Optically modulated elastic waves enter the surface of the inspected material by absorption of thermal radiation. An
inhomogeneous disturbance in the material causes locally enhanced losses. Such material defects heat up at a different
rate than the surrounding more homogenous material and therefore generate differences in thermal contrast. Modulating
the amplitude of the optical stimulus turns defects into thermal wave transmitters. The frequency of the stimulus signal
must be matched to the specific thermal conductivity and mass density of the inspected material. It is possible to locate
defects at different depths below the surface by varying the amplitude of the stimulus wave. Proper detection of thermal
waves resulting from material defects near the surface requires a measurement setup that allows the recording of thermal
images from an infrared camera. The recorded images can then be compared against the phase of the stimulus signal.
This photo-thermal lock-in thermography method not only allows the evaluation of the amplitudal thermal wave
information but more importantly it also allows for the extraction of the phase information. The phase shift between the
stimulus signal and the captured thermal wave is directly correlated to the thermal propagation time.