The thermographic images of laser heated spots or lines are perturbed by nearby cracks, providing NDE techniques for crack detection. Scanning with a laser line, rather than a laser spot, results in a substantial reduction in inspection time. 3D finite difference modelling results are presented that show the sensitivity of the laser line thermography technique to cracks of varying lengths, depths and openings. A novel crack imaging technique is presented that is based on assembling second spatial derivative thermal images of a scanned laser line. Experimental results show the new technique to image cracks with openings as small as a few micrometres. The scanning time of the laser line thermography technique is shown to be over an order of magnitude smaller than that of the laser spot thermography technique whilst producing crack images of similar quality.

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