Movement of an optical mouse may be tracked by providing a source of non-coherent light in a movable housing for illuminating a work surface at a low angle of incidence, providing a two dimensional array of photo detectors for producing arrays of data elements responsive to light reflected from surface...http://www.google.com/patents/US7907120?utm_source=gb-gplus-sharePatent US7907120 - Optical mouse with uniform level detection method
