Pulsed, lock-in and frequency modulated thermography are three alternative nondestructive evaluation techniques. The defect imaging performance of these techniques are compared using: matched excitation energy; the same carbon fiber composite test piece and infrared camera system. The lock-in technique suffers from blind frequencies at which phase images for some defects disappear. It is shown that this problem can be overcome by using frequency modulated (chirp) excitation and an image fusion algorithm is presented that enhance phase imaging of defects. The signal-to-noise ratios (SNRs) of defect images obtained by the three techniques are presented. For the shallowest defects (depths 0.25 and 0.5 mm, 6 mm diameter), the pulsed technique exhibits the highest SNRs. For deeper defects the SNRs of the three techniques are similar in magnitude under matched excitation energy condition.
Chatterjee, K., Tuli, S., Pickering, S. G., & Almond, D. P. (2011). A comparison of the pulsed, lock-in and frequency modulated thermography nondestructive evaluation techniques. NDT and E International, 44(7), 655-667. https://doi.org/10.1016/j.ndteint.2011.06.008