GRIN Medium
The GRIN medium has a quadratic transverse variation of refractive index, with either a maximum or minimum on the axis that produces a lensing effect. The element has similar matrices for both work planes, but gradient constants n2 can be set independently for tangential and sagittal planes.
Unlike GRIN Lens, this element doesn’t imply any interfaces. To get proper results, you should put appropriate interface elements at both sides of the medium.
When the gradient constant n2 is set to zero, the element becomes simple Space Filled With Medium.
See also
GRIN Lens, Elements Matrices, ../catalog, ../elem_props, Gradient-index optics in Wikipedia