Eye Tracker Could Improve AR Displays

A battery-free eye tracker developed at Dartmouth College uses NIR lights and photodiodes, instead of cameras, to make the system energy efficient and less bulky.

The eye tracker integrated into a regular pair of glasses. The system relies on NIR lights and photodiodes for eye-tracking and is powered by two thin solar cells on the arms of the glasses. Courtesy of DartNets Lab.

The wearable eye tracker tracks both the 2D position and diameter of the pupil. Small photodiodes on the lens frame sense light reflected by the eyeball. The reflected light is then used to infer the pupil position based on the pupil’s light absorption.

NIR LEDs are used to illuminate the eye from various directions. The NIR LEDs, as point lights, emit ultrashort light pulses onto the eye from different directions, while the photodiodes sense reflected NIR light from their vantage points. The system exploits characteristics of different eye movement stages and adjusts itself accordingly to increase energy savings.

“By detecting the type of eye movement, the system can adapt sensing and computation,” said researcher Tianxing Li. “Some movements have predictable trajectories, allowing the system to infer subsequent pupil position and minimizing energy use.”

The researchers built a prototype with off-the-shelf hardware components and integrated it into a regular pair of glasses. Experiments with 22 participants showed that the system can achieve 0.8-mm mean error in tracking pupil position and 0.3-mm mean error in tracking pupil diameter at 120-Hz output frame rate, consuming 395 µW mean power, which is supplied by two solar cells on the arms of the glasses.

The battery-free eye tracker can be powered by energy harvested from indoor lighting and is easy to integrate into a regular pair of glasses. “We took a minimalist approach that really pays off in power use and form factor,” Li said. “The new system opens a wide range of uses for eye-tracking applications.”

The new technology could improve player controls for gaming and allow for more accurate image displays. By allowing for a more precise measurement of eye position, it could result in more efficient rendering of images by display systems, and could one day eliminate the need for hand controllers.

The study was conducted exclusively indoors because strong IR light outdoors can saturate light sensors in the current prototype. Future research will include adapting the light sensor gain in the system for outdoor use and improving detection of certain rapid eye movements.

“This is an exciting advancement for gamers, developers, and other users of smart glasses,” said professor Xia Zhou. “It’s the first-ever eye tracker that can fit into your everyday glasses and run without batteries.”

The eye tracker is being introduced at MobiCom 2018, Oct. 29 to Nov. 2, in New Delhi, India.

The paper, Battery-Free Eye Tracker on Glasses, is available at https://home.cs.dartmouth.edu/~xia/papers/mobicom18-eye.pdf.

A wearable eye tracker on glasses tracking both the pupil size and position, all without a battery. Courtesy of DartNets Lab.