Monocular/binocular Monocular data shown is based on data from each eye individually. Binocular data is the average of
the two eyes.
Gaze precision Describes the spatial angular variation between individual and consecutive gaze samples. Gaze
precision can be measured under various conditions. Gaze precision is sometimes also specified as
‘spatial resolution’. For more details, see Appendix II
Gaze accuracy Describes the angular average distance from the actual gaze point to the one measured by the eye
tracker. Gaze accuracy can be measured under various conditions. For more details, see Appendix II.
Sampling rate Number of data samples per second. The TobiiT/X Eye Trackers series have a stable data-rate of 60,
120 or 300 Hz; that is 60, 120 or 300 data samples per second are collected for each eye.
Sampling variability Sampling variability specifies the maximum difference between the stated sampling rate and the
actual sampling rate that can occur during an eye tracking test.
Processing latency Describes the time required by the eye tracker processor to perform image processing and eye gaze
computations.
Total system latency The duration from mid-point of the eye image exposure, to when a sample is available via the API on
the client computer (assuming a dedicated Gigabit Ethernet connection). This includes half of the
image exposure time, plus image read-out and transfer time, processing time and time to transfer the
data sample to a client computer.
Timestamp precision via sync-out
port
The temporal deviation of the signal on the sync-out port relative to the beginning of the actual
exposure of the eye image.
Timestamp precision as specified
in each data sample
The temporal deviation of the timestamp in the data sample received by the client application. This
includes any offset in the clock sync between the eye tracker processing unit and a typical client
computer.
Time to tracking recovery for
blinks
When a subject blinks, the eye tracker loses the ability to track eye gaze because the eye is covered
by the eye lid. 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.
Time to tracking recovery after
lost tracking
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
milliseconds elapses during which the eye tracker is unable to detect the eyes in close proximity to
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 in such a situation. If
the eye tracker is unable to detect the eyes of the subject even after about one minute, the system
will enter a “slow search” mode, leading to larger recovery times.
Freedom of head movement Describes an area (height × width in cm) where at least one eye is within the eye tracker’s field of
view.
Operating distance Describes the minimum and maximum distances between the subject’s eyes and the surface covering
the eye tracker sensors at which eye tracking can be done while maintaining robust tracking.
Max head movement speed Describes the maximum head movement speed allowed while maintaining robust tracking. The
specified number is for sideways head movement.
Max gaze angles The maximum gaze angle for which the eye tracker can perform robust and accurate tracking on
both eyes. The gaze angle is the angle ABC with A = center of the eye tracker (midpoint between
the two eye tracking sensors), B = eye position (midpoint between the left and the right eye) and C
= stimuli point.
Eye tracking technique Tobii Eye Trackers use two different techniques to determine eye position: 1. Bright pupil eye tracking,
where an illuminator is placed close to the optical axis of the imaging device, causing the pupil to
appear lit up (the same phenomenon that causes red eyes in photos). 2. Dark pupil eye tracking
where the illuminator is placed away from the optical axis, causing the pupil to appear black.
Eye tracking processing unit Gaze data calculations are performed by firmware embedded in the eye tracker. Different
applications can be connected over a LAN connection as clients to the eye tracker system to gather
eye gaze data and other data in real-time, perform calibrations, etc.
Appendix I: Glossary
Definitions
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