The invention relates to an optical
system, in particular for microlithography, having at least one optical element (100, 200, 300, 400) that comprises a substrate (105, 205, 305, 405) and at least one optically effective layer
system (110, 210, 310, 311, 410) situated on this substrate (105, 205, 305, 405), and at least one monitoring unit (120, 220, 240, 320, 340) for monitoring a degradation state of this layer
system (110, 210, 310, 311, 410), wherein the monitoring unit (120, 220, 240, 320, 340) comprises a
detector (121, 221, 241, 321, 341) for detecting non-directed
radiation (130, 230, 330, 430) that emanates from the optical element (100, 200, 300, 400) during the operation of the optical system, wherein the non-directed
radiation (130, 230, 330, 430) detected by the
detector comprises
fluorescence radiation that is caused or modified in the event of an interaction of the layer system of the optical element with used light pre-sent during the operation of the optical system.