Читать книгу Eye Tracking the User Experience - Aga Bojko - Страница 11

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 recently evolving appearance-based systems. Instead of relying on infrared light, these low-cost solutions use off-the-shelf webcams to extract and track eye features on the face. Webcam eye tracking is most often employed in remote testing, during which participants use their computers at home or at work without having to come to a lab.

One of the current constraints of webcam eye tracking is poorer accuracy as compared to the standard infrared devices. The accuracy decreases even further when participants move around or move their computer—something that’s difficult to control in a remote session (see Figure 1.4). In addition, the rate at which the gaze location is sampled by webcams is relatively low, which greatly limits data analysis.

FIGURE 1.4 Some of the challenges of remote research with webcam eye tracking stem from the researcher’s inability to control the test environment.