Eye tracking has extensively been in use since its advent. It can be narrated as a means whereby the movements of a human eye are recorded while he is reading some text or is merely looking at something. The frequency with which a particular spot on the screen is observed is noted and calculations are then used to assess the usability of a particular thing with the observation that it has been assigned with. It is used in passive and active modes both. How that is done is narrated in the following few lines:
Passive MODE
• This mode assists greatly in the assessing of behavioral patterns. For example during simulation lectures for pilots and rocket fliers the movements of the eyes of the learners gives an idea of the learning patterns of individuals as well as their concentration paradigms.
• The creation of individualized reading aids is made possible by assessing when a person is reading and when the gaze of the person has stopped on a particular spot for longer than a normal duration of time.
• Buyers attention can be noted down in order to assist market researchers.
Active MODE
• On screen keyboards and mouse use the data collected via eye tracking in order to assess the next command that is being given by the user. This is useful specifically in the case of such systems being in the use of disabled people.
• Patients in clinics/hospitals may be provided with eye tracking facility enabled systems that would record their input whenever patients cannot speak, touch or move.

However, there are certain constraints to the use of eye tracking that can be summarized as follows:
Physiological hindrances: Enough light may not be reflected via iris of the eye. Eye lashes or eye lids may be covering the iris of the eye.
Eye glasses or eye contacts may be hindering the execution of eye tracking.
SUMMARIZING THE CURRENT STATE OF EYE-TRACKING TECHNOLOGY:
The present studies and researches conducted in the field of Eye Tracking are limited to the use of eye-tracking as an input medium as discussed and narrated earlier in the text. The time and location movements of fixations and Saccades of the participating individuals’ eye movements are the most commonly used measurements that are used to record eye tracking.