The algorithms used by Tobii Eye Trackers allows for calculations of the position of the eyes as well as the pupil size.
The optical sensor registers an image of the eyes which then is used to calculate parameters to adjust the eye model algorithms. As the eye model used by Tobii Eye Trackers provides data about the distance between the eye and the sensor, the firmware can calculate the pupil size by measuring the diameter of the pupil on the image and multiply it with a scaling factor.
Several definitions exist regarding what should be defined as the size of the pupil. In the eye model used by Tobii Eye Trackers the pupil size is defined as the actual, internal physical size of the pupil. However, in most scientific research the actual size
In the eye trackers that use both bright and dark pupil tracking techniques, the reported pupil size changes according to the technique deployed by the eye tracker during data collection, introducing noise in the analysis. As a result we recommend the use of either the Tobii TX300 Eye Tracker or the Tobii T60/120, X60/120 Eye Trackers set to "Low Illumination" mode to measure pupil size.