Figure 3.

The simplified model of the human eye. The pupil is actually a hole in the iris, allowing light to enter the retina. The corneal surface can be modeled as a sphere partly embedded in the eye. Pc and Cc refer to centers of the pupil and corneal sphere through which the optical axis traverses. The radius of the corneal sphere is denoted with p. nc and nair refer to refractive indices of cornea and air. Fovea is the spot on the retina with the highest density of photoreceptors. The axis that (approximately) traverses the gaze point, Cc, and the fovea is called the foveal axis (to be precise, the visual axis and the optic axis intersect at the nodal point of the eye which is within 1 mm of the cornea center (Guestrin and Eizenman, 6)). δ is the gaze distance along the foveal axis between Cc and gaze point.