31 / 33
Appendix C Glossary
Accuracy
The angular average distance from the actual gaze point to the one
measured by the eye tracker.
Binocular calibration
The eye tracker collects data from both eyes at the same time and
processes the data for each eye independently. The calibration is valid
when it succeeds in collecting and processing data from both eyes.
Binocular eye tracking
Tracks and reports data for both left and right eye.
Blink recovery time
(time to tracking
recovery for blinks)
When a participant blinks, the eye tracker loses the ability to track eye
gaze because the eye is covered by the eyelid. If the pupil is occluded
for only a short period (a few hundred milliseconds), the system will
regain tracking immediately when the pupil becomes visible again, but
only if the subject has maintained approximately the same head
position during the blink. Data during blinks are only lost when the
pupil is occluded, i.e., during the eye lid movement itself or when the
eye is closed.
Bright pupil tracking
The process of capturing and processing eye images, with a set of
illuminators that are placed close to the optical axis of the camera. This
causes the pupil to appear lit up in the image (this is the same
phenomenon that causes red eyes in photos). The eye tracking
algorithms identify the pupil by searching for a bright elliptical form in
the image.
Dark pupil tracking
The process of capturing and processing eye images, with a set of
illuminators that are located further from the optical axis of the
camera.
This causes the pupil to appear darker than the rest of the eye in the
image. The eye tracking algorithms identify the pupil by searching for
a dark elliptical form in the eye image.
Data sample output
Type of data provided by the eye tracker.
Freedom of head
movement
Describes the region in space where the participant may move their
head and still have at least one eye within the eye tracker’s field of view
(track box).
Gaze recovery time
An eye tracker working in a natural user environment may occasionally
lose track of the subject’s eyes, e.g., when the subject completely turns
away from the tracker. If a period of a few hundred millisecond elapses
during which the eye tracker is unable to detect the eyes near where
they were last detected, the eye tracker will start searching for the eyes
within the entire head movement box. The stated measurement is the
typical time to tracking recovery once the eyes return to the field of
view of the cameras again, i.e., when the subject is within the track box
limits, with the eyes open and facing the eye tracker.
