The sensor does not respond to sunlight, light bulbs, fluorescent lights or flying sparks. False tripping is possible, however, with UV radiating light sources. Included in this are, for example, halogen and mercury vapor lamps, lightning, arc during welding work and reflections of the above radiation sources. In addition, smoke, dust and water vapor, as well as contamination of the UV pane, absorb the ultraviolet radiation and therefore negatively effect the response behavior of the sensor. The device has potential-free changeover for malfunctions and alarm and can be set to very short reaction times.