IEC 61300-3-50:2013 describes the procedure to measure the crosstalk of optical signals between the ports of a multiport M x N (M input ports and N output ports) fibre optic spatial switch. The crosstalk is defined as the ratio of the optical power at an output port which comes from the unconnected input port, to the optical power at the output port which comes from the connected input port.Keywords: measure the crosstalk of optical signalsThe contents of the corrigenda of January 2015 and of July 2015 have been included in this copy.