User manual

Appendix A: Using the EyeLink 1000 Analog and Digital Output Card

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• For gaze position data, the data range is scaled to the display coordinates,

which are 640 by 480 at startup, and may be changed via link commands.

The data range setting is -0.2 to 1.2, allowing 20% extra range for fixations

that map to outside the display. This extra data range allows for slippage or

for identification of fixations outside the display. Scaling to recover gaze

position data is more complex, as the numerical value is partially dependent

on the display coordinates. The following formulas do the conversion in

several stages, with R being the voltage range proportion, and S being the

proportion of screen width or height.

R = (voltage-minvoltage)/(maxvoltage-minvoltage)

S = R*(maxrange-minrange)+minrange

Xgaze = S*(screenright-screenleft+1)+screenleft

Ygaze = S*(screenbottom-screentop+1)+screentop

7.5 Pupil Size Data

For pupil size, either pupil area or pupil diameter may be monitored. These are

very high-resolution measurements, with resolution as small as 5 microns

(0.005 mm). Pupil size measurements are affected by eye position, due to the

optics of the eye and camera, and should be considered to be measured in

arbitrary units, with a pupil size of zero being represented by the lowest analog

voltage.

7.6 Timebase and Data Strobe

The EyeLink 1000 eye tracker samples eye position every 0.5, 1, 2 or 4 ms and

outputs analog data at 2000, 1000, 500, or 250 hz (depending on your tracker

setting and system licensing). This combination of fast sampling rate and non-

continuous output differs from most eye-tracking systems with analog outputs,

which either output continuous analog data (such as limbus-tracking systems)

or output samples at a lower rate, such as 50/60 Hz video-based tracking

systems. This causes the EyeLink analog output to rapidly step between data

values, which means that sampling at fixed intervals makes it likely that

samples might be missed, sampled twice, or the transition between samples

might be recorded instead. Since the EyeLink 1000 tracker and most data-

acquisition systems rely on interrupt-driven software sampling and output, it is

possible that time base jitter could result in missed samples, or repeated