Editing Eye tracking (section)

=== Optical tracking ===
{{See also|Video-oculography}}
[[File:EYE-SYNC eye-tracking analyzer.JPG|thumb|An eye-tracking [[head-mounted display]]. Each eye has an LED light source (gold-color metal) on the side of the display lens, and a camera under the display lens.]]

The second broad category uses some non-contact, optical method for measuring eye motion. Light, typically infrared, is reflected from the eye and sensed by a video camera or some other specially designed optical sensor. The information is then analyzed to extract eye rotation from changes in reflections. Video-based eye trackers typically use the corneal reflection (the first [[Purkinje images|Purkinje image]]) and the center of the pupil as features to track over time. A more sensitive type of eye-tracker, the dual-Purkinje eye tracker,<ref>{{cite journal|doi=10.1364/AO.24.000527|pmid=18216982|last=Crane|first=H.D.|author2=Steele, C.M.|s2cid=10595433|title=Generation-V dual-Purkinje-image eyetracker|journal=Applied Optics|volume=24|issue=4|pages=527–537|year=1985|bibcode=1985ApOpt..24..527C}}</ref> uses reflections from the front of the cornea (first Purkinje image) and the back of the lens (fourth Purkinje image) as features to track. A still more sensitive method of tracking is to image features from inside the eye, such as the retinal blood vessels, and follow these features as the eye rotates. Optical methods, particularly those based on video recording, are widely used for gaze-tracking and are favored for being non-invasive and inexpensive.