IEC 61280-4-1:2009 is applicable to the measurement of attenuation of installed fibre-optic cabling using multimode fibre, typically in lengths of up to 2 000 m. This cabling can include multimode fibres, connectors, adapters and splices. In this standard, the fibre types that are addressed include category A1a (50/125 µm) and A1b (62,5/125 µm) multimode fibres, as specified in IEC 60793-2-10. The attenuation measurements of the other multimode categories can be made, using the approaches of this standard, but the source conditions for the other categories have not been defined. The main changes with respect to the previous edition are: - An additional measurement method based on optical time domain reflectometry (OTDR) is documented, with guidance on best practice in using the OTDR and interpreting OTDR traces. - The requirement for the sources used to measure multimode fibres is changed from one based on coupled power ratio (CPR) and mandrel requirement to one based on measurements of the near field at the output of the launching test cord. - Highlighting the importance of, and giving guidance on, good measurement practices including cleaning and inspection of connector end faces.