IEC 61300-3-39:2011 selects non-angled physical contact (PC) optical connector plugs for use as the reference plug in the return loss RL measurement and to define an acceptance return loss value RL_a for use in plug acceptance testing. This procedure is for use to guarantee a certain return loss value RL when two plugs have been successfully tested against the reference connector when randomly mated. This second edition of IEC 61300-3-39 cancels and replaces the first edition published in 1997 and constitutes a technical revision. Changes from the previous edition are structure of the standard, general description and the Annex.