Patent ID: 7522157

Claim:
A graphic input device for measuring a displacement of a pen-shaped pointing device relative to a writing surface in two directions in space, wherein displacement measuring means in each direction comprise: A. means in the pointing device for transmitting ultrasonic (US) continuous waves (CW) propagating in the air in generally that direction, such as part of the US waves illuminate the surface; B. means on the writing surface for reflecting part of the US waves back, generally towards the pen-shaped pointing device; C. means in the pen-shaped pointing device for receiving US waves reflected off the surface from that general direction; D. means for measuring a phase of the received waves, wherein a change in the phase is proportional to the pen-shaped pointing device's displacement relative to the writing surface in that direction; E. means for electronic separation of US waves corresponding to one direction in space from US waves corresponding to the other direction in space.