Eyelink pupil size requires calibration

[alvinwmtan]

Eyelinks use an internal unit for their pupil size measurement, which is fine if we want relative units, but does not translate into absolute units (which is commonly used, especially to compare across systems). To do so, labs will need to run a calibration: printing a black circle of known absolute size, holding it at head distance away from the Eyelink, and conducting a measurement. This should be relatively brief (and labs may have already done so for other experiments), but we will need to collate the data for the pupillometry spinoff.

Source: https://www.sr-research.com/support/thread-154.html (walled behind account creation; relevant section in screenshot below)

[rhepach]

Having looked into this a bit more, and after discussions with colleagues who have used or use Eyelink-systems, it seems that there is no straightforward fix to this. There is a long and a short answer. The short answer is that studying pupil dilation changes is likely not possible with these systems if remote tracking mode was used. Even relative changes in pupil size are difficult to interpret if participants changed their position to the eye tracker from baseline to test.