Fundamental information about eye tracking
Eye tracking is the measurement of the movements of the eye, with the purpose of recording what a subject is looking at. All of our 4 SMI eye trackers use 'dark pupil' technology, where the gaze of the eye is tracked with an infra-red camera by identifying the pupil and the reflection of an infra-red light source on the cornea. The Tobii eye tracker, in the CALM lab, uses a combination of dark and bright pupil technology.
Eye trackers come in two varieties: 'contact' and 'remote'. With contact eye trackers the camera and light source are fixed to the head, or the head is on a chin rest to limit movements relative to the camera. In remote systems the camera and light source are in a fixed location, at some distance from the subject, enabling some head movements. Contact systems are more precise and reliable, in general.
Another important feature of an eye tracker is the sampling frequency. To be able to follow the eye during saccades the minimum frequency necessary is about 200 Hz. For 'heat maps', which map summed gaze duration by location, or for AOI dwell time analyses lower frequencies are sufficient.
All our eye trackers are able to output the x and y coordinates of the screen location the subject is looking at, and the diameter of the pupil. That last value will be 0 during a blink.
Basic types of eye movements
Eye movements can be classified into several types, but this is not an unambiguous classification. Different algorithms will give different outcomes for the number of fixations and the total fixation time. Eye tracking data will also always contain unclassifiable bits, for example during a blink.
Fixations
Fixations are low-velocity movements that occur when the subject is gazing at a fixed object. Fixations will contain small movements when the eye drifts slightly away from the fixation point, and corresponding fast corrections.
Saccades
Saccades are high-velocity movements that happen in between two fixations. Saccades can only be measured with high-speed eye trackers, and require a tracking frequency of about 300 Hz or higher.
Pursuit
Pursuit, or 'smooth pursuit' happens when the eye follows a moving object. These are medium velocity movements.