Effectiveness of Limbal Eye Tracking over Pupil Eye ... - CustomVis

Effectiveness of
Limbal Eye Tracking
over
Pupil Eye Tracking
• Paul P van Saarloos
• Tarak Pujara

Proprietary
Interest
Paul van Saarloos holds Patents on both Pupil and
Limbal based eye tracking
He is also a shareholder and employee of
CustomVis plc (Laser uses Limbal based eye
tracking)

Eye Tracking
How do we measure
Performance

Speed
and
Accuracy

Which is Easier to See
Pupil or Limbus

Method
1. Analyze video of eye during surgery.
2. 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

Results
450 consecutive frames were measured.
The average difference between the
Eye Tracker output and Manual
measurement was:
38 – 63 microns
Range due to image pixel size

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

Published Data
Very little data exists on eye tracker Accuracy
Talyor et al 1 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 al 2 also reported 100 micron accuracy for
a pupil based eye tracker, but only on artificial
eyes.
1. “Determining the accuracy of an eye tracking system for laser refractive
surgery”, N Taylor et al, J Refractive Surg. 2000, 16(5):S643-6
2. “Automatic eye tracker for excimer laser photorefractive keratectomy”
P Gobbi et al, J Refractive Surg. 1995 11(3):S337-42

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

Summary
• Limbal based Eye Tracking is
about twice as accurate as
Pupil based Eye Tracking

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.