Thermal waves are used in nondestructive testing for defect analysis by lock-in-thermography. A thermal wave field contains useful information about material and component properties, such as layer thickness or reflection coefficients of internal boundaries. Only the exact knowledge of the thermal wave field enables quantitative evaluation of these parameters. For example, a calculated wave field can be used for deconvolution of lock-in-thermography images as it provides a point spread function and therefore gives a better lateral resolution. For a better knowledge of thermal wave fields, basic experiments concerning interference effects are presented in this paper.