  There is a widespread assumption that the peak velocities of visually guided
saccades in the dark are up to 10~\% slower than those made in the light.
Studies that questioned the impact of the surrounding brightness conditions,
come to differing conclusions, whether they have an influence or not and if so,
in which manner. The problem is of a complex nature as the illumination
condition itself may not contribute to different measured peak velocities
solely but in combination with the estimation of the pupil size due to its
deformation during saccades or different gaze positions. Even the measurement
technique of video-based eye tracking itself could play a significant role. To
investigate this issue, we constructed a stepper motor driven artificial eye
with fixed pupil size to mimic human saccades with predetermined peak velocity
\& amplitudes under three different brightness conditions with the EyeLink
1000, one of the most common used eye trackers. The aim was to control the
pupil and brightness. With our device, an overall good accuracy and precision
of the EyeLink 1000 could be confirmed. Furthermore, we could find that there
is no artifact for pupil based eye tracking in relation to changing brightness
conditions, neither for the pupil size nor for the peak velocities. What we
found, was a systematic, small, yet significant change of the measured pupil
sizes as a function of different gaze directions.
