By making the user wear a special contact lens, it is possible to make quite accurate recordings of the direction of gaze. Two lens techniques exist: by engraving one or more plane mirror surfaces on the lens, reflections of light beams can be used to calculate the position of the eye. And by implanting a tiny induction coil into the lens, the exact positioning of the lens can be recorded through the use of high-frequency electro-magnetic fields placed around the user's head.
These techniques hardly seem tractable for everyday use; the health issues concerning high-frequency electro-magnetic fields are not yet resolved, and alone the requirement that the user must wear special contact lenses, possibly connected to wires, seems to make the technique too cumbersome for non-laboratory tasks. Perhaps some day "contacts prepared for eye tracking" will be an option at the optometrist, but we have doubts that a majority of people not already wearing contacts will do so solely to communicate with their television sets!