Absolute instruments and perfect imaging in geometrical optics
  We investigate imaging by spherically symmetric absolute instruments that provide perfect imaging in the sense of geometrical optics. We derive a number of properties of such devices, present a general method for designing them and use this method to propose several new absolute instruments, in particular a lens providing a stigmatic image of an optically homogeneous region and having a moderate refractive index range. 
Tomas Tyc, Lenka Herzanova, Martin Sarbort and Klaus Bering
physics.optics