Continuous gradient index and shell models of the human lens

Because of the complexity of tracing rays through a gradient refractive index medium, the human eye's lens is sometimes approximated by a shell structure with constant refractive index within shells. In the shell model, power arises from a combination of an axial variation in index and the curvatures of the shell surfaces. We develop an equation which gives the power due to the gradient index of the lens, and use this to choose shell models that have the same power as the continuous gradient index model. Some types of shell models are described and evaluated.