IEC 61300-3-6:2008 presents procedures for the measurement of the return loss (RL) of a fibre optic device under test (DUT). RL, as used in this standard, is the ratio of the power (Pi) incident on, or entering, the DUT to the total power reflected (Pr) by the DUT, expressed in decibels. This third edition cancels and replaces the second edition published in 2003. It constitutes a technical revision. The changes with respect to the previous edition are to reconsider the constitution of the document and launch conditions for multimode fibres.