Abstract: An optical transducer for producing a measurement signal which is modulated in accordance with the position of a physically movable member in the form of a modulating mirror and which also produces a reference signal the intensity of which is independent of the position of the modulating mirror. Both the modulating mirror and a further optical element which is arranged to provide the reference signal are arranged on the optical axis along which a light beam is directed by an input lens. Thus the system can tolerate normally experienced vibrations transverse to the optical axis and can provide for a substantial degree of movement of the modulating mirror.

Abstract: A segmented optical system for use in an alarm system. A plurality of optical elements such as mirrors are arranged so that each of them directs radiation from a predetermined direction towards a common focal point. Each optical element defines a respective discrete field of view to a detector at the focal point, adjacent fields of view overlapping so that an object moving from within one field of view towards an adjacent field of view enters an area which is common to the two adjacent fields of view. There is thus a stepwise change in the intensity of radiation reaching the detector from a single source when that single source crosses the boundary of a field of view and yet there is no space between adjacent fields of view from which no radiation can be received by the detector.