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Quantitative Comparison of Thermographic Data Sequences

Acceptance of thermography for nondestructive testing (NDT) has grown considerably in the past few years. In a typical implementation scenario, the inspector evaluates results visually in order to determine whether or not a repair is required or a part is acceptable. In online quality and process control applications, this approach is too slow, too subjective and not sensitive enough to be practical. In manufacturing, it is often necessary to automate the test process and compare a part to a discontinuity free “gold standard” sample, in order to make an instantaneous accept/ reject decision or adjust process parameters. In such cases, a part may be rejected because of structural, compositional or cosmetic irregularities. The issue is not whether a sample is good or bad, but whether or not it is identical to the master. Conventional thermographic analysis techniques can be difficult to apply in this situation, as direct comparison of entire image sequences is required. However, sequence comparison can be simplified, and even automated, by reducing an entire thermographic time sequence to a single fingerprint image, which is rotationally and translationally invariant and immune to shot to shot energy variations, ambient temperature variations and other common thermographic artifacts.

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