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Camera-based eye trackers are the mainstay of today's eye movement research and countless practical applications of eye tracking. Recently, a significant impact of changes in pupil size on the accuracy of camera-based eye trackers during fixation has been reported Wyatt 2010. We compared the pupil-size effect between a scleral search coil based eye tracker (DNI) and an up-to-date infrared camera-based eye tracker (SR Research Eyelink 1000) by simultaneously recording human eye movements with both techniques. Between pupil-constricted and pupil-relaxed conditions we find a subject-specific shift in reported gaze position exceeding 2 degrees only with the camera based eye tracker, while the scleral search coil system simultaneously reported steady fixation. This confirms that the actual point of fixation did not change during pupil constriction/relaxation, and the resulting shift in measured gaze position is solely an artifact of the camera-based eye tracking system. We demonstrate a method to partially compensate the pupil-based shift using separate calibrations in pupil-constricted and pupil-dilated conditions, with pupil size as an index to dynamically weight the two calibrations.
Drewes et al. (Wed,) studied this question.