June 2013
Volume 54, Issue 15
Free
ARVO Annual Meeting Abstract  |   June 2013
Effects of changes in pupil centration and pupil size on the output of video eye trackers
Author Affiliations & Notes
  • Ulrich Wildenmann
    Section of Neurobiology of the Eye, Tuebingen University, Tuebingen, Germany
  • Frank Schaeffel
    Section of Neurobiology of the Eye, Tuebingen University, Tuebingen, Germany
Investigative Ophthalmology & Visual Science June 2013, Vol.54, 188. doi:
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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)

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Abstract

Purpose: To evaluate potential measurement errors that could be induced in video eye trackers when pupil centration changes with pupil size.

Methods: 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).

Results: 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.

Conclusions: 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.

Keywords: 667 pupil • 668 pupillary reflex  
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