METHOD OF MEASURING RELATIVE MOVEMENT OF AN OBJECT AND AN OPTICAL INPUT DEVICE OVER A RANGE OF SPEEDS

A method and optical module for measuring relative movement of an input device and object (15) along at least one measuring axis. A laser device (3) having a laser cavity is provided for generating a measuring beam (13) in respect of each measuring axis. The measuring beam (13) is used to illuminate the object (15) and measuring beam radiation reflected from the object (15) and re-entering the laser cavity generates a self-mixing effect in the laser and causes changes in operation of the laser cavity. A detector (4) is used to generate a measurement signal representative of these changes and an electronic processing circuit (18) selects, in dependence on the speed of relative movement, one of at least two parameters of the measurement signal for use in determining the speed and direction of relative movement.