Abstract

Although a number of devices are currently in use for monitoring eye position, none is both accurate and convenient to use. Methods based on the use of contact lenses can provide high accuracy but have obvious inconveniences. Other techniques—e.g., skin-mounted electrodes, or eyeglass-mounted photoelectric pickups—are relatively convenient, but eye position can be measured to an accuracy of no better than about 0.5° to 1°. A novel eye-tracking instrument has been developed that makes use of two Purkinje images. The instrument operates in the infrared, so that it does not interfere with normal vision; it requires no attachments to the eye; it has a sensitivity and accuracy of about 1 min of arc, and operates over a two-dimensional visual field of 10° to 20° in diameter. The basic principle of the instrument is described, and operating records are shown.

© 1973 Optical Society of America

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References

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  1. The Eye, edited by H. Davson (Academic, New York, 1962), Vol. 4, Table II, p. 111.
  2. Purkinje reflections are referenced to the optical axis of the eye. This is different from the visual axis of the eye, which centers on the fovea. For simplicity here, however, we will simply refer to the eye axis, meaning the optical axis when referring to the Purkinje reflections and the visual axis when referring to where the eye is looking. The angular separation between the optical and the visual axes varies from subject to subject, and must be accounted for in aligning and calibrating the instrument.
  3. D. A. Robinson, J. Physiol. (Lond.) 174, 245 (1964).
  4. M. H. Katcher, thesis, University of California, Berkeley (1972).
  5. H. B. Barlow, Q. J. Exp. Psychol. 15, 36 (1963).
    [CrossRef]
  6. G. S. Brindley, J. Physiol. (Lond.) 147, 194 (1959).
  7. L. A. Riggs and A. M. L. Schick, Vision Res. 8, 159 (1968).
    [CrossRef] [PubMed]

1968 (1)

L. A. Riggs and A. M. L. Schick, Vision Res. 8, 159 (1968).
[CrossRef] [PubMed]

1964 (1)

D. A. Robinson, J. Physiol. (Lond.) 174, 245 (1964).

1963 (1)

H. B. Barlow, Q. J. Exp. Psychol. 15, 36 (1963).
[CrossRef]

1959 (1)

G. S. Brindley, J. Physiol. (Lond.) 147, 194 (1959).

Barlow, H. B.

H. B. Barlow, Q. J. Exp. Psychol. 15, 36 (1963).
[CrossRef]

Brindley, G. S.

G. S. Brindley, J. Physiol. (Lond.) 147, 194 (1959).

Katcher, M. H.

M. H. Katcher, thesis, University of California, Berkeley (1972).

Riggs, L. A.

L. A. Riggs and A. M. L. Schick, Vision Res. 8, 159 (1968).
[CrossRef] [PubMed]

Robinson, D. A.

D. A. Robinson, J. Physiol. (Lond.) 174, 245 (1964).

Schick, A. M. L.

L. A. Riggs and A. M. L. Schick, Vision Res. 8, 159 (1968).
[CrossRef] [PubMed]

J. Physiol. (Lond.) (2)

D. A. Robinson, J. Physiol. (Lond.) 174, 245 (1964).

G. S. Brindley, J. Physiol. (Lond.) 147, 194 (1959).

Q. J. Exp. Psychol. (1)

H. B. Barlow, Q. J. Exp. Psychol. 15, 36 (1963).
[CrossRef]

Vision Res. (1)

L. A. Riggs and A. M. L. Schick, Vision Res. 8, 159 (1968).
[CrossRef] [PubMed]

Other (3)

M. H. Katcher, thesis, University of California, Berkeley (1972).

The Eye, edited by H. Davson (Academic, New York, 1962), Vol. 4, Table II, p. 111.

Purkinje reflections are referenced to the optical axis of the eye. This is different from the visual axis of the eye, which centers on the fovea. For simplicity here, however, we will simply refer to the eye axis, meaning the optical axis when referring to the Purkinje reflections and the visual axis when referring to where the eye is looking. The angular separation between the optical and the visual axes varies from subject to subject, and must be accounted for in aligning and calibrating the instrument.

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Figures (10)

Fig. 1
Fig. 1

Schematic diagram of the eye: PR, Purkinje reflections; IL, incoming light; A, aqueous; C, cornea; S, sclera; V, vitreous; I, iris; L, lens; CR, center of rotation; EA, eye axis; a ≈ 6 mm, b ≈ 12.5 mm, c ≈ 13.5 mm, d ≈ 24 mm, r 7.8 mm.

Fig. 2
Fig. 2

Location of the first and fourth Purkinje images for (a) collimated light on the eye axis and (b) collimated light at angle Δ from optic axis of the eye: EA, eye axis; FT, first Purkinje image; FH, fourth Purkinje image; L, lens; C, cornea. The dark section of arc is the equivalent mirror for the fourth Purkinje reflection.

Fig. 3
Fig. 3

Schematic of the eye-tracker optical system: VT, visual target; R, allowed range of eye movements; IA, input axis; CA, collecting axis; CA, extension of collecting axis; S, light source; S1, artificial pupil imaged at pupil of eye; CW, chopper wheel; S2, source of Purkinje pattern, imaged at infinity; DC, dichroic mirror; M, front surface mirror; Mx and My motors that drive M in x and y direction, respectively; BS, beam splitter; P1 and P4, quadrant photocells; A4, aperture in front of P4. Focal lengths of lenses L2, L3, and L4 are 60, 150, and 90 mm.

Fig. 4
Fig. 4

Photocell connections: Mx and My, signals that drive the two-dimensional mirror in x and y, Px and Py, motors that drive P4 in x and y directions.

Fig. 5
Fig. 5

Feedback circuit around each motor: D.S., drive signal; M.D., motor driver; M, motor; MM, mechanical motion; M.S., motion sensor, Hewlett–Packard DCDT displacement transducer; O, output signal.

Fig. 6
Fig. 6

Upper, horizontal eye movements recorded from first-Purkinje-image tracker; lower, horizontal eye movements recorded from fourth-Purkinje-image tracker. The upper trace shows the wandering baseline, typical of corneal image trackers. Both traces record two 3° horizontal eye movements. During the interval T–T, the subject’s head was translated 0.5 mm horizontally, which shows in the upper trace but not the lower. Arrows at the bottom indicate eye blinks.

Fig. 7
Fig. 7

An x, y recording of eye movements (movements of P4) over an 11 × 11 array of illuminated points. Adjacent points were separated by 1.8°. The ragged traces on the horizontal axis, left of center, are due to eye blinks.

Fig. 8
Fig. 8

(a) Fixation record; (b) repetitive traverse of the corners of a 1 2 ° square; (c) diagonal saccadic movement with 3° horizontal and vertical components. Upper, horizontal movements; lower, vertical movements.

Fig. 9
Fig. 9

Location of the third Purkinje image with respect to the first and fourth images: IA, input axis; EA, eye axis; CA, collecting axis.

Fig. 10
Fig. 10

Auxiliary optical system used in stabilization experiments.

Equations (1)

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S 7 sin Δ .