Eye Tracking the User Experience. Aga Bojko

only one eye, while binocular systems record both eyes. Theoretically, monocular eye trackers should be sufficient because both eyes tend to move together, so by looking at the position of one eye, you can also determine the position of the other eye. Monocular systems are usually also more affordable than binocular ones.

      However, UX research can benefit from binocular eye tracking in a few ways. First, binocular tracking increases accuracy and precision due to the averaging of the data from both eyes. Second, if one eye temporarily loses tracking (perhaps due to the participant moving a little outside of the head box), information doesn’t get lost because there are always data from the other eye. Finally, knowing the position of both eyes helps correct the parallax error in wearable eye trackers.

       Pupil Illumination Methods

      In Chapter 1, “Eye Tracking: What’s All the Hoopla?” we discussed how eye tracking works. If you remember, the typical eye tracker knows where someone is looking based on the position of the person’s pupil center in relation to the corneal reflection. Infrared light-emitting diodes illuminate the participant’s face, and cameras in the eye tracker detect and record those two eye features.

      What Chapter 1 didn’t mention was that depending on where the diodes are positioned with regard to the cameras, two different methods can be used to illuminate the eyes: bright-pupil method and dark-pupil method (see Table 3.4). Both methods have the same goal: to create as much contrast as possible between the pupil and the iris (the colored part of the eye) in order to make the pupil easily distinguishable. However, the effectiveness of each method depends on the conditions under which it is used and the physical characteristics of the participant’s eyes. Most eye trackers offer either bright-pupil tracking or dark-pupil tracking, but some have both and automatically select the one that provides higher accuracy in a given situation.

TABLE 3.4 COMPARISON OF BRIGHT- AND DARK-PUPIL METHODS

	Bright-Pupil Method	Dark-Pupil Method
How It Works	When the infrared light source is in line with the optical axis of the camera, the pupil appears brighter than the iris because the reflection from the retina is directed toward the camera. (This is the same phenomenon that causes the red-eye effect in photos.)