With the thermal wave method we can create a strong temperature oscillation near the surface, but no localized strong oscillation at an arbitrary depth in the sample (this follows from the heat conduction equation). In mathematical terms, the measured current is a convolution of the pyroelectric coefficient p(x) with the temperature distribution:

where I~ is the pyroelectric current and D the thermal diffusivity. Unfortunately, solving for p(x) is an ill-conditioned problem. Various methods exist for retrieving p(x):