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eye tracking basics

Eye Tracking Basics | Eye Structure

To understand how eye tracking works, it is important to have a basic knowledge of the structure of the eye.

  • pupil—an aperture, or opening, in the iris that allows light to enter into the eye unless viewed exactly on-axis with a light source, the pupil appears black
  • iris—the colored part of the eye, opens and closes to control the pupil size
  • cornea—a thin, film-like tissue that covers the eye
    • on its way to the pupil, light passes through the cornea
    • the cornea is mostly transparent; however, some light is also reflected from the cornea
  • sclera—the white part of the eye

Some eye tracking systems, such as ETVision, use a tracking technique known as “Pupil to CR” Tracking that uses the relationship between two eye features, the black pupil and mirror reflections from the front surface of the cornea (Corneal Reflections, or CRs), to compute gaze within a scene.


pupil-to CR- eye tracking technique

“Pupil to CR” eye tracking

Light from a pair of harmless near infra-red LEDs on the head gear illuminate the eye area. The near IR light is almost invisible to the user, so it does not cause a distraction, however it is visible to the eye camera. The mirror reflections of these light sources from the front surface of the cornea (CRs) appear in the camera image as a pair of bright dots. When the eye rotates in its socket, the center of the pupil moves relative to the spots. By comparing the relative position of the pupil and the CRs, the eye tracking system can compute the pointing direction of the eye visual axis relative to a coordinate frame centered at the scene camera.

During periods when the headgear remains stationary on the head, eye gaze direction can also be computed from pupil or CR position alone, and this can allow the system to continue to compute gaze when only a subset of the eye image features can be successfully identified. By computing the point at which the lines of gaze from the two eyes converge, the gaze point can be represented as spot on the scene camera image.

Since the geometry of the eye structures and the position of the optics with respect to the face vary somewhat from person to person, gaze computation accuracy can be improved by incorporating data from calibration points (points at which the person wearing the optics is known to be fixating a particular point on the scene camera image). Excellent accuracy is often achieved with a single calibration point, but can sometimes be improved with additional points.

Eye tracking has many specific applications for academic research, HCI, market research (ergonomics & design), sports performance, virtual reality, etc., and can easily be added to any study to increase feedback from participants in any environment.

Adapted from the ETVision Manual 

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Watch Eye Tracking Fundamentals now or visit our Webinars and Video Tutorial pages.

BIOPAC offers a wide array of wired and wireless equipment that can be used in your research. For more information on solutions for recording and analyzing life science signals and measurements, visit the Research > Applications section on the BIOPAC website.

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