Abstract: A method for determining the relative motion of a surface with respect to a measurement device comprising: illuminating the surface with incident illumination such that illumination is reflected from portions of the surface; placing a partially reflecting object, which is part of the measuring device, adjacent to the surface, wherein part of the incident illumination is reflected or diffracted by the object, as a reference illumination; coherently detecting the illumination reflected from the surface, utilizing the illumination reflected from or diffracted by the object as a local oscillator, to form a signal; determining the relative motion of the surface from the signal; varying the phase of the illumination reflected from or diffracted by the object with respect to that reflected from the surface; and determining the direction of relative motion parallel to the surface based on a characteristic of the signal caused by said varied relative phase.