Patent ID: 7227531

Claim:
A data input device for use with a tracking surface, said tracking surface being optically rough, said device comprising: a coherent light source for projecting a coherent light beam onto said tracking surface to reflect a speckle pattern from the coherent light beam striking said tracking surface; an optic positioned in the coherent light beam for altering a wavefront profile of the coherent light beam so that the coherent light beam has a substantially constant phase relative to a plane perpendicular to said beam at a tracking distance from said optic, said tracking distance being different than a focal length of the optic, whereby said tracking surface reflects a substantially stable speckle pattern from the altered coherent light beam when the coherent light source and tracking surface are separated by about said tracking distance, wherein said optic alters the wavefront profile of said coherent light beam so that the coherent light beam is substantially out-of-phase across the coherent light beam within a plane perpendicular to said beam at an out-of-phase distance from said optic, such that locating the tracking surface at about said out-of-phase distance from said optic creates a boiling speckle pattern; a detector associated with the coherent light source for detecting said speckle pattern reflected by the tracking surface and for imaging the detected speckle pattern; and a controller responsive to the detector for correlating the speckle pattern either to track relative movement between the coherent light beam and the tracking surface when the detector detects said substantially stable speckle pattern or to suspend tracking of relative movement between the coherent light beam and the tracking surface when the detector detects said boiling speckle pattern.