It is a common requirement to want to mark the onset of visual stimuli. This can be done by sending event markers via USB from the stimulus presentation computer, but you can also have StimTracker detect and mark the onset of visual stimuli directly. This requires some work but offers two advantages: one, it is more accurate because it avoids delays that are inherent to USB signals, and two, it is software-independent.
The vast majority of experiments use a white or a black background, so StimTracker ships with two light sensors, one in black and the other in white. This is intended to minimize any distractions to the participant.
Placing the light sensor right above the stimulus would likely distract the participant. When preparing the trial, it is recommended that you also display a square or a circle near the edge of the screen. The circle needs to be only about 4 or 5 millimeters in diameter.
Once the trial is setup correctly and you are satisfied with the location of the marker circle on the monitor, simply peel the self-sticking tape off the light sensor and attach it to the monitor. It is recommended that the light sensor cover completely the circle in order to avoid distracting the participant.
See Using StimTracker for information on setting the threshold. StimTracker can detect up to 4 light sensors and you can set the thresholds independently for each sensor.