An apparatus for controlling the position of a screen pointer includes an at least partially coherent light source for illuminating an imaging surface, thereby generating reflected images. The apparatus includes a navigation sensor for generating digital images based on the reflected images, performing a movement computation based on the digital images, generating movement data based on the movement computation that is indicative of relative motion between the imaging surface and the apparatus, wherein the movement computation has a low sensitivity to effects in the digital images caused by particle contamination.