have demonstrated significant potential for the inspection of civil engineering structures, composite materials, insulation systems, and concealed defects. Among active imaging approaches, silicon-based FMCW radars have become particularly attractive owing to their compactness, low cost, and high range resolution. However, their deployment may be restricted in some environments because of regulations governing electromagnetic emissions or concerns regarding interference with nearby electronic systems.

Passive imaging represents an attractive alternative for these applications. By exploiting naturally emitted thermal radiation or an external incoherent thermal source, passive systems eliminate the need for radio-frequency transmitters while avoiding regulatory constraints and electromagnetic interference. In our previous work, a passive millimeter-wave sensor operating around 100 GHz was successfully employed for augmented-reality imaging in civil engineering, enabling the visualization of hidden structures located behind insulating panels.

In this work, we investigate the use of an ultra-sensitive passive detector combined with a heated blackbody source for transmission imaging in a civil engineering context. Unlike conventional continuous-wave imaging systems, the proposed approach relies on broadband incoherent thermal emission rather than coherent electromagnetic illumination. Consequently, it benefits from a simple experimental configuration while avoiding coherence-related artefacts and emission constraints.

The proposed concept was experimentally evaluated using two plasterboard panels placed between the thermal source and the passive sensor. Raster-scan measurements were performed to reconstruct transmission images of concealed objects. Experimental results demonstrate that the proposed system is capable of producing high-quality images with a spatial resolution of approximately 2 mm. These results highlight the potential of passive blackbody-based millimeter-wave imaging as a compact, regulation-free solution for non-destructive inspection in civil engineering and other industrial applications.