An optoelectronic sensor (10) has a circumferential front screen (42) comprising a curvature both in a circumferential direction and in a transverse height, thus focusing light reflected at the inside of the front screen (42). Test light passes from a test light transmitter (50a-f) through the front screen (42) to a reflector (52a-f) and subsequently onto a test light receiver (56a-b). A decreasing light transmissivity of the front screen (42) is detected based on a decrease of a signal generated by the test light in the test light receiver (56a-b). The test light receiver (56a-b) is arranged on a same side of the front screen (42) as the reflector (52a-f) such that the test light path (54a-f) leads from the reflector (52a-f) via reflection on the inside of the front screen (42) to the test light receiver (56a-b).