The behavior of orthotomic systems of rays, rays associated with a system of wavefronts, is analyzed from the point of view of classical geometrical optics. The rays themselves are described in terms of the ray equation derived from Fermat’s principle. A condition for an aggregate of rays to comprise an orthotomic system is found. Some consequences of this condition on the geometric properties of wave-fronts are found. The resemblance of some of these to the Maxwell equations is noted.
O. N. Stavroudis, "Orthotomic Systems of Rays in Inhomogeneous Isotropic Media," Appl. Opt. 10, 260-263 (1971)