Patent ID: 7122781

Claim:
A method for measuring relative motion between an illuminated portion of a surface (S) and an optical sensing device comprising a photodetector array ( 420 ), said method comprising the steps of: a) detecting by means of said photodetector array a first light intensity pattern of said illuminated portion of the surface at a first point in time; b) comparing light intensity between neighbouring pixels of said photodetector array and determining edge direction data from said detected first light intensity pattern, which data is descriptive of light intensity differences between said neighbouring pixels and includes: a first edge condition, or positive edge, defined as a condition wherein the light intensity of a first pixel is less than the light intensity of a second pixel; and a second edge condition, or negative edge, defined as a condition wherein the light intensity of said first pixel is greater than the light intensity of said second pixel; c) storing said edge direction data or data extracted therefrom; d) detecting a second light intensity pattern of said illuminated portion of the surface at a second point time; e) comparing light intensity between neighbouring pixels of said photodetector array and determining said edge direction data from said detected second light intensity pattern; and f) determining a measurement of the relative motion between said optical sensing device and said illuminated portion of the surface based on a comparison of said edge direction data determined at steps b) and e) or based on a comparison of data extracted from said edge direction data.