Thickness determination and prediction of wet paint using terahertz time-domain spectroscopy

We present an approach to determine the thickness of wet paint using terahertz time-domain spectroscopy (TDS). For calibration, an additional distance sensor is used to measure the layer thickness indirectly. This allows to calculate the material properties (index of refraction and extinction coefficient) during the entire drying process, which are described in this work with the Lorentz dispersion model. From this calibration data set, the layer thickness can be determined at any time during the drying process and, if the material properties change significantly, also the current drying condition. For the tested paint, this also allows to predict the coating thickness in dry state.