Abstract: The invention relates to a novel thermoelectric sensor for determining or measuring the thermal power of radiation, especially laser radiation, with at least one thermoelectric detector or sensor element which has an active detector surface of a thin active layer of a crystalline solid with anisotropic thermoelectric power and in which the surface perpendicular of the layer does not coincide with one of the primary anisotropy directions, on the thin active layer there being at least two contacts for tapping a voltage dependent on the thermal output.