Abstract Due to the increasing application of CFRP in mass application, an economic repair process gains importance. As the typical repair process starts with the defect detection, the accuracy of the measurement system is a limiting factor of the repair process. Lock-in thermography is a promising approach for non-destructive defect detection. Since the phase images contain a superposition of depth information, the determination of the defects' depth is a challenge. Based on the complex wave field, this paper introduces a method to determine the thermal properties and the defects' depth in CFRP structures using lock-in thermography.