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Comparison of image correlation algorithms for hidden damage laser speckle photometry

This paper explores laser speckle photometry (LSP), a recent optical-based image analysis tool, as a method for detecting barely visible impact damage (BVID) in composite structures. This non-contact optical-based method provides the potential for large-scale scanning of aircrafts in real time to unearth BVID which would typically go unnoticed during routine inspections. In exploring LSP, various image correlation algorithms were tested to determine the most effective. The typical error-based correlation algorithm of mean squared error (MSE) was compared to two more advanced algorithms, normalized cross-correlation (NCC) and structural similarity (SSIM) index. Thermal LSP was conducted on a composite honeycomb panel with a surface dent of maximum depth 0.5 mm. The underlying damage was around 30 mm in diameter. When compared to the baseline experiments conducted using C-Scan and a laser Doppler vibrometer (LDV), it was found that only with limited cooling (around 2 seconds) did the algorithms produce consistently accurate results. Among the three imaging conditions, MSE and certain configurations of SSIM provided results that aligned the best with the baselines. Upon extended cooling, LSP produces too many false positives to be useful regardless of correlation algorithm used. Nevertheless, LSP shows great promise as a real-time non-destructive inspection tool not only in the aerospace industry but also in industries such as additive manufacturing where defects are prevalent.