What is Eye Tracking?

Eye-tracking is a technique for recording and analyzing eye movements used in areas such as cognitive science, psychology, human-computer interaction (HCI), market research, medical research and other fields.

Today, most eye-trackers use such basic technology as “video-oculography” (VOG), namely the filming of the position of the eye and its movement by means of a digital camera. More modern eye-trackers use a lighting system of the face (and, therefore, the eyes), infrared (IR) or near infrared (NIR), to better delineate the outline of the pupil and to one (or more) reflexes, known as “corneal reflex” (SLR Corneal, CR).

The resulting vector from the report (dynamics) of these two parameters (after a calibration period) can then be used to obtain the position of the eyes in comparison to certain elements on the display.

Traditional Recording Techniques

The dissemination and application of techniques for analyzing eye movements were limited by several technological constraints; some examples would be the invasiveness of certain methods, and the complexity of others.

A by no means exhaustive list follows:

Electric-oculography
This method uses the potential corneal-retinal continuousness. The eye is an electric dipole, whose positive (anterior) and negative (posterior) are located closer to one of two registrant electrodes depending on the position of the eye at any given time. It thereby determines a potential difference in variability between the electrodes placed on the orbital top and bottom of the internal and external margins.

Magnetic-oculography
Eye movements are derived from changes in the magnetic field in which the subject is immersed. Such changes are due to the fact that the eye of the subject is asked to wear a special contact lens that contains a conductive coil. It is a highly invasive technique and it is necessary to administer a local anesthetic to the subject. The presence of an ophthalmologist is necessary.

Video-oculography
The first techniques for videos and photos of the eye were based on individual photos or films in the 16mm format obtained through head-mounted devices. The records were then analyzed, manually, frame-by-frame. Knowing at any given time the material presented and visual direction of the subject, it was possible to make inferences about visual behavior. These techniques have evolved: in particular, Tobii trackers use techniques which combine VOG and include infrared reflection techniques.

Infrared Reflection Techniques
This method of refraction is based on infrared images of the eye that measure the amount of light reflected from the surface. The cornea is transparent to visible light, but reflects infrared. When the eyes are moving up, the top of the cornea moves, thereby affecting the amount of reflected light capable of being measured by photo detectors. The pupil can then be used to measure the horizontal and vertical position of the eye. The IR light is absorbed completely by the liquid part of the eye and the light cannot therefore be extended to the retina and reflected from the rear.