Stefan Kohlbecher Stanislavs Bardins Klaus Bartl Erich Schneider Tony Poitschke Markus Ablaßmeier Calibration-free eye tracking by reconstruction of the pupil ellipse in 3D space. 135-138 2008 ETRA http://doi.acm.org/10.1145/1344471.1344506 conf/etra/2008 db/conf/etra/etra2008.html#KohlbecherBBSPA08