a distance measuring method for measuring surfaces uses a laser source having a frequency that can be modulated to tune a wavelength of a laser beam in a wavelength range . the laser beam is generated with a coherence length to provide a measuring beam and is emitted at the surface , located within a specified distance range , as a measuring beam . the measuring beam is back - scattered by the surface and is received again and used to interferometrically measure the distance from a reference point to the surface . the specified distance range lies at least partly outside of the coherence length . one portion of the laser beam is temporally delayed with respect to another portion , such that the one optical path difference caused by the delay matches the optical path difference that corresponds to a distance in the specified distance range plus or minus the coherence length of the laser .