Abstract: The displacement of a first object (6) relative to a second object (28) is measured by differential measurements from a photodetector (17) and an associated evaluation device (18). The photodetector (17) receives a measurement beam of radiation from a source (9) via a reflector (5) secured to object (6), and a reference beam of radiation from a source (10) via a reflector (26) secured to object (28). Sources (9, 10) are closely adjacent and in the focal plane of a collimating lens (13) through which both beams pass. Both beams are focused onto the detector (17) by a focusing lens (21). Because both beams pass through lenses (13, 21) movements thereof do not affect the displacement measurement.