Continuous wave terahertz imaging for NDT: Fundamentals and experimental validation. (February 2021) - Costa, Frederico B.
Abstract: Continuous wave terahertz (CW THz) imaging, is a variant of terahertz imaging that has been gaining scientific and technological relevance in multiple areas. In this paper the fundamental phenomena of CW THz were studied and a mathematical model was developed that successfully describes the Fabry–Perot interference for such a system, opening the possibility for measurement of thicknesses and surface curvatures. The capabilities of the system were tested using different types of defects, such as voids, water infiltrations and thin metallic wires. The interactions between different materials, features and the radiation beam were numerically studied using finite element method and the results agreed with the experiments. By comparing the results with other Non-Destructive Testing methods, it was found that CW THz imaging is particularly interesting to image water infiltrations and composite materials that incorporate conductive wires. Graphical abstract: Highlights: Fabry–Perot effect is crucial to understand Continuous Wave Terahertz Imaging. An analytical model comprising Fabry–Perot and attenuation phenomena was validated. Numerical simulation provided valuable insight on experimental THz NDT results. 35 μ m metallic wires and water infiltrations were detected in dielectric parts. THz radiation presented high reliability compared to some other NDT techniques.