IEC 61300-3-19:1997 describes the test to determine the dependence of return loss of a singlemode fibre optic component on the state of polarization (SOP) of the light passing through the component. Return loss is the absolute value of the ratio in decibels of the total reflected power to the incident power from an optical fibre link or system. Since the SOP of light in a component is generally indeterminate and in fact often changing as a function of time, a component that exhibits polarization dependence will have varying return loss in a system. This procedure can be applied to any singlemode passive component and interconnecting device, including attenuators, isolators, branching devices, switches, connectors, and splices, if applicable.