The present invention relates to a projection exposure system for microlithography with a light source for generating working light (9), an illumination system for illuminating a mask having structures to be imaged and a projection objective for imaging the structures of the mask onto a substrate, the illumination system and / or projection objective being at least one The light guide element (14) comprises which operating light (11) differs from the working light (9) and which has a metallic protective layer (17).
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