Resolution studies for thermal wave imaging

Results are presented from experimental studies of thermal wave scattering from subsurface targets in solids scanned with an optical probe (laser) beam. The technique is under development for defect detection through mirage effect monitoring. The specimens scanned were two rectangular plastic channels embedded in a 3 mm thick Al block, a plastic-coated metal sphere covered with a thin metal film and a metal wire encased in plastic. In the case of the metal bar, the probe beam scanned the surface at an angle perpendicular to the plastic channel. The thermal signatures derived at different wavelengths of the probe beam were compared with theoretical predictions. The predictions and experimental data showed good qualitative agreement. The data indicated that the intrinsic spatial resolution in the extreme near-field limit is independent of the thermal wavelength and is controlled by the depth of the subsurface defect.