We study theoretically and experimentally the ability of photothermal techniques to thermally characterize the anisotropic materials whose principal axes are not contained at the sample surfaces. We first determine the thermal diffusivity tensor when the directions of the principal axes are known. Inversely, we calculate the directions of the principal axes in a sample whose thermal diffusivities are given. Measurements validating our theory are presented for a sample of pyrolytic graphite (PG), which has a high anisotropy, by means of two photothermal techniques: IR radiometry and the mirage effect.
"Photothermal characterization of anisotropic materials with buried principal axes," Optical Engineering 36(2), (1 February 1997). https://doi.org/10.1117/1.601211