The relative position (M) of two members (33, 34) is sensed by scanning (S) an optical beam (25) over an encoded pattern (20) carried by one of the members, and by using a read head (21) secured to the other member to receive an optical signal transmitted from the encoded pattern (20). This optical signal is received by an optical position sensor to discriminate from the frequency of the signal, the position of the read head (21) relative to the encoded pattern (20). As the relative position is detected from the frequency of the signal rather than its intensity, components can be replaced without recalibration. The optical beam (25) is scanned by using a light source of variable wavelength which is passed through a device, such as a diffraction grating or a gradient index lens, for deflecting the beam (25) dependent on its wavelength.