Lockin-Thermography: Principles, NDE-applications, and trends

A review is given about Lockin-Thermography, about its photoacoustic and photothermal roots, about the principle and modern applications for nondestructive testing using different kinds of options. 1. What is Lockin-Thermography? As Lockin-Thermography is based on thermal waves, a short excursion to this kind of waves and some remarks on the way they were used seems to be appropriate. Some indications about promising futural developments are provided as well. 1.1. Basics of thermal waves When Fourier was involved in planning the water supply tubing for Paris, he was concerned with the problem how deep the tubes should be buried in the soil to prevent freezing in winter. So he dealt with the periodical temperature cycles on the surface and how deep they extend into the soil. He found out that the process is described by a linear differential equation whose solution is a highly attenuated wave where thermal diffusivity µ is the only parameter involved [1]. In order to solve the linear differential equation for non-sinusoidal boundary condition (daily and annual temperature cycle), he decomposed it into a sum of sine functions and superposed the solutions. Fourier became famous for developing the mathematical principle of solution which is broadly applicable. His solution is the “thermal wave” that describes the deviation T of temperature from its local average,