Nondestructive evaluation of defect in optically opaque material by polyvinylidene difluoride film sensor

In this paper, we propose an one-dimensional model to analyze a photopyroelectric signal of optically opaque materials measured by a polyvinylidene difluoride (PVDF) film sensor in a thermal wave microscope. This model considers three layers (sample, air gap and the PVDF film) and the reflections of the thermal wave at the interfaces of each layer. In order to check the validity of the proposed model, we measure the inclination of the frequency dependence of the signal measured by the PVDF film and compare it with a theoretical value. When laser light is irradiated onto a graphite surface on the PVDF film, the measured inclinations are -1.6 for a 9 μ m PVDF film, -1.8 for a 28 μ m film and -2.0 for a 110 μ m film. These inclinations were comparable with the theoretical inclinations in the cases of all the film thickness of the PVDF film considered. In addition, we describe an artificially internal hole which assume to be a defect in the graphite sample can be evaluated using PVDF film sensor