Synchronization of a Removable Optical Element with an Eye Tracker: Test Case for Heterophoria Measurement

Heterophoria describes the deviation of the optical axes in the absence of binocular fusion. Eye trackers (ET) can provide an objective assessment but are not broadly used clinically. We examined the feasibility of combining an infrared (IR) pass-filter, IR detector, and an off-the-shelf ET. The proposed setup was validated against the broadly used cover test (CT). Furthermore, the setup was used to examine whether testing conditions can affect the measurements.

An IR detector was attached to a handheld IR-pass filter that blocks visible light to provide occlusion while passing IR light for eye tracking. The detector senses the IR illumination of the eye tracker, creating a recordable signal of the occluder position synchronized with eye positions acquired by the SMI Red250 tracker. The mean of three measurements of each condition, three versus ten seconds occlusion, the occluded eye, and ET versus CT results were compared using the Wilcoxon test, correlation and Bland and Altman plots. Differences between measurements that were within 2^Δ were considered clinically insignificant.

Thirty normally-sighted subjects (mean age 24.50 ± 2.20, range 20-28) with heterophoria ranging between 14^Δ exophoria and 4^Δ esophoria were recruited. There was no significant difference between the occluded eyes. However, there was a difference between 3 and 10 seconds' cover duration. The CT data were more similar to the 10 seconds cover duration, although differences were less than the clinical resolution of 2^Δ.

An inexpensive off-the-shelf ET can be used to measure heterophoria with controlled testing parameters.

Our study demonstrated a robust technique for synchronization of an optical element such as an IR cover, with an off-the-shelf commercial eye tracker. The synchronization of optical elements with eye tracking, which has been described here for heterophoria, can be adapted for other clinical measurements.