An apparatus for determining the temperature of a substrate, in particular of a semiconductor wafer during a heating thereof by means of a first radiation source is described. Furthermore, an apparatus and a method for thermally treating substrates are described, in which the substrate is heated by means of at least one first radiation source. The apparatus comprises a first grating structure having grating lines, which are opaque with respect to a substantial portion of the radiation of the first radiation source, wherein the grating structure is arranged between the first radiation source and the substrate, and a drive unit for moving the first grating structure. Furthermore, a first radiation detector is provided, which is directed directly onto the surface of the substrate facing the grating structure, and a device for determining radiation emitted by the substrate due to its own temperature and for determining the temperature of the substrate on the basis of the radiation detected by the first radiation detector.