A method of operating a scanner, in particular a laser scanner, for monitoring an at least two-dimensional protected field which is divided into a plurality of cyclically scannable monitored fields, wherein the scanner comprises at least one light transmitter and at least one light receiver, comprising the steps: - transmitting transmitted light beams into the protected field by the light transmitter; - receiving light which is reflected or remitted by an object which may be present in the protected field by the light receiver; - generating a received signal for the respective monitored field on the basis of the received light, with all the monitored fields of the protected field being scanned once within a scanning cycle; - determining a frequency of occurrence at which received signals are generated; and - determining a degree of detection safety at least on the basis of the frequency of occurrence, with the degree of detection safety representing a quantifying measure for the environmental conditions under which the scanner is operated, and with the value of the degree of detection safety being increased as the frequency of occurrence falls and being decreased as the frequency of occurrence rises, - wherein a determination is made on the basis of the received signals generated over a predefined number of multiple evaluations of consecutive scanning cycles whether a detected object satisfies at least one predefined object detection criterion for an unauthorized object and an object detection signal is generated when the object detection criterion is satisfied, with the predefined number of multiple evaluations of consecutive scanning cycles being determined at least on the basis of the degree of detection safety, with the object detection criterion comprising that a determination is made for the predefined number of multiple evaluations of consecutive scanning cycles that at least one detected object has a predefined minimum size and/or is in particular stationary in at least one of the monitored fields, and with the predefined number of multiple evaluations of consecutive scanning signals being increased when the degree of detection safety is decreased and/or the predefined number of multiple evaluations of consecutive scanning cycles being decreased when the degree of detection safety is increased; and/or - wherein the scanner has a rotatable light deflection unit for deflecting the light of the light source into the protected field and is configured for monitoring one or more scanning areas, with the monitored fields being of sector shape, with the magnitude of the angular resolution at which the protected field is scanned being adjustable in dependence on the degree of detection safety, and with the angular resolution being decreased when the degree of detection safety is increased and/or the angular resolution being increased when the degree of detection safety is decreased. - transmitting transmitted light beams into the protected field by the light transmitter; - receiving light which is reflected or remitted by an object which may be present in the protected field by the light receiver; - generating a received signal for the respective monitored field on the basis of the received light, with all the monitored fields of the protected field being scanned once within a scanning cycle; - determining a frequency of occurrence at which received signals are generated; and - determining a degree of detection safety at least on the basis of the frequency of occurrence, with the degree of detection safety representing a quantifying measure for the environmental conditions under which the scanner is operated, and with the value of the degree of detection safety being increased as the frequency of occurrence falls and being decreased as the frequency of occurrence rises, - wherein a determination is made on the basis of the received signals generated over a predefined number of multiple evaluations of consecutive scanning cycles whether a detected object satisfies at least one predefined object detection criterion for an unauthorized object and an object detection signal is generated when the object detection criterion is satisfied, with the predefined number of multiple evaluations of consecutive scanning cycles being determined at least on the basis of the degree of detection safety, with the object detection criterion comprising that a determination is made for the predefined number of multiple evaluations of consecutive scanning cycles that at least one detected object has a predefined minimum size and/or is in particular stationary in at least one of the monitored fields, and with the predefined number of multiple evaluations of consecutive scanning signals being increased when the degree of detection safety is decreased and/or the predefined number of multiple evaluations of consecutive scanning cycles being decreased when the degree of detection safety is increased; and/or - wherein the scanner has a rotatable light deflection unit for deflecting the light of the light source into the protected field and is configured for monitoring one or more scanning areas, with the monitored fields being of sector shape, with the magnitude of the angular resolution at which the protected field is scanned being adjustable in dependence on the degree of detection safety, and with the angular resolution being decreased when the degree of detection safety is increased and/or the angular resolution being increased when the degree of detection safety is decreased.