A real-time measurement of relative position data and/or of geometrical dimensions of a moving body by lighting unit and a detector unit, wherein the moving body is movably guided relative to both of the units. Light beams are transmitted from the lighting unit towards the detector unit; the moving body protrudes between the lighting unit and the detector unit into the volume flooded by the light beams, so that the shadow border of the shadow thrown by the moving body extends over the detector unit. The detector unit includes a two-dimensional optical position detector, which is designed as a two-dimensional optical waveguide containing photo-luminescent particles. Signals from the two-dimensional optical waveguide are read out by a plurality of small-area photoelectric sensors spaced apart from one another, the strength of the signals correlating with the intensity of the light at the sensor location in waveguide mode.