Here, β is the volume thermal-expansion coefficient, cp is the specific heat of the medium, N is the ratio of acoustic impedances of the absorbing and transparent media, and τ is the running time. As follows from (4), in the limit of an acoustically free boundary (N ≫ 1), the OA signal represents the rarefaction phase following the compression signal front. In the limit of an acoustically rigid boundary (N ≪ 1), the wave reflected from the medium boundary is also compressive. The most important conclusion from (4) is that the leading edge of the generated OA signal repeats the distribution of heat release in the medium and thereby the distribution of the laser fluence. In the case of a purely absorptive character of the medium under study (a scattering-free medium), the in-depth light penetration along z-axis obeys the Bouguer–Lambert–Beer law