first eye tracking devices appeared in the early 1900s. These eye trackers were intrusive because they relied on electrodes mounted on the skin around the eye or on the use of large, uncomfortable contact lenses that study participants had to wear. Non-intrusive eye tracking techniques started emerging shortly thereafter. They involved recording light that was reflected on the eye or filming the eyes directly.
The advances in eye tracking technology since then have focused on reducing the constraints posed by eye trackers on research participants, while increasing the precision and accuracy of these devices, as well as making data analysis easier. At the same time, eye tracking research has deepened researchers’ understanding of the relationship between the different aspects of eye movements and the human cognitive processes.
The first application of eye tracking to UX-related research dates back to 1947, when Paul Fitts and his colleagues investigated how pilots used the information provided by instruments in the cockpit to land a plane.1 At the time, however, eye tracking was still primarily used by academic and medical researchers. It wasn’t until the late 1900s and early 2000s when the technology, mostly due to its improved affordability and usability, became more widespread among practitioners.
To learn more about the evolution of the eye tracking technology and details of how it works, you should check out Duchowski’s Eye Tracking Methodology.2
What Is Eye Tracking, Anyway?
You are hopefully reading this book not because you want to build an eye tracker, but because you want to make use of eye tracking in your research. If that is the case, you do not need to know exactly how the hardware works to be successful in using it, just like you do not need to know what is under the hood of your car to be a good driver. However, as a professional, you should be at least somewhat well versed on the topic.
If you are already involved in eye tracking research, then you probably know what I mean. I am often asked about how eye tracking works by research stakeholders, other UX practitioners, study participants, and even my friends. And how can I blame them for their curiosity? Eye tracking is indeed fascinating.
Imagine that someone at a party overhears you mentioning eye tracking. Let’s call him John.
JOHN (wrinkling his forehead): Eye tracking? What is that?
YOU: Eye tracking is the process of determining where someone is looking. It can also measure the characteristics of eye movements and the eye itself, such as the size of the pupil. To conduct eye tracking, you need special equipment called an eye tracker.
JOHN: An eye tracker?
YOU: Yes, an eye tracker. It’s a piece of hardware that records your eye movements as you look at a computer screen, a physical object, or even your surroundings in general. Some eye trackers are affixed to a pair of glasses or a special hat you can wear. Others can be placed in front of you, like those that are attached to computer monitors.
JOHN: This sounds pretty cool. But how does it work?
YOU: The eye tracker shines infrared light onto your face, and then it records two things: the reflection of the infrared light from the retina, which helps find the center of your pupil, and the reflection of the infrared light from the cornea, which is called corneal reflection.
JOHN: Retina? Pupil? Cornea? You kind of lost me there.
YOU: The retina, pupil, and cornea are parts of the eye. Let me show you the eye diagram that I carry in my wallet for occasions such as this one (proudly taking the eye diagram from your wallet [see Figure 1.1]). The retina is a light-sensitive tissue in the back of the eye. The pupil is a black-looking opening that allows light to enter the retina. The cornea is the transparent front part of the eye.
FIGURE 1.1 The human eye.
JOHN (nodding): Uh-huh.
YOU: If you look at my eyes right now, you will see the corneal reflection of the light in this room in each of them. If I keep my head still and look to the left, to the right, up, and down (demonstrating), the corneal reflection doesn’t move—only the pupil does. You can see that the relationship between the pupil center and corneal reflection changes (see Figure 1.2).
FIGURE 1.2 The relative position of the pupil and corneal reflection changes when the eye rotates but the head remains still.
JOHN: So where you are looking can be determined from the location of the pupil center relative to the corneal reflection.
YOU: Exactly. Now, if I move my head slightly while looking at the same spot (demonstrating), the relationship between the pupil center and corneal reflection remains the same (see Figure 1.3). Even though I’m moving, the eye tracker would know I’m looking at the same spot.
FIGURE 1.3 The relative position of the pupil and corneal reflection does not change when the head moves but the person is looking at the same spot.
JOHN: So what’s inside of the eye tracker that allows it to do something like that?
YOU: Modern commercial eye trackers consist of two main components. The first one, a source of near-infrared light, creates the reflection in the eye. The second component is a video camera sensitive to near-infrared light. The camera is focused on the eye and records the reflection. The software then figures out the location of the gaze and superimposes it onto an image of what you were looking at, such as a Web page.
JOHN: Why is infrared light needed? Wouldn’t regular light work?
YOU: The trick is to use a wavelength that is invisible to people, and thus not distracting, yet reflected by the eye.
JOHN: But isn’t infrared light dangerous?
YOU: Any light wavelength—ultraviolet, visible, and infrared—can be harmful in high intensities, but the exposure from the eye tracker is just a tiny fraction of the maximum exposure allowed by safety guidelines. There is no danger, even if I were to track your eyes for hours.
This is when you and John realize that everyone else who was initially listening to your conversation has already walked away, and you decide to rejoin the party.
Webcam Eye Tracking
While most commercial eye trackers are based on the infrared illumination approach described in this chapter, it is important to mention the
