Abstract: A detector contains a housing with at least one window for allowing radiation to enter, at least one outlook sensor for sensing entered radiation, a unit for processing outlook sensor signals, and outlook mirrors that are shaped and mounted in the housing for reflecting onto the outlook sensor radiation from outside detection zones better than radiation from elsewhere. At least some of the outlook mirrors face the window and in operative orientation neighbor each other vertically. The detector further contains one or more window sensors for sensing radiation indicative of the window being masked or having been damaged and a unit for processing window sensor signals. A gap between at least two of the outlook mirrors allows radiation to travel between the window and at least one window sensor or accordant window sender or both.