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Ulrich Wildenmann, Frank Schaeffel; Effects of changes in pupil centration and pupil size on the output of video eye trackers. Invest. Ophthalmol. Vis. Sci. 2013;54(15):188.
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© ARVO (1962-2015); The Authors (2016-present)
To evaluate potential measurement errors that could be induced in video eye trackers when pupil centration changes with pupil size.
Software was developed under Visual C++ to track both pupil center and corneal center at 87 Hz sampling rate while pupil constrictions were elicited by a torch. Corneal center was determined by a circle fit through the pixel positions detected at the corneal margin by an edge detection algorithm. Standard deviations for repeated measurements were ± 0.03 mm for horizontal pupil center position and ± 0.03 mm for horizontal corneal center positions. The output of the software was checked against manual measurements in videographs of the eyes. Ten subjects were tested (5 female, 5 male, age 25 - 58 years).
At 4 mm pupil size, the pupil was nasally decentered relative to the corneal center by 0.18 ± 0.19 mm in the right eyes and -0.14 ± 0.22 mm in the left eyes. Vertical decentrations were 0.30 ± 0.30 mm and 0.27 ± 0.29 mm, respectively, always in superior direction. Decentrations were similar at the largest pupil sizes observed in each of the subjects (right and left eyes: horizontal 0.17 ± 0.20 mm and -0.12 ± 0.22 mm, and vertical 0.26 ± 0.28 mm and 0.20 ± 0.25 mm). Interestingly, pupil centration changed with pupil size (average change per mm pupil size change: for the right eyes horizontally 0.01 ± 0.02 mm and vertically 0.04 ± 0.05 mm, for the left eyes -0.02 ± 0.04 mm and 0.07 ± 0.33 mm, respectively). Pupil decentration with fully dilated pupils had no predictive value for centration changes that occurred during pupil constriction.
The pupil center was significantly decentered relative to the corneal center in nasal and superior direction. Decentration of the pupil increased during pupil constriction. Assuming a Hirschberg ratio of 12 deg/mm, a change in pupil center position of 0.1 mm would be equivalent to a change in the output of a Purkinje image-based eye tracker by 1.2 deg. Since pupil centration can change by more than 0.5 mm in some subjects, the induced measurement errors can exceed 5 deg.
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