Thermographic evaluation of Coating Thickness with Different Modulation Waveforms

Modulated thermography has been successfully applied to coating thickness evaluations. In addition to lock-in thermography (LIT), three different more waveforms are used to alternate the heating patterns. Among them, frequency modulated thermal wave imaging (FMTWI) belongs to frequency modulation, Barker coded lock-in thermography (BC-LIT) belongs to phase modulation, Barker coded frequency modulated thermal wave imaging (BC-FMTWI) belongs to the combination of frequency modulation and phase modulation. A two-dimensional simulation model is established with COMSOL. To better compare the detectability in coating thickness, Gaussian white noise is introduced into the temperature signal. After Cross-correlation (CC), both CC-Delay and CC-Phase demodulation methods are used to calculate the SNR of different waveforms. The results show that CC-Phase is more preferable for detecting the thickness of thin coatings for all modulating waveforms. FMTWI and BC-FMTWI can effectively increase the detection range in thickness. However, LIT still has better SNR in its sensitive thickness range.