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.