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In this paper, the use of a signal to noise ratio is proposed for the quantification of the goodness of two selected lock-in thermography processing techniques. The single Fourier Transform (DFT) and the Harmonic Approximation (HA) were applied to experimental data from a CFRP specimen containing 25 defects (Teflon ® inclusions) at different depths and having several sizes. The obtained phase images were compared using this SNR parameter value, and DFT showed better results than HA, for which a reduction of approximately 6dB was quantified. This reduction affects consideraly the automated detection capabilities of smaller and depper defects, for which a SNR value higher than one (0 dB) is required. Active infrared thermography applied to the non-destructive testing (NDT) of materials offers a reliable, straightforward and fast means for retrieving structural information from a specimen. In the lock-in technique, which is also known as thermal wave imaging, the heat introduction occurs periodically with a certain frequency, and the local surface temperature modulation is evaluated for the detection of anomalies that could indicate the presence of a defect or inhomogeneity. The temporal evolution of the surface temperature is stored in a thermographic images sequence (thermograms). Defect enhancement techniques must be applied in order to produce a successful thermal inspection. Two specific image processing techniques, one based on the Discrete Fourier Transform (DFT) and the other based in the Harmonic Approximation (HA) of the Fourier Transform [1-2], have been applied to lock-in thermographic inspection showing both advantages and disadvantages. The present work describes how to quantify the goodness of each technique in defect detection using a signal to noise ratio (SNR). The performance of the proposed method has been evaluated on a carbon fibre reinforced plastic (CFRP) specimen containing several defects, having different surface geometries, and being subjected to a sinusoidal excitation. This evaluation is done for sequences obtained with different excitation frequencies.

Signal to noise ratio (SNR) comparison for lockin thermographic data processing methods in CFRP specimen