To measure the distance to a moving reflective surface by providing a processor which can be operated to deliver a positional signal indicative of the distance to the reflective surface.

A first light source disposed on the first side of an axis transmits first light onto a reflective surface. The reflective surface 12 reflects a part of light onto a first photodetector 18 having a first area which can be operated to receive the first light reflected on the reflective surface 12. The first photodetector 18 delivers first and second signals as the positional action of the first light received in the first area. A second light source transmits second light onto the reflective surface which reflects a part of the second light onto a second photodetector 24. The second photodetector 24 delivers third and fourth signals as the positional action of the light received in the second area. A processor 26 coupled with the first and second photodetectors 18, 24 receives that signal. The processor delivers a positional signal indicative of the distance to the reflective surface as the action of the first to fourth signals.