IEC 60793-1-40:2019 RLV contains both the official IEC International Standard and its Redline version. The Redline version is available in English only and provides you with a quick and easy way to compare all the changes between the official IEC Standard and its previous edition. IEC 60793-1-40:2019 establishes uniform requirements for measuring the attenuation of optical fibre, thereby assisting in the inspection of fibres and cables for commercial purposes. Four methods are described for measuring attenuation, one being that for modelling spectral attenuation: – method A: cut-back; – method B: insertion loss; – method C: backscattering; – method D: modelling spectral attenuation. Methods A to C apply to the measurement of attenuation for all categories of the following fibres: – class A multimode fibres; – class B single-mode fibres. Method C, backscattering, also covers the location, losses and characterization of point discontinuities. Method D is applicable only to class B fibres. Information common to all four methods appears in Clauses 1 to 11, and information pertaining to each individual method appears in Annexes A, B, C, and D, respectively. This second edition cancels and replaces the first edition published in 2001. This edition constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition: a) Improvement of the description of measurement details for B6 fibre; b) Improvement of the calibration requirements for A4 fibre; c) Introduction of Annex E describing examples of short cable test results on A1 multimode fibres. Keywords: measuring the attenuation of optical fibre