A method for determining the relative motion of a surface with respect to a measurement device comprising: illuminating the surface from a source with incident illumination, such that illumination is reflected from portions of the surface toward a detector; spatially filtering the reflected illumination such that the phase of the detected optical illumination from a given scatterer on the surface is substantially constant or linearly related to the translation of the surface; generating a signal by the detector responsive to the illumination incident on the detector; and determining the relative motion of the surface from the signal.