A detector system for spatially resolved detection of a gas substance in an area is described. The detector system includes a detector comprising an image sensor; a band filter arranged in an optical beam path before the detector for transferring a beam with a wavelength spectrum including an absorption wavelength corresponding to the gas substance, a telescope, a polarizing beam splitter, and an interferometric stage including a retarder for creating an optical path difference for measuring absorption dips due to the presence of the gas substance. The retarder includes multiple birefringent media arranged with the optical axes relative to each other so that at least one increases an optical path difference and at least one decreases an optical path difference between the polarized beam components, and the thicknesses of the birefringent media are tuned to minimize a focal shift between the polarized beam components.