An interpretation of Choudhary and Felsen´s perturbation method for tracing the phase paths of evanescent fields

A method for constructing the complex eikonal of an evanescent optical field, given its values on some surface, is described. It proceeds by successive approximations whose steps consist of tracing real rays in appropriate fictitious media. The first step, valid for weakly evanescent fields, is essentially a method previously described by Choudhary and Felsen.