The eikonal distribution on a lens surface

The derivation of the formula describing the eikonal distribution on the surface of a dielectric axisymmetrical lens for an arbitrary direction of displacement of a source from the focus is presented. In this case, the second focus of the system is at infinity. The formula takes into account three terms of expansion of the eikonal in terms of the powers of the source displacement. The accuracy of the formula obtained is analyzed by comparing the results of calculations of the eikonal on the surface of an aplanatic lens obtained using this formula and the results of exact geometrical-optics calculation.