Abstract: The present invention provides an input device for positioning a cursor on a computer screen by movement of a ball whose rotary movements are transformed into electronic signals, which is unique in that the angular rotations of the ball can be detected by an optical detection means by use of substantively coherent light in such a manner that the rotation of the ball is converted into data values with respect to an x-axis and a y-axis, said axes being preferably perpendicular to each other. This provides a compact detecting device which is suitable for the construction of a computer mouse with the shape of a ball-point pen.