A user input device, including a user input area, light emitters arranged along a first edge of the input area so as to be evenly spaced, light receivers arranged along a second edge of the input area, a curved lens arranged in front of the emitters, such that light emitted by each of the emitters is refracted as it enters the curved lens, and is again refracted as it exits the curved lens, resulting in unevenly spaced collimated light exiting the curved lens and crossing the input area, the collimated light being shifted laterally by a non-zero offset along a direction parallel to the first edge, the offset being a characteristic of the arrangement of that emitter vis-à-vis the curved lens, and a calculating unit for determining location of an object inserted into the input area from outputs of the receivers, based on the characteristic offsets of the emitters.