A lock-in CCD camera based method for thermal diffusivity measurement by an improved photothermal beam deflection slope method

Abstract

Photothermal beam deflection is a well-established technique for measuring thermal diffusivity. A linear relationship that arises in this technique is that given by the phase lag of the thermal wave as a function of the distance to a punctual heat source when unidimensional heat diffusion can be guaranteed. This relationship is useful in the calculation of the sample’s thermal diffusivity, which can be obtained straightforwardly by the so-called slope method, if the pump beam modulation frequency is well-known. The measurement procedure requires the experimenter to displace the probe beam a given distance from the heat source, measure the phase lag at that offset and repeat this for as many points as desired. This process can be quite lengthy in dependence of the number points. Here we present a detection scheme that overcomes this limitation and simplifies the experimental setup using a Web-cam that substitutes all detection hardware utilizing motion detection techniques and software digital signal post-processing. The used Logitec C920 camera does not have on board lock-in capabilities, so specialized self-referenced lock-in software was developed for this application. Basically, a video is recorded for several pump beam excitation cycles and is post-processed so that a phase image is obtained. The usefulness of the method is demonstrated by measurements of test samples.