In this section we will discuss the calibration procedure of Tobii Eye Trackers.
Calibration is the process whereby the geometric characteristics of a subject’s eyes are estimated as the basis for a fully-customized and accurate gaze point calculation.
Before an eye tracking recording is started, the user is taken through a calibration procedure. During this procedure, the eye tracker measures characteristics of the user’s eyes and uses them together with an internal, anatomycal 3D eye model to calculate the gaze data. This model includes information about shapes, light refraction and reflection properties of the different parts of the eyes (e.g. cornea, placement of the fovea, etc.). During the calibration the user is asked to look at specific points on the screen, also known as calibration dots. During this period several images of the eyes are collected and analyzed. The resulting information is then integrated in the eye model and the gaze point for each image sample is calculated. When the procedure is finished the quality of the calibration is illustrated by green lines of varying length. The length of each line represents the offset between each sampled gaze point and the center of the calibration dot. Large offsets (long green lines) can be caused by various factors such as, the user not actually focusing on the point, the user being distracted during the calibration or the eye tracker not being set up correctly. However, the user does not have to keep the head completely still during calibration as long as the focus of the user’s eyes is kept on the moving dots. During the calibration both the light and dark pupil methods are tested to identify the most suitable for the current light conditions and the user’s eye characteristics.