This study highlights potential differences that can arise in gaze position estimates from 1st Purkinje image-based eye trackers based on how individual Hirschberg ratios are calculated. Purpose: To evaluate the accuracy and repeatability of eccentric-viewing, prism-based and theoretical techniques that are routinely used to calibrate Hirschberg Ratio (HR) in 1 st Purkinje image-based eye trackers. Methods: Hirschberg ratios of 28 UK participants (18-40 years) were obtained using the PlusOptix PowerRef3 photorefractor and eye tracker. In the gold-standard eccentric viewing technique, participants viewed eccentric targets (±12°, 4° steps) at 2m. In the prism-based technique, 4-16∆D base-out and base-in prisms were placed in 4∆D steps before an eye occluded with an infrared filter; the fellow eye fixated a target at 1m. Each participant's HR was calculated as the slope of the linear regression of the shift in Purkinje image, relative to the pupil centre, for each target eccentricity or induced prism power. Theoretical HR was calculated from the participant's corneal curvature and anterior chamber depth measures. Data collection was repeated on another visit using all three techniques to assess repeatability. Data were also obtained from an Indian cohort (n=30,18-40 years) using similar protocols. Results: HR ranged from 10.61-14.63°/mm (median: 11.90°/mm) in the eccentric viewing technique. The prism-based and theoretical techniques demonstrated inaccuracies of 12% and 4% relative to the eccentric viewing technique. The 95% limits of agreement of intra-subject variability ranged from ±2.00°/mm, ±0.40°/mm, and ±0.30°/mm for the prism-based, eccentric viewing and theoretical techniques (P>.05). Intraclass correlation coefficients (95%CI) were 0.99 (0.98-1.00) for eccentric, 0.99 (0.99-1.00) for theoretical and 0.88 (0.74-0.94) for prism-based techniques. Similar results were found for the Indian cohort. Conclusion: The prism-based and theoretical techniques both demonstrated relative inaccuracies in measures of Hirschberg ratio, compared to the eccentric viewing technique. The prism-based technique exhibited the poorest repeatability.

No abstract

No abstract

No abstract