Abstract: An optoelectronic sensor (10) for detecting objects in a monitored zone (20) is provided which has the following: a front screen (38); a light transmitter (12) for transmitting a light beam (16); a movable deflection unit (18) for the periodic sampling of the monitored zone (20) by the light beam (16); a light receiver (26) for generating a received signal from the light beam (22) remitted by the objects; at least one test light transmitter (42); at least one test light transmitter (42), at least one test light receiver (44) and at least one test light reflector (48) which span a test light path (46a-b) through the front screen (38); and an evaluation unit (32) which is configured to acquire pieces of information on the objects in the monitored zone (20) from the received signal and to recognize an impaired light permeability of the front screen (38) from a test light signal which the test light receiver (44) generates from test light which is transmitted from the test light transmitter (42) and which is refl