The paper presents a numerical method to detect and characterise defects and inhomogeneities by means of active thermography. The objective is to determine the wall thickness of structure elements with an inaccessible back wall, e.g. of pipes or containers. As test specimens we used 2 cm thick PVC samples with flat bottom holes in the back wall. Flash lamps provided the heating. The inversion method is the Levenberg-Marquardt method, which starts by a rough first guess of the back wall geometry using the echo defect shape method, and then the PVC thickness of specimen profiles was calculated. We validated the result by comparing the measurement data to simulated data for selected geometries.