A method of operating a distance-measuring monitoring sensor for monitoring an at least two-dimensional protected field which is divided into a plurality of sector-shaped, cyclically scannable monitored fields, wherein the monitoring sensor comprises at least one light transmitter and at least one light receiver, comprising the steps: - transmitting a transmitted light beam into the protected field by the light transmitter; - receiving light by the light receiver, with the light being reflected or remitted by an object (18, 20) which may be present in the protected field; - generating a specific received signal for the respective monitored field on the basis of the received light, wherein a respective received signal comprises information on the angular position of the monitored field and on the distance of an object (18, 20) located therein from the monitoring sensor, wherein all the monitored fields of the protected field are scanned once within a cycle; - determining the size of an angular region over which a respective detected object (18, 20) extends; - determining the size of the object (18, 20) on the basis of the determined distance and on the basis of the determined size of the angular region; - classifying the object as a particle or as a large object on the basis of the determined size of the object and of a predetermined threshold value for the size, wherein the object is classified as a particle if it is smaller than the threshold value and is classified as a large object if it is larger than the threshold value, characterized by - checking whether at least one large object is located in at least one of the monitored fields for a predefined first number of sequential cycles; - checking whether at least one particle is located substantially at a stationary angle in at least one monitored field for a predetermined second number of sequential cycles; and - generating an object detection signal on the basis of the received signals when it is found that at least one of the conditions checked in the two checking steps is satisfied. - transmitting a transmitted light beam into the protected field by the light transmitter; - receiving light by the light receiver, with the light being reflected or remitted by an object (18, 20) which may be present in the protected field; - generating a specific received signal for the respective monitored field on the basis of the received light, wherein a respective received signal comprises information on the angular position of the monitored field and on the distance of an object (18, 20) located therein from the monitoring sensor, wherein all the monitored fields of the protected field are scanned once within a cycle; - determining the size of an angular region over which a respective detected object (18, 20) extends; - determining the size of the object (18, 20) on the basis of the determined distance and on the basis of the determined size of the angular region; - classifying the object as a particle or as a large object on the basis of the determined size of the object and of a predetermined threshold value for the size, wherein the object is classified as a particle if it is smaller than the threshold value and is classified as a large object if it is larger than the threshold value, - checking whether at least one large object is located in at least one of the monitored fields for a predefined first number of sequential cycles; - checking whether at least one particle is located substantially at a stationary angle in at least one monitored field for a predetermined second number of sequential cycles; and - generating an object detection signal on the basis of the received signals when it is found that at least one of the conditions checked in the two checking steps is satisfied.