A position-independent dual-focal-spot terahertz spectroscopic method for accurate thickness characterisation of multilayer coatings on anisotropic substrates

In the aerospace automotive and advanced manufacturing industries, precise and non-destructive paint thickness measurement is essential for guaranteeing safety and optimal performance. For this purpose, terahertz (THz) spectroscopy has proven to be a dependable instrument particularly when employing frequency-domain fitting methods. However, when these methods are applied to anisotropic and heterogeneous substrates like carbon fibre-reinforced polymers (CFRPs) their accuracy is often reduced. To address these challenges and enable accurate thickness measurement of multilayer coatings on CFRP substrates this study presents a novel dual-focal-spot THz spectroscopic technique. The method obtains a position-independent equivalent substrate parameter that effectively averages out local anisotropic differences by gathering spectral data from two focal spots with different beam sizes. Effective frequency-domain fitting is possible with this technique even in the presence of structurally complex substrates. The method can accurately determine coating thicknesses as small as 50 μm with measurement errors as low as 2 μm and without requiring the sample to be mechanically rotated according to experimental validation. The application of THz spectroscopy for high-precision non-contact inspection in industrial settings is greatly enhanced by the recommended method.