Composite materials used as structural member in aircraft, vehicles, yachts and vessels, require high performance mechanical properties so their quality control and inspection is essential for security, maintenance and evaluation. Impact damage, as it occurs on the interlaminar scale, is hard to reveal with standard NDT methods. Active thermography is an appropriate method for evaluating such materials, is proposed. The resulting raw data thermographic sequence is processed by the Fast Fourier Transformation (FFT) algorithm. In order to obtain better results, raw data are post processed. The presented approach is based on fitting the temperature decay curves with smooth curve which enables better detection of impact defects. Test specimens are made of glass (GFRP) and carbon reinforced polymer (CFRP).