The basic idea to identify inhomogeneities inside a bulk material from the temperature evolution in time, when the surface is properly heated, was greatly developed in recent time. The most important improvement is due to large computation availability given by updated thermographic digital systems.

This paper is a result of long-term cooperation between CNR-ITEF (Italy) and Tomsk Polytechnic University (Russia). It is targeted to exhibit in a rather illustrative way some possibilities of infrared image processing using both trivial and sophisticated procedures to enhance signal-to-noise ratio. The accent is made onto thermal characterisation of subsurface defects including Dynamic Thermal Tomography (DTT). DTT allows not only to localize defects in space and depth, but also to select only a certain layer for the inspection, enhancing therefore the NDT reliability.

The development of analytical and numerical models of the direct and inverse thermal problem are extremely important for Thermal NDT. These tools allow not only to obtain quantitative evaluation of the defect and a better understanding of the underlying physic, but more important, to simulate the test and therefore to predict temperature maximum value on the surface and the required heating for a desired inspection depth, in the particular material.

Experimental results, including data reduction algorithms, in the inspection of composite and porous materials are presented. The reported inspection of carbon fiber reinforced plastics in Aerospace Industry and the detection of delaminations under frescos are examples of traditional and emerging activities. Another application is the estimation of corrosion of metals. In these cases different approaches have been experimented, dealing with analytical instead of heuristic inversion function.
The use of Neural Networks to classify defects by their depth is also described. This new approach is very promising in this field especially when a theoretical model of the temperature history in time could be computed for the particular target.

The heating function versus time and the experimental set up have been considered, according to different applications. A description of the most important requirements of the thermographic system, the heating device and the controlling equipment is given. The data logging system, the compression and storing of the involved huge amount of data, the processing and displaying device are illustrated as well. The paper will contain a review of thermal NDT features and conclusions on the areas where this method could play screening or independent role.