Abstract: A process, apparatus and heat microscope for testing the properties of materials by the photothermal effect includes generating a laser beam with a laser light source integrated into a portable measuring head, emitting the laser beam toward a region of a surface of a material sample to be tested, and focussing the laser beam to a desired measurement point diameter at a target light spot with optics at an end toward the laser beam, for absorbing a proportion of the amount of light energy with irradiated volume elements of the material sample and emitting infrared light signals from the surface of the volume elements and volume elements adjacent thereto. The emitted IR light signals are conducted to an optical decoupling element for conducting the emited IR light signals further and largely suppressing components of the laser beam reflected at the surface of the sample.