The reflective optical element with a measuring device

The invention relates to a reflective optical element comprising a substrate (1), a reflective coating (3) and a measuring device (5). The measuring device (5) is designed such that it is at least partially disposed between the coating (3) and the substrate (1). This ensures that all areas of the optical element are incident on the relevant radiation effect reflecting, and in addition, the radiation can be measured at different points. This has the further advantage that no additional loss occurs characterized in that a portion of the radiation must be hidden for measuring and monitoring purposes. the proportion of radiation is used instead of this, which is anyway not reflected.