Function performance in response to pattern recognition on an optical navigation device such as a finger mouse

An optical navigation device (fig 1, 100), comprising a radiation source capable of producing a beam of radiation; a sensor for receiving an image; and an optical element for identifying movement of an object on a first surface to thereby enable a control action to be carried out. The optical element is such that a whole of the imaged area of the first surface is substantially covered by the object in normal use. The device is operable to receive from the object on the first surface an input describing a pattern (400), to compare the received pattern to a stored reference pattern and to perform a predetermined function (e.g. device (mobile phone) unlock) if the received pattern and stored reference pattern are substantially similar. The pattern may be a continuous line, the device storing the continuous line as a set of turning points in chronological order.