Eyetracker Calibration Project

 

Youngmin Kim, Wayne Wang, and Francois Guimbretiere

 

 

We have improved the calibration of the eyetracker for our future perception studies in graphics. This project was done upon the preliminary calibration setup by Wayne Wang (CS undergraduate in UMD) under Dr. Francois Guimbretiere in human computer interaction lab.

 

We used the ISCAN ETL-500 eye-tracker which can record eye movements continuously at 60 Hz. The experiment was carried out on a 17-inch LCD display with a resolution of 1280x1024, placed at a distance of 24 inches, subtending a visual angle of approximately 31.4 degrees horizontally. The subjects had a chin rest to minimize head movements and to maintain calibration.

 

The standard calibration of ETL-500 eye-tracker was not sufficiently accurate for our purposes due to non-linearities in the eye-tracker-calibrated screen space. Therefore we have used a two-step calibration process in which the first step is the standard calibration with 5 points on the screen and the second step involves a more densely-sampled calibration phase with 13 additional points. Our calibration included asking the subjects to successively look at and click on 13 points presented sequentially on the screen. This gave us an accurate correspondence between the eye-tracker space and the monitor space for that subject. We then triangulated the monitor's screen space using these 13 points and 4 corner points from the first phase calibration. Such a triangulation allowed us to accurately get a position on the monitor by interpolating inside the triangle where the subject was looking. After this we tested the accuracy of the calibration by asking the subjects to look at 16 randomly selected points on the screen. Most of subjects were able to successfully calibrate to within an accuracy of 30 pixels (about .75 degree) for each of the 16 points.