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and transformed pupil-CR data might exceed this range, but in general this
range will be similar to that of the camera image. Raw data should be
assumed to be in arbitrary units.
For HREF data, the entire data range is assumed to be -30000 to +30000. This
is about 127°. This should never be exceeded. The default range setting is
therefore 0.0 to 1.0. The HREF data may be recovered from the voltage by the
following formula:
HREF = (voltage-(minvoltage+maxvoltage/2)*60000/(maxvoltage-minvoltage)
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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
8.4 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 cameras, and should be considered to be measured in
arbitrary units, with a pupil size of zero being represented by the lowest analog
voltage.
8.5 Time base and Data Strobe
The EyeLink II eye tracker samples eye position every 2 or 4 msec and outputs
analog data at 250 or 500 Hz. This combination of fast sampling rate and non-
EyeLink II User Manual version 2.12 (07/02/2006) © 2002-2006 SR Research Ltd. 100