This part of IEC 1300 describes the procedure to measure the far-end crosstalk of light between channels of a multiport MxN or 1xN fibre optic passive components (switch, WDM or, in particular cases, fan-out). The far-end crosstalk is defined as the ratio of the optical power that goes out from a given output port to an optical power that goes out from another output port, nominally isolated from the previous one. For WDMs the signal travelling in the two output paths have different wavelengths and the far-end crosstalk corresponds to the ratio of the optical power, at a given wavelength, that goes out from the corresponding output port to an optical power, at the same wavelength, that goes out from another output port, nominally isolated from the previous one at that wavelength. For WDM components the term isolation is more commonly used than far-end crosstalk.