Patent ID: 7737948

Claim:
A method of sensing relative movement between a data input device and a surface by determining displacement of optical features in a succession of images of the surface, the method comprising: emitting coherent light from a laser; collimating the coherent light so as to form a collimated illumination beam with a predetermined diameter, D, and is transmitted to the surface at a normal angle thereto; generating a speckle pattern by impingement of the coherent light from the collimated illumination beam on the surface; and detecting the images of the surface using a sensor array comprising a comb array, wherein the laser and the sensor array are both configured on a same planar substrate that is configured to lie parallel to the surface, and the predetermined diameter, D, of the collimated illumination beam satisfies an equation D=(h λ)/(ξ Λ cos 2 θ), wherein h is a height from the planar substrate to the surface, λ is a wavelength of the coherent light, ξ is a fraction between 0.25 and 0.5, Λ is a period of the comb array, and θ is an angle subtended by the beam from a point on the sensor.