Abstract: Movement of a body part or other movable object is measured by attaching a retro-reflecting landmark (8) to the body part. An observation field (4) containing the landmark (8) is scanned using a beam (20) of radiation from a lamp (10) and reflecting it with a rotatable mirror (18) to scan the field. The retro-reflected light (42) is detected and the time of retro-reflection by reference to the phase of rotation of the mirror (18) is measured to provide a signal related to the angular position of the landmark (8) in the field (4). More than one landmark (8) can be observed simultaneously by using landmark (8) with edge filtering characteristics and appropriate arrangements for discriminating retro-reflected light (42) according to spectral characteristics.