Photothermal Radiometry – Principle and Applications

Photothermal radiometry has been proved being a versatile tool for the non-contact and non-destructive material inspection in near-surface zones. Although these near-surface areas represent only a few percent of the workpiece’s volume, they influence significantly its quality, functional behaviour, reliability and life time. In particular, they determine decisively the mechanical properties of precisely manufactured and/or surface finished parts. In photothermics, the propagation and penetration depths of optically excited thermal waves, the thermal response at the surfaces on optical excitation of opaque materials give sets of information about the outer and inner structures of the inspected materials. This paper focuses on photothermal radiometry and its potential to assess the thermal material properties and their changes which might be induced by a mechanical impact and/or by a thermal treatment. Special emphasis is given on the industrial applicability of the photothermal radiometric technique. Two problems of industrial relevance are illustrated: the thickness determination of optically opaque layers and the evaluation of hardness depths and profiles in steel specimens.