Most of the studies presented in this book use camera-based gaze trackers, used to monitor where the user is looking on screen or in the 3D environment. While a gaze tracker may resemble a blackbox device that miraculously determines the point of gaze, it actually consists of several hardware and software components with specific purposes. This chapter gives a brief introduction to the typical setup, composition and operating principles of eye and gaze tracking and it provides a simplified overview of camera-based gaze trackers that will be of use to all readers of this book.
TopBasic Setup
A typical gaze tracking system setup includes a video camera to record the movements of the eye(s) and a computer to analyse the gaze data. In addition, infrared (IR) light sources are often also used to help improve the accuracy of gaze position tracking. The details and reasons for the use of IR are explained below.
With remote systems, the user sits in front of the monitor on which the information is presented and the gaze tracking system maps the user’s gaze vector as screen co-ordinates. The gaze position information can then be exploited by any application that has become gaze-aware. Figure 1 shows a fairly typical setup, with the camera placed below the computer screen, IR emitters located on both sides of the camera and a user sitting in front of and looking at the screen. For demonstration purposes, instead of a gaze-aware application, the figure shows the setup screen which the user uses to verify that the system is tracking both eyes properly.
Figure 1. The SeeTech system (www.see-tech.de) tracks both eyes, which can be more accurate than single eye tracking (© 2008 COGAIN. Used with permission)
Since the tracking is based on eye movements, the system must have an unobstructed view of the eye and the pupil. Remote systems usually have set limits for the optimal distance and angle at which the gaze tracker is able to track the eye reliably. There are also differences between different gaze tracking systems in how much head movement they tolerate and where on the screen they are most accurate. Other potential reasons why the tracker may (momentarily) lose the eye include, for example, varying light conditions, additional reflections caused by glasses or contact lenses, drooping eyelids, heavy makeup, thick glasses, coloured contact lenses, or even narrow framed glasses which can partially block the line of view to the pupil (Goldberg & Wichansky, 2003; Bojko, 2005). Modern trackers have improved substantially in their ability to track almost anybody. However, certain limitations still remain. For example, the user might squint while laughing or accidentally block the camera with their hand. Fortunately, current trackers are often able to resume tracking as soon as the view of the eye(s) is restored.