This part of IEC 60793 establishes uniform requirements for measuring the mode field diameter (MFD) of optical fibre@ thereby assisting in the inspection of fibres and cables for commercial purposes. The MFD represents a measure of the transverse extent of the electromagnetic field intensity of the mode in a fibre cross section@ and it is defined from the far-field intensity distribution as a ratio of integrals known as the Petermann II definition. The definitions of MFD are strictly related to the measurement configurations. The mathematical equivalence of these definitions results from transform relationships between measurement results obtained by different implementations summarized. Four methods are described for measuring MFD: ?? method A: direct far-field scan; ?? method B: variable aperture in the far field; ?? method C: near-field scan; ?? method D: bi-directional backscatter using an optical time domain reflectometer (OTDR). All four methods apply to all categories of type B single-mode fibre shown in IEC 60793-2 and operating near 1 310 nm or 1 550 nm. Method D is not recommended for the measurement of fibres of unknown type or design. Information common to all four methods is contained in clauses 1 to 8@ and information pertaining to each individual method appears in annexes A@ B@ C and D@ respectively.