Are pupil size calculations possible with Tobii Eye Trackers?

Hardware Neuroscience Eye tracking technique

Knowing the size of the pupil whether it changes over time is often used when studying emotional responses to stimuli. 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

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 allowing an external software (e.g. Tobii  Studio)  to  record  the  pupil  size  variation during an eye tracking session.

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.

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