A projection illumination unit (1) is provided with an illumination device (3), comprising a light source, in particular a UV-light emitting laser, a projection lens (7), a reticle plane in which a reticle (5) is arranged, a wafer plane in which a wafer (2) is arranged and a flushing gas system, for flushing the interior of the projection lens (7) and/or the external regions, in particular, the reticle plane and the wafer plane. At least one device (25,32,33) for the monitoring of at least one flushing gas flow is provided, which interrupts the radiation from the light source to the projection lens (7) when the flushing gas flow rate drops below a given level. The device (25,32,33) for the monitoring of the flushing gas flow is connected to the light source by means of a controller. The controller (27) is provided with a timer arrangement (36).