Several definitions exist regarding the size of the pupil. If wanting to compare data between different studies and eye tracker manufacturers, it is important to understand the different definitions.
In the eye model used by Tobii Eye Trackers the pupil size is defined as the actual, internal physical size of the pupil and not the size it appears to be when looking at the eye from the outside. It is important to keep this in mind as the same internal pupil size can appear to vary if you see it from the outside depending on from which angle you see the eye from. This is because the front of the eye basically is a lens through which you see the pupil. Depending from which angle you look through the lens, the pupil appears more or less distorted.
However, in most scientific research the actual size of the pupil is less important than its variations in size over time. Tobii Eye Trackers output pupil size information for each eye together with each gaze point through the Pro SDK allowing for recording of the pupil size variation during an eye tracking session.
The pupil diameter data is provided for the left and the right eye individually and is an estimate of the pupil size in millimeters.