The invention relates to an optical sensor for monitoring a monitored zone having a detection device for observing a detection zone including the monitored zone, said detection device including a transmission device for transmitting transmission light into the detection zone and a receiver for receiving light reflected back or remitted back from the detection zone or transmitted through the detection zone and having at least one test object within the detection zone of the optical sensor. In accordance with the invention, the optical sensor has a device for avoiding and/or eliminating the contamination of the at least on test object.