Generally, lens to eye ball and the experiment can

Generally, the eye tracker devices measure/determine the eye ball
position in several ways that can be classified in three categories: contact lens
based, electrooculogram based and video based. The first category includes
invasive eye trackers that use contact lens with mirrors (Yarbus, 1967) or
magnetic search coil (Kenyon, 1985). The eye trackers that uses contact lens
with mirrors implies an entire process of attaching the lens to eye ball and the
experiment can last only a short period of time (measured in minutes), Fig. 1.
The eye trackers with magnetic search coil requires two soft contact lens and
between a coil of wire with 13 mm diameter. The twisted pair of wires from
search coil was connected to a magnetic coil system (Kenyon, 1985) for
measuring the intensity of magnetic field variation, as it is presented in Fig. 2.
These eye trackers were used specially used by the scientists for research of
physiology and dynamic of eye movements. Despite the vast improvements and
the accuracy obtained, the systems were not widespread because of invasive
process of attaching the lens and because the head had to be kept still in order
not to affect the measurements.
74 Robert Gabriel Lupu and Florina Ungureanu
Fig. 1 – Contact lens with mirrors (Yarbus, 1967).
Fig. 2 – Contact lens with magnetic search coil (Photo courtesy of Chronos Vision).
The eye trackers from second category measure the eye balls biopotentials
using electrodes placed near the eye. Because of very high nerves
density of retina, the eye ball is polarised (Fig. 3 a). The movement of the eye
cause the surrounding electric fields to move as well. These voltages can be
measured by placing electrodes near the eye (Fig. 3 b). The amplitudes of
acquired signals depend on position of the eye. Thus is possible to determine
the eye positions and used in human computer interaction. The disadvantages
are the costs of signals amplifiers and the presence of electrodes on subject face.