Correcting errors in eye-position data arising from the distortion of magnetic fields by display devices

Abstract

Eye-position recordings have been used to produce detailed information about sampling and search behavior. The melding of magnetic head-tracking technology with eye-tracking systems has allowed for freer head movement, but not without problems. Although the requisite calibrations made before and after a reading might indicate good eye-tracking accuracy, point-of-gaze errors can occur if the reader leans toward a display, such as a film alternator. The error results from the characteristic that large metal masses can distort magnetic fields, leading to false magnetic sensor data. Corrections for this type of error have involved mapping the exact location and orientation of the magnetic sensor in the magnetic field by using a precision positioning device. We have devised a much cheaper method that relies on a fixture that holds the magnetic sensor in identical positions, with and without the distorting effect.

Tracking the line of primary gaze in a walking simulator: Modeling and calibration

“…The validation experiments show that, in agreement with Caldwell et al (2000), the use of polynomial distortion compensation can improve the accuracy of gaze point calculations ( Figures 6C and 6D). In addition, benefit was also seen from geometric calibration.…”

“…above the pegboard plane. Other devices for placing a sensor at known locations have also been developed, including a fixture made entirely of Plexiglas (Caldwell et al, 2000) and an optical tracking system (Ikits, Brederson, Hansen, & Hollerbach, 2001). …”

“…Our walking simulator uses a magnetic tracker (Blood, 1990) to measure location and orientation of the participant's head. This type of device, also used in biomechanics (Day, Dumas, & Murdoch, 1998), virtual reality (Livingston & State, 1997) and eye-tracking research (Caldwell, Berbaum, & Borah, 2000), is subject to environmental magnetic distortions that worsen with the distance between the magnetic transmitter and the magnetic sensor. Compensation for this distortion is required if measurements are to be taken from near the limits of the tracker's range or if the tracker is to be used in close proximity to large metallic objects (Nixon, McCallum, Fright, & Price, 1998).…”

“…These distortion compensation techniques generally require custom equipment and are not included in popular commercial tracking systems. For example, in a study in which the eye movements of radiologists were investigated, Caldwell et al (2000) used an ASL eye-tracking system, but they employed a modified version of the ASL EYEPOS software and a custom calibration fixture to correct for magnetic distortion. This method allowed for more accurate gaze tracking near a metallic device, but few details of the software modifications were published.…”