We propose a generalization of the two-dimensional eikonal-limit cloak
derived from a conformal transformation to three dimensions. The proposed cloak
is a spherical shell composed of only isotropic media; it operates in the
transmission mode and requires no mirror or ground plane. Unlike the well-known
omnidirectional spherical cloaks, it may reduce visibility of an arbitrary
object only for a very limited range of observation angles. In the
short-wavelength limit, this cloaking structure restores not only the
trajectories of incident rays, but also their phase, which is a necessary
ingredient to complete invisibility. Both scalar-wave (acoustic) and transverse
vector-wave (electromagnetic) versions are presented.Comment: 17 pages, 12 figure
