Simultaneous Multi-frequency lock-in Thermography: A new flexible and effective Active Thermography scheme

A new active thermography scheme is here introduced, referred as ”‘Multi-Frequency Thermography”’, which uses an optimized multi-tone signal for simultaneously implementing lock-in analysis on a discrete and arbitrary set of linearly spaced frequencies. Such a signal, which modulates the intensity of the heating source here being a LED system in the visible range, results from the non-trivial summation of a desired number of odd and even harmonics of a fundamental tone, each of them having a specific initial phase value but equal amplitude, so as to deliver the same energy amount for all the chosen frequencies. In this way, a discrete set of thermal waves having different diffusion lengths are simultaneously excited within the inspected sample to probe different depths into it. With respect to standard lock-in thermography, it is demonstrated that the proposed approach can extract amplitude and phase features for all the excited frequencies from a single measurement, which lasts as long as a lock-in implemented at the fundamental tone. Both quantitative and qualitative comparisons with standard lock-in thermography are here reported, showing an excellent agreement. Hence, this new active thermography scheme can provide several advantages in practical implementations of thermography nondestructive evaluation.