Pupil Foreshortening Error

[Michael Wimmer]

Hi Tobii!

I am using a HP Omnicept G2 Reverb and record pupil size data during a video game. Is the algorithm for pupil size calculation considering gaze angle-dependencies in the pupil size? If no or not completely, is there a gaze range in x and y direcion you can recommend for a save use of the metric?

[Grant [Tobii]]

Hi @miwi, we cover in some detail the image-based algorithm used to determine pupil size which is independent of gaze angle @

https://www.tobiipro.com/learn-and-support/learn/eye-tracking-essentials/is-pupil-size-calculations-possible-with-tobii-eye-trackers/

So there is no gaze range as such, but as the document indicates, it is better use % of pupil size difference rather than any absolute value.

[Michael Wimmer]

Hi, thank you for your answer. Let me specify my question a little bit:

The data I collected suggest that at the moment when a user changes his gaze direction, that also the pupil size changes. Could it be that the measured pupil size changes just because of people looking around?

[Grant [Tobii]]

Hi @miwi, yes pupil size can vary significantly between dark and light but also there is constant subtle variation when scanning an environment. However, if the pupil size is changing much more than expected, it may be worth looking at environmental conditions during recording such as ambient lighting, spotlights, etc.

[Michael Wimmer]

Hi,

Thank you! However, I could see the dependency of the pupil size on the view angle (especially in X direction) even in completely controlled, constant conditions (only looking to the left/right with a black background). Recorded variations of the pupil size due to these saccades are around +/-0.2mm. Are you aware of such behavior?

[Grant [Tobii]]

Hi, After consulting internally, I’m afraid we do not have any particular metrics to demonstrate the relationship between gaze angle and reported pupil size. It may be worth posting on HP own forum in case another user there has performed a similar analysis.

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