1. Effectiveness of Accuracy of Eye Trackers in Laser Refractive Surgery
American Society of Cataract and Refractive Surgeons
4-9 April, 2008
Effectiveness of Accuracy of Eye Trackers in Laser Refractive Surgery
Dr Paul van Saarloos, PhD
Dr. Mukesh Jain, PhD
Dr Tarak Pujara, MS 1

2. CustomVis introduces a new generation of Eye Tracking
Dual Limbus based tracking with 1000Hz closed loop response
Gaze tracking.
Cyclorotation by automatic iris recognition.
Elevation independent scanning
(Z data tracked).
6 degrees of eye motion tracked!

3. Iris Recognition Cyclorotation automatic correction

4. Purpose
Pupil based eye trackers need to image through the dry treated corneal surface.
In addition to that, plume and laser flashes could affect the accuracy of such eye tracker.
The Pulzar Z1, Solid State Laser uses a limbal based eye tracker.
This study is designed to determine if a limbal based eye tracker provides an improved accuracy in tracking the eye.

5. Eye Tracking  How do we measure Performance?5

6. Speed and Accuracy 6

7. Which is Easier to See? Pupil or Limbus?7

8. Method Analyze video of eye during surgery.
Manually measure the center of Limbus in each frame.
3. Compare to eye tracker output for same time period and average difference.
4. Compare to published measurements of accuracy for pupil eye trackers 8

9. Results 38 microns 450 consecutive frames were measured.
The average difference between the Eye Tracker output and Manual measurement was:
38 microns 9

10. Results continued..
Pupil based eye tracker had an accuracy of mm for an intact cornea and 0.1mm for a cornea with a flap removed. Accuracy is expected to reduce further once the laser starts. (Results taken from previous studies)
While limbal based eye tracker had an
accuracy of 0.04mm during surgery.

11. Pupil Tracking Problems:
LASIK Bed Irregular (Eye Tracker can not see pupil well).
Excimer laser dries corneal surface making it even less optical.
Ablation plume and laser flashes also affects accuracy.
Pupil center moves up to 0.7mm as it changes size. 11

12. Published Data Very little data exists on eye tracker Accuracy
Talyor et al1 using a special pupil eye tracker to enhance accuracy, measured errors of 100 microns after a flap was cut (laser was not fired)
Gobbi et al2 also reported 100 micron accuracy for a pupil based eye tracker, but only on artificial eyes.
“Determining the accuracy of an eye tracking system for laser refractive surgery”, N Taylor et al, J Refractive Surg. 2000, 16(5):S643-6
“Automatic eye tracker for excimer laser photorefractive keratectomy” P Gobbi et al, J Refractive Surg (3):S337-42 12

13. To put accuracy measurements in context:
Bueeler and Mrochen* measured an average induced error of 100 microns when the eye tracker response was reduced from instantaneous to 1 Hz.
(Based on measured eye movements during surgery and computer simulations)
Pupil based Eye Tracking creates as much error as a very slow eye tracker!
* “Simulation of Eye-tracker Latency, Spot Size, and Ablation Pulse Depth on the Correction of Higher Order Wavefront Aberration With Scanning Spot Laser Systems”, Bueeler and Mrochen, J Refractive Surg. 2005; 21:28-36 13

14. Summary
Limbal based Eye Tracking is about twice as accurate as Pupil based Eye Tracking 14

15. Conclusion
Limbal Based Eye Tracking has the potential to be significantly more accurate than Pupil Based Eye Tracking during LASIK.
Limbal Based Eye Tracking allows more accurate registration of Custom Treatments.
Accuracy is as important as speed for eye tracking. 15