IEC 61300-3-40:1998 describes the procedure to measure the ability of an optical fibre connector to maintain a given extinction ratio across the connection in pm fibre. In this test we limit the measurement to the most common case of nearly linearly polarized light propagating in pm fibre. The term "extinction ratio" used here refers to the proportion of light propagating in the two orthogonal polarization axes of the fibre and is more properly defined as "polarization crosstalk". "Extinction ratio" is the commonly used term. This procedure has been designed for use with dedicated equipment in a manufacturing environment and thus incorporates a rather elaborate test configuration. It should be noted that other more direct test configurations can be used in suitably equipped optical laboratories. These may be the subject of additional test procedures in the future. It should be noted that this test procedure is not designed for long term continuous testing; drift of polarization alignment can occur particularly in the single-mode fibre. Hence, repeated rather than continuous measurement is required for long term testing of components.