1. A method comprising: obtaining a first image of an eye in a bright-pupil imaging mode in which said image comprises a retinal retro-reflection; obtaining a second and differing image of the eye in a dark-pupil imaging mode in which said image comprises a corneo-scleral retro-reflection; calculating at least one image quality metric for the image of the eye in the bright-pupil imaging mode; calculating at least one image quality metric for the image of the eye in the dark-pupil imaging mode; comparing the calculated image quality metric of the first image in the bright-pupil imaging mode with the calculated image quality metric of the second image in the dark-pupil imaging mode; selecting, based on comparison of the calculated image quality metrics, one of the bright-pupil imaging mode and the dark-pupil imaging mode; and based on the selected bright-pupil imaging mode or dark-pupil imaging mode, perform tracking of the eye.