relay grin lens VS cylindrical glass

[haoran]

This optic-principle related question is making me thinking now, and I hope you could help me out.

The relay grin lens should be 0.5 or 1 pitch and to bring all the light from bottom to the top of it. I wonder why not just leave the space empty or use some cylindrical glass (with homogeneous index) for that function. I imagine it would be like to see from air or through a piece of glass from the top.

Hope it is not too naive. Thanks!

[Daniel Aharoni]

Hi Haoran,
Great question. Firstly, a relay GRIN lens generally will be slightly shorter than 0.5pitch or 1pitch. This is due to wanting the intermediate image to be formed just beyond the top surface for the relay GRIN lens.

One could imagine using a plane glass cylinder instead of a relay GRIN lens but this effectively reduces the NA of the system. In other words, GRIN lenses can transfer light across a long distance without limiting the angle of accepted light.