Three-dimensional nonparaxial characterization of physical point sources

Optical point sources are considered theoretical idealizations. However, it is pertinent to characterize feasible physical point sources for the current micro- and nano-optics. It is shown that certain three-dimensional (3D) distributions of real and virtual point emitters provide a nonparaxial energy cone with the same geometry of the cone provided by the single ideal real point emitter placed at the midpoint of the 3D distribution. These cones fit along arbitrary distances from the 3D distribution up to very short ones (a fraction of the wavelength). A generalized criterion is proposed to characterize the physical point source as a feasible 3D object. Some implications are discussed.