How Eye Tracking Works

by Zoja Savkovic

It might seem like eye tracking is bit magical, but it's really just hardware and algorithms. The humble computer mouse is actually quite similar, in that it's also just hardware and algorithms, but we probably don't associate as much magic with it because it's so widespread.

It's interesting that studies of eye movement were done as early as the eighteen hundreds - according to Wikipedia. Eye trackers were then made as early as the nineteen hundreds.

This post will focus on the tobii5 eye tracker. But before that a bit of an overview of eye tracking in general.

Overview

There's a huge amount of theory around eye tracking and eye gaze, such as how sometimes eye movement can be smooth or more erratic.

Somehow all of this research and theory resulted in these broad categories of eye trackers:

  • non intrusive optical tracking, such as with video
  • intrusive eye tracking, where things are actually attached to the eye, where these attached things are actually tracking the movement of the eyeball, or sensing electric potential which can be used to ascertain movement too

The rest of this post will focus on non intrusive video optical tracking such as in the tobii5.

Video Based Eye Trackers

Reflections

Video based eye trackers (see the link for some pictures which might help) seem simple, some kind of signal is sent out, reflected and then geometry is used to measure things and find out where eyes are looking at. But what exactly is reflected from?

Well the cornea and the lens have reflections from the inside and outside. This makes four reflections, and they have a fancy name: Purkinje images.

So these are the reflections that are used!

This is all given a fancy name called pupil centre corneal reflection.

Signals Sent Out And Sensed

Infrared light is apparently used because the contrast of parts of the eye in an image is greater when this wavelength is used compared to visible light. When I say image I suppose I actually mean like a collection of signals put together to form an image, since infrared can't really be seen as an image. Think of the radar image of weather, it's a picture but it's not like an scene that we see in every day life.

So that's it - a lot of things have put together to allow eye tracking to work!