Abstract: A signal is generated for producing a video display carrying superimposed images, at least one of which is derived from a system for detection of non-visual signals, such as an ultrasonic inspection device or a backscatter detector of penetrating radiation. Accurate registration of the superimposed images is achieved by providing for the system for detection a moveable probe indicating the location of inspection by the detector system, a point source of light or other radiation to which a video camera responds on the probe which point source is `on` when the detector response is positive, and a comparator and memory system in which is built up a store of all those image locations where the camera observed the point source `on`. Output from the memory system is mixed with the camera video signal so that a bright point is produced at each of those locations thereby to produce the superimposed image.