Researchers at the University of Washington have developed such a tool, called FingerIO. FingerIO uses sonar to track two-dimensional finger movement to within 8 millimeters. Since the input surface—like a table, or a forearm—is now larger, making fine manipulations on a small screen is easier to do.
FingerIO uses the smartwatch or smartphone speaker to emit an inaudible sound signal which "bounces" off the finger, creating "echoes" that are picked up by the gadget's microphone. That signal bounces off the finger, and those "echoes" are recorded by the device's microphones and used to calculate the finger's location in space.
Since FingerIO uses sonar, it works even when the device is obstructed by something, or stuck in a pocket. Researchers used a certain type of sonar signal called Orthogonal Frequency Division Multiplexing, which is stronger than typical sonar signals.
FingerIO isn't perfect—for the time being, it only operates in two dimensions, and can only track the motion of one finger at a time. But if the research team can tackle these challenges, we may well be able to manipulate our smart devices by gesturing in midair, without even giving them a glance.