Free Head Motion Eye Gaze Tracking Techniques

Abstract

In this chapter we look at a passive eye monitoring system, or simply eye tracker (ET), as a mono-camera system that measures the position and orientation of the subject’s eye. The most common image based ET technique is known as the pupil-corneal reflection (PCR) technique, because it uses a reflection on the surface of the cornea (the first Purkinje image) generated by an external light source that is used as a “reference” point. The two dimensional image vector defined by the center of the pupil (or the iris) and the corneal reflection is used to estimate the gaze direction, after a simple calibration procedure that defines the mapping from image coordinates to screen coordinates1. To facilitate pupil detection and tracking in real-time, an active differential lighting scheme is commonly used [421]. By placing a light source near the camera optical center, a bright pupil image is generated, while a second light source, distant from the camera center, generates a dark pupil image. The two light sources can be synchronized with the video frame rate, so that at every other frame the camera captures a bright or a dark pupil image, and the pupil can be easily segmented as a high contrast region from the subtraction of the bright and dark pupil images.