A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). The lithographic apparatus has an inspection apparatus with an EUV radiation source. The radiation source emits a radiation beam that includes coherent radiation of a specific wavelength. The beam propagates to illumination optical system, which focuses the radiation beam into a focused beam of illuminating radiation. The illumination optical system illuminates a three-dimensional product structure on the substrate, which scatters the illuminating radiation. On the surface of a detector, the radiation scattered by the product structure forms a diffraction pattern that is used to reconstruct data describing the three-dimensional product structure.