Abstract: The present invention provides an off-axis rotary joint for optical signal transfer. An annular waveguide has optical discontinuity means, such as a plurality of circumferentially equally spaced apart notches directed inwardly thereof, which discontinuities intercept a light beam emanating from a light source near the waveguide. The discontinuities direct light into the waveguide, along which it travels until it meets an internal reflecting member which, in turn reflects the travelling light out of the waveguide to a suitable detector. The light source and the waveguide/reflector/detector are relatively rotatable so that the signal strength at the detector is an indication of the position of the light source relative to the waveguide. The invention has practical applicability in apparatus, such as turrets and CT scanners, in which the central shaft or axis area must be occupied by equipment or personnel rather than componentry of the rotary joint.