Theory and experiment of IR lock-in thermography with image sequence processing

Lock -in thermography is an active infrared thermography technology which is based on thermal wave processing and surface temperature analysis. It is especially suitable for the non-destructive test of subsurface defects of composite materials and compound structure component. The principle of lock-in thermography was described, and the FEM was used to investigate the behavior of thermal waves in homogeneous plates with several holes in subsurface as defects and the plate with a finite thickness under convective conditions. In experimental study, the lamp with sine modulated law was used for active heat source into sample, and the image sequences of thermal wave were collected with Jade MWIR 550 FPA infrared thermography. The digital filter algorithm was researched and Savitzky-Golay digital smooth filter algorithm was used to reduce high frequency noise signal. And then the FFT transform algorithm was researched to obtain the image of amplitude and phase of thermal wave on the surface of sample. The program of IR lock -in thermography was developed by Visual C ++ development platform. The experimental results show that the selecting proper excited frequency can acquire the defect characters.