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Photothermal radiometric detection and imaging of surface cracks
Authors:Irving Kaufman  Pan-Tze Chang  Hsueh-Shun Hsu  Wen-Yuan Huang  Daw-Yang Shyong
Affiliation:(1) Department of Electrical and Computer Engineering, Arizona State University, 85287 Tempe, Arizona
Abstract:This paper discusses surface crack detection by photothermal radiometric imaging (PRI). In PRI, also called dynamic infrared scanning, a surface is scanned with a spot of heat. Imperfections are detected by radiometrically sensing changes in the surface temperature of a small area in the vicinity of this spot. In the work described, cracks narrower than 25 µm (0.001 in.) in a lightly rusted steel surface have been detected. Indiscrete scanning an amplitude modulated heating beam is moved in steps, remaining at each location long enough to measure amplitude and phase of the AC temperature. Incontinuous scanning a constant intensity heating beam is moved continuously while the temperature deviations are measured. This paper presents methods of calculating amplitude and phase of surface temperature for discrete scanning and instantaneous temperature for continuous scanning across a surface crack. For a steel surface scanned by a watt-level laser beam, predicted surface temperature deviations when crossing the crack are several degrees Celsius, with expected radiometrically detected power several orders of magnitude above the detector noise. In experiments performed, both techniques easily detected narrow cracks in a smooth, clean surface. Discolorations and pits, on the other hand, generate a disturbing type of ldquosurface noise.rdquo This noise was minimized by differential detection. Based on results obtained, continuously scanned PRI with a fan-type heating beam and array detection could become a viable way of mapping surface cracks at practical scanning speeds.
Keywords:Photothermal  imaging  radiometric measurement  infrared scanning  crack detection  crack mapping  moving spot of heat  surface temperature  differential detection  laser heating
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