Eye angles are usually defined by specific branches. The pupillary axis is an axis of useful reference, which corresponds to a line perpendicular to the surface of the cornea and goes through the center of the pupil.
To identify the pupillary axis, guide the eye in a way that the corneal reflection of a source of light)image from Purnkinje) appear confused with the center of the entrance pupil (Pe). Indeed, the reflection is then aligned with the local radius of curvature of the corneal surface, which is by definition perpendicular to the tangent to the corneal surface to the point under consideration, confused with the center of the pupil.
The pupillary axis to set certain angles, whose most important clinical is the Kappa angle. This axis is used for reference in a centered optical system (ex: standard camera lens), refractive lenses are aligned on a common centerline. In fact, this axis is confused with the axis which connects him between the diaphragm (pupil) and the top of the front lens, while being perpendicular to the surface of this lens at this point. The eye is considered an optical system with two main lenses (cornea/lens) and one diaphragm separating them (IRIS pupil) not aligned on a common axis, the line joining the center of the pupil with a point located on the front of the cornea and such as pupillary axis passing through is locally perpendicular to the top of the cornea is used as the axis of reference.