Thermographic reconstruction of internal heat sources by means of virtual sound waves

Thermographic reconstruction of internal heat sources using the virtual wave concept. Using an infrared camera for thermographic reconstruction has many advantages compared to ultrasound reconstruction: no coupling media is needed and it makes it possible to measure the temperature of many surface pixels both simultaneously and without contact. The main drawback is the strong degradation of spatial resolution as imaging depth increases, which results in blurred images for structures lying deeper. In this work, it is shown that image reconstruction methods from ultrasonic imaging can be employed for thermographic signals. Before using these imaging methods, a virtual signal is calculated locally for each camera pixel by applying a one-dimensional reconstruction ("local transformation") for the thermographic signal at this pixel position. The local transformation describes all the irreversibility of the heat diffusion process and can be used for every sample shape and dimension. This is demonstrated by two-dimensional reconstructions from numerical simulations and from thermographic measurements, where parallel steel-rods, embedded in an epoxy matrix are heated by induction using eddy current pulses.