Eye tracking is the process of measuring either the point of gaze ("where we are looking") or the motion of an eye relative to the head. An eye tracker is a device for measuring eye positions and eye movements. Eye trackers are used in research on the visual system, in psychology, in cognitive linguistics and in product design. There are a number of methods for measuring eye movements. The most popular variant uses video images from which the eye position is extracted.

The most widely used current designs are video-based eye trackers. A camera focuses on one or both eyes and records their movement as the viewer looks at some kind of stimulus. Most modern eye-trackers use contrast to locate the center of the pupil and use infrared and near-infrared non-collimated light to create a corneal reflection (CR). The vector between these two features can be used to compute gaze intersection with a surface after a simple calibration for an individual.

Two general types of eye tracking techniques are used: Bright Pupil and Dark Pupil. Bright Pupil tracking creates greater iris/pupil contrast allowing for more robust eye tracking with all iris pigmentation and greatly reduces interference caused by eyelashes and other obscuring features. It also allows for tracking in lighting conditions ranging from total darkness to very bright. The ASL 6000 eyetracker used at YNiC is a bright-pupil eye tracker.

Figure B.1. A Pupil Being Tracked.

Figure B.2. An Example Reading Path.

The images above show how the eyetracking software 'locks' onto the pupil with a cross hair (Figure B.1). The image in (Figure B.2) shows the x-y output (in pink) of the eye gaze path detected by the eyetracker overlaid onto the stimulus (in this case some German text).