This part of 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. See equation (1). The definitions of MFD are strictly related to the measurement configurartions. The mathematical equivalence of these definitions results from transform relationships between measurement results obtained by different implementations summarized in figure 1 as follows: 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