QUANTITATIVE CHARACTERIZATION OF SUBSURFA CE INHOMOGENEITIES BY THERMAL WAVES

Photothermal measurement techniques offer new possibilities for the no ndestructive evaluation of material inhomogeneities below the surface of opaque solids. Many defects such as buried structures, cracks or lo calized deviations from the bulk material are closely connected with c hanges in thermal conductivity or specific heat, and therefore, become accessible by photothermal means. This paper is focused on the quanti tative characterization of subsurface defects with respect to strength , location and depth. The imaging process is described by means of the photothermal point spread function, which is subsequently used for th e retrieval of the subsurface defect from frequency dependent photothe rmal measurements.