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 each comprise a transceiver unit carrying said plurality of light emitting elements and light receiving elements, said first and second transceiver units 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 each comprise a transceiver unit carrying said plurality of light emitting elements and light receiving elements, said first and second transceiver units 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. <EOS>