Home Forums Eye Tracking Devices About the sampling rate of tobii

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• #21451
ET
Participant

Hello !

As a MSc students, in the scholl for eye optometry and vision sceince, I have a lot of data generated by tobii eye tracker 4C.
I red in an article that the eye velocity can reach up to 400 degrees per second.
The sampling rate of the tobii is 90Hz, and the capture-light-sensor Hz (CMOS or CCD, I don’t know which one is in the tobii) is few MHz.
calculating 400 / 1000000 is 0.0004 degrees , means that for 0.0004 degrees, the pupil can be blured by the “mechanism” of the tobii.
The algorithm “can think” that, for one direction in which the eye is move for, that the pupil is larger.
I found an evidence (still, in a doubt) for that, when I look in my data, wherever the eye is move (left / right), the pupil in the same direction is larger.

Am I correct ? Is there a possible situation for falsy data by the tobii ?

TNX
Eliran

#21514
Grant [Tobii]
Keymaster

Hi @elta, thank you for your query. The issue you raise is less to do with sampling frequency but rather geometry.

When the eye is not looking straight towards the camera, the pupil appears as an ellipse, and there is refraction in the cornea as well that complicates things.
Tobii does a better job than most trackers since we do not estimate pupil directly from the image, but we reconstruct a complete 3d model with cornea surface, refraction and all. From this model, we extract the actual pupil diameter in millimeters (as opposed to the apparent pupil diameter in pixels that most other trackers do).

It is not perfect, for several reasons. Imperfections in the image is not the major problem. imperfection in the geometric model is a bigger source of errors.
In the 4C for example, some images are BP and some images are DP. This is also a source of problems. the contrast of the pupil edge is different for bright and dark pupil images. Stereo camera Pro trackers (Fusion and Spectrum) does not cycle between DP and BP like the consumer trackers partly to avoid this problem.

Let us know if you have any further queries. Best Wishes.

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