The invention relates to an optical sensor (1) for detectomg an object in a monitoring area. The optical sensor (1) has a has a transmitting/receiving unit (6) arranged inside a housing (3) and rotatable around a rotational axis. The transmitting/receiving unit (6) includes a transmitter (8) to transmitted light rays (7) and a receiver (10) to receive received light rays (9). The transmitted light rays (7) and/or the received light rays (9) are guided through a window (12) in the housing (3). The transmitting/receiving unit (6) forms a distance sensor which works according to a light transmission time method. The transmitter (8) emits the transmitted light rays (7) in a transmitted light pulse form. The transmisstion time from the transmitted light pulse to a to-be-detected object (13) is detected.