1. A method for measuring relative motion between an illuminated portion of a surface and an optical sensing device, wherein the optical sensing device comprises a coherent light source and a photo-detector device, wherein the photo-detector device includes an array of pixels associated with comparators for comparing light intensity between neighbouring pixels, said method comprising the steps of: (a) illuminating said surface portion at a determined flash rate using said coherent light source; (b) detecting a speckled light intensity pattern detecting a speckled light intensity pattern using said photo detector device, defining first and second axes, of said illuminated portion of the surface for each flash and determining positive and negative edges in each detected speckled light intensity pattern along said first and second axes by using said comparators and by introducing an offset at one input of the comparators before determining said positive and negative edges; (c) extracting Peaks and Nulls for each of a plurality of detected speckled light intensity patterns by respectively defining successions of a positive edge and a negative edge and successions of a negative edge and a positive edge; (d) determining a measurement of the relative motion between the optical sensing device and said illuminated portion of the surface based on a comparison of extracted Peaks and Nulls of successive detected speckled light intensity patterns; and (e) a step of keeping only pairs including a neighbouring Peak and Null and ignoring individually occurring Peaks and Nulls.