The Basics of Eye Tracking
An eye tracker estimates the point of gaze with extreme accuracy using image sensor technology that finds the user’s eyes and calculates the point of gaze with mathematical algorithms. In other words, the eye tracker works much like you would if you face another person and estimate at what they are looking just by observing their eyes.
Pupil center corneal reflection
Most modern eye trackers are based on the fundamental principle of corneal-reflection tracking.
- One or several near-infrared illuminators, invisible to the human eye, create reflection patterns on the cornea of the eyes.
- At high sampling rates, one or multiple image sensors register the image of the user’s eyes.
- Image processing is used to find the eyes, detect the exact position of the pupil and/or iris, and identify the correct reflections from the illuminators and their exact positions.
- A mathematical model of the eye is used to calculate the eyes’ position in space and the point of gaze.
Dark and bright pupil reflection
Two different illumination setups can be used with pupil corneal reflection eye tracking:
Bright pupil eye tracking - illuminators are placed close to the optical axis of the imaging sensor, which causes the pupil to appear lit up (this is the same phenomenon that causes red eyes in photos)- Dark pupil eye tracking - illuminators are placed away from the optical axis causing the pupil to appear darker than the iris.
Some Tobii eye trackers use both bright and dark pupil methods to calculate the gaze position. These eye trackers can accommodate larger variations in experimental conditions and ethnicity than an eye tracker using only one of the techniques described above.
Calibration
Before an eye tracking session starts, the system calibrates the user’s eyes. During calibration, the eye tracker measures characteristics of the user’s eyes that are required to accurately calculate gaze direction.
Tobii eye trackers use a 2, 5 or 9-point calibration and the procedure takes only a few seconds. No adjustments of the equipment or the positioning of the user is required. Tobii eye trackers can even use a so-called calibration-free setup, where calibration is carried out undetected by the user.
Freedom of head movement
The head movement box is the imaginary box in which a user can move his/her head and still be tracked by the device. Tobii eye trackers have different head movement boxes depending on model. A larger head movement box allows the user more freedom to move in front of the eye tracker and provides greater comfort.
Tobii’s eye trackers have a unique tolerance for head movement during tracking. Head movement within the eye tracking box has very little impact on gaze data accuracy and the system can even track users when one eye is outside the sensors’ visual field.