SR Research: EyeLink Remote

"The EyeLink Remote system is designed for areas of eye tracking research where a head rest or head mount is not desirable but high accuracy and resolution are still important.

Head distance is accurately measured at 500 Hz using a small target sticker placed on the participants’ forehead. The use of a distance measure that is independent of the eye allows for head position to be tracked even during blinks, providing an extremely fast 2 msec blink recovery delay. The fast head position sampling rate also allows for head tracking during very quick head movements. (website)

Click image for a video demonstration

Rather impressive 500 Hz sampling rate which enables saccade detection algorithms and fast head movements. The recovery time from blinks is specified at only 2 milliseconds. However, the target sticker on the forehead is a workaround from actual face tracking. OK in the lab but not feasible in everyday, real world gaze interaction scenarios. (besides that my guess is that this tracker does not come cheap due to the high speed cameras used)