Setting Pupil Thresholds - SR Research EyeLink Portable Duo User Manual

exclude regions of the camera image (e.g., frame of the glasses, eyebrow) that
may otherwise be detected as a pupil/CR reflection pattern. If the search limit
area isn't placed on the center of the pupil, press "A" or the "Align Eye Window"
button to center it. The size and shape of the search limit area can be adjusted
by first having the zoomed eye image selected and then pressing ALT and cursor
keys on the Host keyboard together (ALT + ⇑ or ⇓ to adjust the height; ALT + ⇐
and ⇒ to adjust the width). The position of the search limits can be adjusted
with SHIFT and cursor keys or by clicking in the global view to move them to
the click location.
The operation of the Remote Mode is influenced by ambient lighting. In general,
the pupil shrinks under bright light and dilates in a dark environment. It's
important that users check the pupil size reported in the thumbnail camera
image periodically throughout recording. If a yellow size warning is constantly
observed, it is likely that the pupil size is too small and as a result, the recorded
data may be noisy. If this happens, first check whether the participant is seated
at the recommended eye-target distance of 520 mm. Dimmer room lighting will
also help alleviate this issue.
Now proceed to section 3.7 "Calibration". For the best recording accuracy in the
Remote Mode, please use a 13-point calibration type.

3.3 Setting Pupil Thresholds

The camera image of the eye(s) should now be clear, with the pupil centered
when the participant looks at the eye image on the Display PC. The pupil
threshold may now be automatically set by pressing the 'Auto' button or the 'A'
key when the camera image is selected. The pupil of the eye should be solid
blue, with no other color in the image when the threshold is properly set. If
large areas other than the pupil are colored, the participant may have blinked
during the 'Auto Threshold' procedure: press A to do Auto Threshold again.
If the participant wears eyeglasses, reflections may block the image of the pupil.
You may need to make slight adjustments to the eye tracking unit (e.g., moving
the camera closer to the participant, lowering the position of the camera,
and/or adjusting the angle of the camera) so that reflections from the glasses
will not interfere with pupil acquisition (the idea is to make the plane of the
glasses less perpendicular to the line of sight of the eye tracker; however please
be advised that not every participant with glasses will be trackable). If the
eyeglasses have an anti-reflective coating, image contrast may be poor and
pupil tracking may be noisy.
The pupil threshold should be checked by closely examining the blue area in
the zoomed camera image. Figure 3-7 shows the symptoms to look for. If the
52 An EyeLink Portable Duo Tutorial: Running an Experiment
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