An apparatus comprising: an optical signal source (214, 706) configured to generate an optical signal and comprising a plurality of light emitting diodes; an optical signal detector system (216, 708) configured to detect the optical signal and comprising a plurality of light detection sensors; a switching system (218, 702) configured to direct the optical signal from the optical signal source (214, 706) to inputs in an optical coupler (202, 714) and direct the optical signal from outputs in the optical coupler (202, 714) to the optical signal detector system (216, 708); and a controller (220, 711) configured to automatically control the switching system (218, 702) to select the inputs in the optical coupler (202, 714) for transmission of the optical signal into the optical coupler (202, 714), select the outputs in the optical coupler (202, 714) for measuring the optical signal output from the optical coupler (202, 714), and measure the optical signal from the outputs selected to form measurements. characterized in that the switching system is configured to sequentially direct current to each of the plurality of light emitting diodes, each light emitting diode (300, 706) connected to a different input of the optical coupler (202, 714), wherein each light emitting diode (300, 706) is adapted to transmit the optical signal to a respective input of the optical coupler (202, 714), wherein the switching system is further configured to sequentially activate each of a plurality of switches, respectively connected to the light detection sensors, so that the optical signal detector system is adapted to receive the optical signal, from the outputs of the optical coupler (202, 714). an optical signal source (214, 706) configured to generate an optical signal and comprising a plurality of light emitting diodes; an optical signal detector system (216, 708) configured to detect the optical signal and comprising a plurality of light detection sensors; a switching system (218, 702) configured to direct the optical signal from the optical signal source (214, 706) to inputs in an optical coupler (202, 714) and direct the optical signal from outputs in the optical coupler (202, 714) to the optical signal detector system (216, 708); and a controller (220, 711) configured to automatically control the switching system (218, 702) to select the inputs in the optical coupler (202, 714) for transmission of the optical signal into the optical coupler (202, 714), select the outputs in the optical coupler (202, 714) for measuring the optical signal output from the optical coupler (202, 714), and measure the optical signal from the outputs selected to form measurements. characterized in that the switching system is configured to sequentially direct current to each of the plurality of light emitting diodes, each light emitting diode (300, 706) connected to a different input of the optical coupler (202, 714), wherein each light emitting diode (300, 706) is adapted to transmit the optical signal to a respective input of the optical coupler (202, 714), wherein the switching system is further configured to sequentially activate each of a plurality of switches, respectively connected to the light detection sensors, so that the optical signal detector system is adapted to receive the optical signal, from the outputs of the optical coupler (202, 714).