To specify the technique ability to provide the depth of analysis in our experimental condition, it is important to consider that in the rapid scan mode, the FTIR-PAS method provides the thermal diffusion length (cm) as , in which D is the sample thermal diffusivity (), is the mirror velocity (), and the incident radiation wavelength (cm). This parameter is the dimension over which the thermal wave decays to of its original amplitude and has been used in the analysis as approximately the sampling depth, where the PAS signal is generated. In our experimental condition, the sample can be approached as comprised of two layers with a hybrid interface, resulting in effective values for the thermal diffusivity. It depends on each values, according to the incident radiation wavelength. For the dentin in the tubular direction, ,22 while for the adhesives, the TL measurements performed in this work provided similar values, . Since the exciting radiation was incident on the adhesive side of the samples, we estimate using the adhesives thermal diffusivity values. Thus, using , we calculate, for example, the penetration depth at () to be higher than 5.0 µm.