Method and system for determining the separation distance between a body and the surface of an object by means of low coherence optical interferometry techniques under distortion due to sub-sampling

Method and system for determining separation distance between an object and a processing or measuring tool involve generating a measurement beam of low coherence optical radiation, leading the measurement beam towards the object and the reflected measurement beam towards an optical interferometric sensor assembly in a first direction of incidence, generating a reference beam of low coherence optical radiation, and leading the reference beam towards the optical interferometric sensor assembly in a second direction of incidence, superimposing the measurement and reference beams on a common region of incidence, detecting position of a pattern of interference fringes between the measurement and reference beams on the region of incidence, and determining difference in optical length between a measurement optical path and a reference optical path on position of the pattern of interference fringes along an illumination axis to determine current separation distance between the processing or measuring tool and the object.