My Tobii Pro X3-120 shows pupil size every third sample.
The Tobii Pro X3-120 runs a fixed sequence of illumination:
Dark Pupil - Dark Pupil - Bright Pupil
The pupil size is slightly different from dark and bright pupil, and more respondents will give better results in bright pupil mode.
To avoid non-significant variations in the pupil size between the dark and bright pupil illumination modes, the Pro X3-120 output gives the pupil size only for the bright pupil sample (i.e. every third sample).