Abstract: Apparatus for transmitting an information signal from one to the other of two relatively rotatable members comprises one or a plurality of light sources radiating into a light guide having an output face which extends over a substantial arc of a circle around which are spaced a plurality of optical detectors. The arc of the circle subtended by the output face of the light guide is greater than the circumferential separation between adjacent optical detectors (assuming they are regularly spaced). Various modifications may be made to the light guide in order to attenuate or enhance the transmission or reflection characteristics, particularly at its surface, so as to obtain a uniform illumination along the whole of the arc of the output face thereof from illumination by one or more discrete light sources.