Abstract
The pupil aperture of the iris is subject to refraction by the cornea, and thus an outside observer sees a virtual image: the “entrance pupil” of the eye. When viewed off-axis, the entrance pupil has an elliptical form. The precise appearance of the entrance pupil is the consequence of the anatomical and optical properties of the eye, and the relative positions of the eye and the observer. This paper presents a ray-traced, exact model eye that provides the parameters of the entrance pupil ellipse for an observer at an arbitrary location and for an eye that has undergone biologically accurate rotation. The model is able to reproduce empirical measurements of the shape of the entrance pupil with good accuracy. I demonstrate that accurate specification of the entrance pupil of a stationary eye requires modeling of: corneal refraction, the misalignment of the visual and optical axes, the thickness of the iris, and the non-circularity of the pupil aperture. The model specifies the location of the fovea in the posterior chamber, and this location varies for eyes modeled with different spherical refractive error. The resulting dependency of the visual axis upon ametropia agrees with prior empirical measurements. The model, including a three-dimensional ray-tracing function through ellipsoidal surfaces, is implemented in open-source MATLAB code.