Accurate Gaze Direction Measurements With Free Head Movement for Strabismus Angle Estimation. - Nicole M. Bakker; Boris Lenseigne; Sander Schutte; Elsbeth B. M. Geukers; Pieter P. Jonker; Frans C. T. van der Helm; Huib J. Simonsz (IEEE Trans. Biomed. Eng., 2013)