Calibration-free eye gaze tracking

Optics: eye examining – vision testing and correcting – Eye examining or testing instrument – Objective type

Reexamination Certificate

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C345S157000

Reexamination Certificate

active

06578962

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to the determination of a user's eye gaze vector and point of regard by analysis of images taken of a user's eye. The invention relates more specifically to eye gaze tracking without the need to calibrate for specific users' eye geometries and to subsequently recalibrate for user head position.
BACKGROUND OF THE INVENTION
Eye gaze tracking technology has proven to be useful in many different fields, including human-computer interfaces for assisting disabled people interact with a computer. The eye gaze tracker can be used as a mouse emulator for a personal computer, for example, helping disabled people to move a cursor on a display screen to control their environment and communicate messages. Gaze tracking can also be used for industrial control, aviation, and emergency room situations where both hands are needed for tasks other than operation of a computer but where an available computer is useful. There is also significant research interest in eye gaze tracking for babies and animals to better understand such subjects' behavior and visual processes. Commercial eye gaze tracking systems are made by ISCAN Incorporated (Burlington Mass.), LC Technologies (Fairfax Va.), and Applied Science Laboratories (Bedford Mass.).
There are many different schemes for detecting both the direction in which a user is looking and the point upon which the user's vision is fixated. Any particular eye gaze tracking technology should be relatively inexpensive, reliable, unobtrusive, easily learned and used and generally operator-friendly to be widely accepted. The corneal reflection method of eye gaze tracking is increasing in popularity, and is well-described in the following U.S. patents, which are hereby incorporated by reference: 4,595,990, 4,836,670, 4,950,069, 4,973,149, 5,016,282, 5,231,674, 5,471,542, 5,861,940, 6,204,828. These two articles also describe corneal reflection eye gaze tracking and are also hereby incorporated by reference: “Spatially Dynamic Calibration of an Eye-Tracking System”, K. White, Jr. et al., IEEE Transactions on Systems, Man, and Cybernetics, vol. 23, no. 4, July/August 1993, p. 1162-1168, referred to hereafter as White, and “Effectiveness of Pupil Area Detection Technique”, Y. Ebisawa et al., Proceedings of the 15
th
Annual International Conference of IEEE Engineering in Medicine and Biology Society, vol. 15, October 1993, p. 1268-1269.
Corneal reflection eye gaze tracking systems project light toward the eye and monitor the angular difference between pupil position and the reflection of the light beam. Near-infrared light is often employed, as users cannot see this light and are therefore not distracted by it. Usually only one eye is monitored, and it isn't critical which eye is monitored. The light reflected from the eye has two major components. The first component is a ‘glint’, which is a very small and very bright virtual image of the light source reflected from the front surface of the corneal bulge of the eye. The glint position remains relatively fixed in an observer's image field as long as the user's head remains stationary and the corneal sphere rotates around a fixed point. The second component is light that has entered the eye and has been reflected back out from the retina. This light serves to illuminate the pupil of the eye from behind, causing the pupil to appear as a bright disk against a darker background. This retroreflection, or “bright eye” effect familiar to flash photographers, provides a very high contrast image. Unlike the glint, the pupil center's position in the image field moves significantly as the eye rotates. An oculometer determines the center of the pupil and the glint, and the change in the distance and direction between the two as the eye is rotated. The orientation of the eyeball can be inferred from the differential motion of the pupil center relative to the glint. The eye is often modeled as a sphere of about 13.3 mm radius having a spherical corneal bulge of about 8 mm radius; the eyes of different users will have variations from these typical values, but individual dimensional values do not generally vary significantly in the short term.
As shown in prior art
FIG. 1
, the main components of a corneal reflection eye gaze tracking system include a video camera sensitive to near-infrared light, a near-infrared light source (often a light-emitting diode) typically mounted to shine along the optical axis of the camera, and a computer system for analyzing images captured by the camera. The on-axis light source is positioned at or near the focal center of the camera. Image processing techniques such as intensity thresholding and edge detection identify the glint and the pupil from the image captured by the camera using on-axis light, and locate the pupil center in the camera's field of view as shown in prior art FIG.
2
.
Human eyes do not have equal resolution over the entire field of view, nor is the portion of the retina providing the most distinct vision located precisely on the optical axis. The eye directs its gaze with great accuracy because the photoreceptors of the human retina are not uniformly distributed but instead show a pronounced density peak in a small region known as the fovea centralis. In this region, which subtends a visual angle of about one degree, the receptor density increases to about ten times the average density. The nervous system thus attempts to keep the image of the region of current interest centered accurately on the fovea as this gives the highest visual acuity. A distinction is made between the optical axis of the user's eye versus the foveal axis along which the most acute vision is achieved. As shown in prior art
FIG. 3
, the optical axis is a line going from the center of the spherical corneal bulge through the center of the pupil. The optical axis and foveal axis are offset in each eye by an inward horizontal angle of about five degrees, with a variation of about one and one half degrees in the population. The offsets of the foveal axes with respect to the optical axes of a user's eyes enable better stereoscopic vision of nearby objects. The offsets vary from one individual to the next, but individual offsets do not vary significantly in the short term. For this application, the gaze vector is defined as the optical axis of the eye. The gaze position or point of regard is defined as the intersection point of the gaze vector with the object being viewed (e.g. a point on a display screen some distance from the eye). Adjustments for the foveal axis offsets are typically made after determination of the gaze vector; a default offset angle value may be used unless values from a one-time measurement of a particular user's offset angles are available.
Unfortunately, calibration is required for all existing eye gaze tracking systems to establish the parameters describing the mapping of camera image coordinates to display screen coordinates. Different calibration and gaze direction calculation methods may be categorized by the actual physical measures they require. Some eye gaze tracking systems use implicit models that map directly from pupil and glint positions in the camera's image plane to the point of regard in screen coordinates. Other systems use physically-based explicit models that take into account eyeball radius, radius of curvature of the cornea, offset angle between the optical axis and the foveal axis, head and eye position in space, and distance between the center of the eyeball and the center of the pupil as measured for a particular user. During calibration, the user may be asked to fix his or her gaze upon certain “known”points in a display. At each coordinate location, a sample of corresponding gaze vectors is computed and used to adapt the system to the specific properties of the user's eye, reducing the error in the estimate of the gaze vector to an acceptable level for subsequent operation. The user may also be asked to click a mouse button after visually f

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