Light curtain for monitoring a protective field, said light curtain comprising: at least one first optical unit having a plurality of light emitting elements and light receiving elements and at least one second optical unit having a plurality of light emitting elements and light receiving elements, wherein the light emitting elements of each optical unit are operable to form a plurality of light barriers with opposing light receiving elements of the respective other optical unit, a control unit for processing output signals generated by said light receiving elements and for generating a defined output signal upon interruption of at least one light barrier, wherein said first and second optical unit (116, 118) each comprise a transceiver unit (102, 104) carrying said plurality of light emitting elements and light receiving elements, said first and second transceiver units (102, 104) having an identical structure, wherein the first optical unit has a first controller module and at least one separately housed, detachable first plug-in module, said first plug-in module having an interconnection to be connected with the first controller module, and general purpose input/output (GPIO) terminals, wherein the first plug-in module is configured to define the functionality of the first optical unit as a sender unit, and wherein the second optical unit has a second controller module and at least one separately housed, detachable second plug-in module, said second plug-in module having an interconnection to be connected with the second controller module, and output signal switching device (OSSD) terminals, wherein the second plug-in module is configured to define the functionality of the second optical unit as a receiver unit. at least one first optical unit having a plurality of light emitting elements and light receiving elements and at least one second optical unit having a plurality of light emitting elements and light receiving elements, wherein the light emitting elements of each optical unit are operable to form a plurality of light barriers with opposing light receiving elements of the respective other optical unit, a control unit for processing output signals generated by said light receiving elements and for generating a defined output signal upon interruption of at least one light barrier, wherein said first and second optical unit (116, 118) each comprise a transceiver unit (102, 104) carrying said plurality of light emitting elements and light receiving elements, said first and second transceiver units (102, 104) having an identical structure, wherein the first optical unit has a first controller module and at least one separately housed, detachable first plug-in module, said first plug-in module having an interconnection to be connected with the first controller module, and general purpose input/output (GPIO) terminals, wherein the first plug-in module is configured to define the functionality of the first optical unit as a sender unit, and wherein the second optical unit has a second controller module and at least one separately housed, detachable second plug-in module, said second plug-in module having an interconnection to be connected with the second controller module, and output signal switching device (OSSD) terminals, wherein the second plug-in module is configured to define the functionality of the second optical unit as a receiver unit.