Visual tracking

Eye tracking is a technique used in cognitive science, psychology (notably psycholinguistics), human-computer interaction (HCI), marketing research, medical research, and other areas. The most widely used current designs are video based eye trackers. A camera focuses on one or both eyes and records their movement as the viewer looks at some kind of stimulus. Most modern eye-trackers use contrast to locate the center of the pupil and use infrared and near-infrared non-collumnated light to create a corneal reflection (CR). The vector between these two features can be used to compute gaze intersection with a surface after a simple calibration for an individual.

Two general types of eye tracking techniques are used, Bright Pupil and Dark Pupil. The difference between the two is based on the location of the illumination source with respect to the optics. If the illumination is coaxial with the optical path then the eye acts as a retroreflector as the light reflects off the retina creating a bright pupil effect similar to red eye. If the illumination source is offset from the optical path, then the pupil appears dark.

Bright Pupil tracking creates greater iris/pupil contrast allowing for more robust eye tracking with all iris pigmentation and greatly reduces interference caused by eyelashes and other obscuring features. It also allows for tracking in lighting conditions ranging from total darkness to very bright. However, bright pupil techniques are not effective for tracking outdoors as extraneous IR sources interfere with monitoring.

Eye tracking setups vary greatly; some are head-mounted, some require the head to be stable (for example, with a chin rest), and some function remotely and automatically track the head during motion. Most use a sampling rate of at least 30Hz. Although 50/60 Hz is most common, today many video-based eye trackers run at 240, 350 or even 1000/1250 Hz, which is needed in order to capture the detail of the very rapid eye movements during reading, or during studies of neurology.

Eye movements are typically divided into fixations and saccades, when the eye gaze pauses in a certain position, and when it moves to another position, respectively. The resulting series of fixations and saccades is called a scanpath. Most information from the eye is made available during a fixation, but not during a saccade. The central one or two degrees of the visual angle (the fovea) provide the bulk of visual information; the input from larger eccentricities (the periphery) is less informative. Hence, the locations of fixations along a scanpath show what information loci on the stimulus were processed during an eye tracking session. On average, fixations last for around 200 ms during reading and 350 ms during scene viewing. Preparing a saccade towards a new goal takes around 200 milliseconds.

Scanpaths are useful for analyzing cognitive intent, interest, and salience. Other biological factors (some as simple as gender) may affect the scanpath as well. Eye tracking in HCI typically investigates the scanpath for usability purposes, or as a method of input in gaze-contingent displays or gaze-based interfaces.

Usage
Eye tracking has many applications :
 * Cognitive Studies
 * Medical Research
 * Human Factors
 * Computer Usability
 * Vehicle Simulators
 * In-vehicle Research
 * Training Simulators
 * Virtual Reality
 * Adult Research
 * Infant Research
 * Adolescent Research
 * Geriatric Research
 * Primate Research
 * Sports Training
 * fMRI / MEG / EEG
 * Commercial eye tracking (web usability, advertising, marketing, automotive, etc)