With the pulsed photo-thermal measuring method, a short laser pulse is sent towards the coating, which heats up the coating. The short wave-length laser radiation is changed into long wave-length radiation, so-called heat radiation, when the laser interacts with the coating. The temperature slowly spreads from the surface to deeper coating regions, until it reaches the substrate. As long as energy is supplied via the laser pulse, the temperature rises. Once the energy supply is terminated, the temperature falls because the heat is rapidly conducted away by the substrate. The receiver optics record the temperature changes on the coating surface over time. An evaluation unit calculates the coating thickness from this. Thinner coatings are more rapidly heated by the laser than thicker coatings. The change over time is used as a measure for the coating thickness. The measuring method depends significantly on the fact that the coating and the substrate have different thermal conductivities.

Fig.1: Principle of the photo thermal measuring method