Imaging with Optically Generated Thermal Waves

TLDR: In this article, the question of whether a temperature periodicity in time is correlated with a periodicity along the z-coordinate is investigated. But it is difficult to imagine what a thermal wave could be characterized by a time and space periodicity, since it is not symmetrical with respect to time inversion.

Abstract: (Carslaw I959), with temperature T, coordinate z, mass density p, specific heat c, thermal conductivity k, and time t. This type of equation is known from diffusion. As the equation is not symmetrical with respect to time inversion, it describes an irreversible process which steadily reduces existing gradients. On this basis it is difficult to imagine what a thermal wave could be. A wave is characterized by a periodicity in time and space. The question is, therefore, whether a temperature periodicity in time is correlated with a periodicity along the z-coordinate. If temperature is modulated at the plane z = 0 at the angular frequency cl and with amplitude ATO, then the modulation at z > 0 is (Carslaw & Jaeger 1959)