ABSTRACT
Gradient-index materials are ones in which the index of refraction varies spatially within an optical medium. The variation in the index of refraction can be in the direction of the optical axis (axial gradients), perpendicular to a line (radial gradient), or symmetric about a point (spherical gradient). Light propagating through a radial gradient material with a nearly parabolic profile focuses periodically in the material and can exhibit no axial aberration. Further, because the profile is wavelength dependent, it is possible to balance the chromatic aberrations of the gradient against those of the homogeneous components to eliminate or reduce the total aberration. Gradient-index lens materials made by the ion exchange process are available commercially in glass and have been fabricated in polymers, zinc selenide/zinc sulfide, and germanium. If a gradient is introduced such that the index of refraction decreases in the optical axis direction, the index of refraction at edge of the lens is slightly lower than at the optical axis.
