In our eye movement labs we can measure eye movements of subjects while they observe visual stimuli on a computer monitor and perform oculomotor tasks. Thus we investigate the eye movements conducted while performing certain tasks and how eye movements are controlled by visual stimuli.
We use different types of so-called eyetrackers to measure eye movements. In the video-based systems (EyeLink II & EyeLink 1000), the subject´s eyes are filmed by infrared video cameras. The eye is illuminated with infrared light invisible to people. Because the pupil reflects very little or no light, a dark stain appears in the camera image. The software identifies the position of this dark stain and interpretes it as the position of the pupil. Of course, we do not want to measure the position of the pupil in the camera image of the eye tracker, but the position of the subject´s gaze on the computer monitor. For this, the eye tracker has to be calibrated. During calibration, small dots appear in different places on the screen, and the subject must fixate them. This allows the eye-tracker to "learn" the relationship between the position of the pupil on the camera image and the gaze position on the computer monitor. Using EyeLink II, the subject wears a kind of bicycle helmet with mounted cameras. EyeLink 1000´s camera is integrated in a chin and forehead rest.
The DPI (Dual Purkinje Image) eyetracker uses a different method to measure the gaze position. As with the video-based systems, the eye is illuminated with infrared light, but not the position of the pupil is measured, but the first and fourth Purkinje reflection. Purkinje reflections are reflections of strong light on the borders of the lens and the cornea. The two reflections are centered onto a photosensor through a movable mirror system. The mirrors are continuously adjusted by engines, so the reflections stay centered. Then, the gaze position is calculated based on the mirror positions. In comparison to the video-based method, this method of measurement is more accurate but also more complicated regarding the practical implementation.
