PHOTOACOUSTIC INVESTIGATION OF THE THERMAL-PROPERTIES OF LAYERED MATERIALS - CALCULATION OF THE FORWARD SIGNAL AND NUMERICAL INVERSION PROCEDURE

The structure of layered materials or substances with depth-dependent properties is often difficult to investigate with conventional optical or acoustical methods. Since the photoacoustic technique is based on thermal waves, which have other transmission and reflection properties than optical or acoustical waves, it can provide information on samples for which other methods fail. In this work, a straightforward numerical calculation of the photoacoustical signal for samples with a one-dimensional layered optical and thermal structure is described. An inversion procedure, based on the nonlinear least-squares fit routine MINUIT is shown to be very effective for obtaining depth information from the frequency dependence of the photoacoustic signal.