Optical transducer with movable filter

An optical displacement transducer is supplied via a fibre-optic cable with broad band radiation from an LED driven at reduced power. The transducer has a movable plate one half of which is neutrally transparent and the other half of which is coated with a filter the transmission characteristic of which varies sharply in the band width of the LED. A second fibre-optic cable receives radiation passing through the plate and supplies it to two detectors that are responsive respectively to two different wavelengths. The relative outputs of the two detectors varies according to the proportion of radiation passing through the transparent and filtering regions of the plate. A comparator compares these outputs to indicate the position of the plate.