Abstract: Method for avoiding a degradation of an optical element, wherein the optical element is arranged in a housing, comprising: a) determining a first degradation value; b) determining a second degradation value, wherein the first degradation value and the second degradation value are determined at different times; c) forming a degradation profile based on the first degradation value and the second degradation value; d) calculating a temporal development of the degradation profile; e) determining at least one predicted degradation value based on the calculated temporal development of the degradation profile; f) comparing the at least one predicted degradation value with a predefinable first limit degradation value; g) monitoring for a predefinable first deviation between the at least one predicted degradation value and the first limit degradation value; h) feeding a first decontamination medium into the interior if attainment of the predefinable first deviation is identified.

Abstract: In a method for characterizing at least one optical component of a projection exposure apparatus (1), an intensity distribution of the illumination radiation (2) is detected in a field plane of the projection exposure apparatus (1) with a measuring device (31) and predicted values of an optical parameter are spatially determined therefrom over at least one predefined surface.