An optical system for testing IR or UV sensors, comprises input optics, output optics having a reticule disposed on the associated optical axis, a radiation source which emits radiation in the visual spectral region and in the infrared or ultraviolet spectral regions, and a beam splitter for simultaneously visualizing an object scene illuminated by the radiation source with the reticule through the output optics into the eye of an observer. The input optics comprise a lens, the imaging properties of which in the visual spectral region are equal to the imaging properties in the infrared or ultraviolet spectral region, and the beam splitter is a dichroic beam splitter.