An arrangement for use in a projection exposure tool (100) for microlithography comprises a reflective optical element (10; 110) and a radiation detector (30; 32; 130). The reflective optical element (10; 110) comprises a carrier element (12) guaranteeing the mechanical strength of the optical element (10; 110) and a reflective coating (18) disposed on the carrier element (12) for reflecting a use radiation (20a). The carrier element (12) is made of a material which upon interaction with the use radiation (20a) emits a secondary radiation (24) the wavelength of which differs from the wavelength of the use radiation (20a), and the radiation detector (30; 32; 130) is configured to detect the secondary radiation (24).