  Head-mounted eye trackers promise convenient access to reliable gaze data in
unconstrained environments. Due to several limitations, however, often they can
only partially deliver on this promise.
  Among those are the following: (i) the necessity of performing a device setup
and calibration prior to every use of the eye tracker, (ii) a lack of
robustness of gaze-estimation results against perturbations, such as outdoor
lighting conditions and unavoidable slippage of the eye tracker on the head of
the subject, and (iii) behavioral distortion resulting from social awkwardness,
due to the unnatural appearance of current head-mounted eye trackers.
  Recently, Pupil Labs released Pupil Invisible glasses, a head-mounted eye
tracker engineered to tackle these limitations. Here, we present an extensive
evaluation of its gaze-estimation capabilities. To this end, we designed a
data-collection protocol and evaluation scheme geared towards providing a
faithful portrayal of the real-world usage of Pupil Invisible glasses.
  In particular, we develop a geometric framework for gauging gaze-estimation
accuracy that goes beyond reporting mean angular accuracy. We demonstrate that
Pupil Invisible glasses, without the need of a calibration, provide gaze
estimates which are robust to perturbations, including outdoor lighting
conditions and slippage of the headset.
