24/08/07

Eye tracking technology

Eye tracking is the science of measuring the movement of the eyes usually is response to visual, auditory, cognitive or vestibular stimulus. There are many different fields eye tracking can be used for, ranging from advertising, physiological activities, neurological and psychological studies. Researchers study the movement of the eye both as physiological responses to stimuli and as an indication of cognition.

Types of eye tracking

Orbital eye tracking	this is the movement of the eyes as an absolute measure. The orbital position of the eye refers only to the deflection of the pupil itself.
Gaze position	this is the point in the subjects field of view where the eye is actually looking. Eye tracking systems that determine gaze position do so by applying complicated mathematical algorithms to the measured orbital pupil position.
Torsional data	this is a special kind of eye tacking that tracks the eye in three dimensions. Horizontal deflection, vertical deflection and rotation around a centre axis parallel with the line of sight.

Common types of eye movements

Fixations	These very low velocity eye movements that correspond to the subject staring at a particular point. They contain very small randomly drifting eye movements and quick adjustments to keep the target central.
Pursuit	These occur when the eyes follow a moving target in order to fix the target on the retina. The eyes can smoothly track a moving object, however rapid involuntary movements occur to catch up the target.
Saccades	These are rapid involuntary movements of the eye makes when looking at different points on a stimulus. I.e. reading.
Gaze path	This is generally the path the eye takes while studying a stimulus image. This can be used to describe the chronological ordering of fixations / saccades the pattern the eye takes while studying the image.

Head movements

For eye tracking systems to determine gaze position they must have a method for separating head movement from eye movements.

Calibration

Eye tracking involves instructing the subject to look at known points while the system observes the pupil position at that point. It is important that the calibration occurs at the distance that the eye will be focusing at or when a participant is performing a specific task. The system will develop the necessary algorithm to translate pupil position to gaze position to all points in the area defined by the calibration.

 

                                  

                IView computer screen. Subjects eye and                     Calibration method: use of white screen                        Eye tracking camera is mounted  onto a
           pupil in clear view.                                                    and laser pen.                                                            helmet and secured on the head.