An optical multiplex system has an LED which supplies broad band optical radiation to one end of a fiber-optic cable that extends, via displacement transducers connected in series, to a receiver. Each transducer has a movable filter plate with a neutral region and a region that attenuates radiation in different respective narrow bands of wavelengths. The receiver includes a detector that receives radiation from the other end of the cable via a filter. The filter has a variable narrow band transmission characteristic so that radiation in either one of the narrow bands can be selected and supplied to the detector which thereby gives an output representative of the state of the selected transducer.