How Does an Eye Tracker Work?

Most commercial eye-tracking systems available today measure point-of-regard by the
“corneal-reflection/pupil-centre” method (Goldberg & Wichansky, 2003). These kinds
of trackers usually consist of a standard desktop computer with an infrared camera
mounted beneath (or next to) a display monitor, with image processing software to
locate and identify the features of the eye used for tracking. In operation, infrared light
from an LED embedded in the infrared camera is first directed into the eye to create
strong reflections in target eye features to make them easier to track (infrared light is
used to avoid dazzling the user with visible light). The light enters the retina and a
large proportion of it is reflected back, making the pupil appear as a bright, well
defined disc (known as the “bright pupil” effect). The corneal reflection (or first
Purkinje image) is also generated by the infrared light, appearing as a small, but sharp,
glint.

Once the image processing software has identified the centre of the pupil and the
location of the corneal reflection, the vector between them is measured, and, with
further trigonometric calculations, point-of-regard can be found. Although it is possible
to determine approximate point-of-regard by the corneal reflection alone , by tracking 
both features eye movements can, critically, be disassociated
from head movements (Duchowski, 2003, Jacob & Karn, 2003).

Video-based eye trackers need to be fine-tuned to the particularities of each person’s
eye movements by a “calibration” process. This calibration works by displaying a dot
on the screen, and if the eye fixes for longer than a certain threshold time and within a
certain area, the system records that pupil-centre/corneal-reflection relationship as
corresponding to a specific x,y coordinate on the screen. This is repeated over a 9 to 13
point grid-pattern to gain an accurate calibration over the whole screen (Goldberg &
Wichansky, 2003).