Temperature response for active thermography using laser scanning heating and method of images

Abstract

When scanning a specimen using a line heat source at a constant velocity, a temperature change occurs partly due to specular reflection at the defect interface. Assuming that the reflection of a transient thermal response is similar to that of geometrical optics, we performed waveform analysis using an imaging method. Image points are calculated based on differential geometry; this can also be performed for a curved surface using a general equation. A combination of the steepest descent analysis of a moving heat source problem and a convolution technique successfully yielded waveforms comparable to those of experimental temperature responses. We designed and constructed an active thermographic imaging system in which a linearly focused continuous wave laser beam was scanned perpendicular to the beam as it covered the entire surface of specimens with simulated internal defects. The real-time response was recorded as a temperature waveform at each image pixel. Waveforms were calculated for specimens without or with buried cylindrical defects parallel to their surfaces and compared to the experimental data. The theory well-explains the signal generation mechanism, and excellent agreement was obtained in waveforms. Some discrepancy between theory and experiment indicates more complicated problems in heat and mass transfer.