An optical displacement sensor comprises a light source 2 coupled to an optical fibre 1 the end 5 of which is directed onto a reflector 3 the displacement of which is to be measured. A lens 4 of appropriate focal length is positioned between the end of the optical fibre and reflector 3 to focus the light emitted from fibre 1 onto reflector 3 so that the amount of light reflected back is a function of the position of reflector 3. A signal detector 6 is also coupled to fibre 1 to detect and measure the light reflected back from reflector 3. Additionally a further light source 8, optical fibre 7 and detector 10 are provided with the end 9 of fibre 7 positioned at the focus of lens 4 so that the light reflected back to detector 10 is independent of the position of reflector 3. The ratio of the measurements from the two detectors gives the position of the reflector 3 with compensation for the effects of the deterioration of its reflectance.