Reliability of real-time eye tracking with a commercial mobile eye-tracker

Purpose: The Tobii Pro Glasses 2 is a wearable mobile eye tracker marketed as a valid research tool for natural gaze interaction. This study identified the working range of the device when the head is static, and tested the hypothesis that 3D gaze position (horizontally, vertically and torsionally about the line of sight) as measured by the device is valid in scientific research. Methods: Validation of gaze position was made via physical testing and replication of the gaze estimation algorithm applied by the eye tracker. Five healthy adults with a mean age of 32.4 ± 9.6 years were recruited to view 32 target points in space using a custom-built mobile rig while their chin was held on the chin-rest. The origin was taken as the estimated point between the eyes. The target was placed at varying Z distances (30, 50, 70, 100, 150, 200, 250, 300 cm) for X widths (0, 30, 70 mm) at primary gaze, and placed at Y angles (-45, -30, -20, -15, -10, -5, 0, 5, 10°) at controlled X and Z distances (0, 100 cm). Independent t-test was used to compare the measured and replicated gaze estimation methods, and intraclass correlation coefficient was used to derive the agreement between the two methods. Results: The mean tracker measured gaze location was not significantly different from the replicated gaze estimation algorithm for various Z distances and X widths (each p>0.05). The intraclass correlation coefficient was 0.77, suggesting a good agreement between the measured and the replicated methods. Average error for Z distances 30–100 cm was less than 5 cm (1.5 ± 2.58 cm) at X distances 0 and 30 mm. Error increased exponentially for distances greater than 150 cm (79.5 ± 56.2 cm) at all X distances. No measurements were recorded for gaze angles outside 5° superiorly and 30 inferiorly. Average error for Y angles between 5° superiorly and 15° inferiorly were less than 5°, and average error for all recorded angles 1.72 ± 3.96°. Conclusions: The Tobii Pro Glasses 2 was able to measure the gaze angles up to 5° superiorly and 30° inferiorly from the primary gaze. The distance coordinate error increased exponentially for a distance greater than 150 cm. These findings suggest that the Tobii Pro Glasses 2 produces reliable gaze measures for a working range of up to 150 cm