I've long noticed how many kits will include just a clear-molded lens to add to optics, such as the targeting pod of an apache, binocular lenses on a figure, rifle scopes, or any other number of things. The clear lenses serve their purpose, but sometimes can leave more to be desired, especially if it's a smaller scale and gets warped or discolored from cement.
This is a trick I learned while living in Japan, and seeing some of the finished out Gundam models. I noticed on many of the kits, the gundam's eyes and weapon optics looked eerily realistic from certain angles...and not just the standard painted-on or kit stickers, nor were they clear-molded lenses. After asking around, I learned what the big secret was.
Cell phone privacy film
You know, the stuff that statically adheres to cell phone screens, so when viewed from an angle other than straight on, the screen appears blank or in a wash of mirrored rainbow colors, like oil on water. There are endless types out here, but the type I found gives the specific appearance of realistic optical glass is the rainbow-mirrored finish, in which green or orange seems the predominant color. You can find it here for about 100 yen (around $1.00) per sheet, and one sheet should last for several projects. It's easy to cut, trim and shape with a hobby knife. Most of it has a protective film on at least one side, so it will be protected from nicks and fingerprints until its in-place and ready to be uncovered.
My first use of it was the "special finish" 1/100 scale Zeta Gundam kit from Bandai, which is "pre-painted" in silver flake. I used the sticker eyes included in the kit as a guide to cut the film to size and shape. The film is transparent from the front, so just lay it right on top of your template. I used standard cement to glue it, and it holds well without warping. The one thing I noticed after it was in place was the overly 'mirrored' finish on the completed model, which I solved with a very thinned flat black acrylic wash to tone it down. Now the results are perfect!
I think to take it a step further and add depth to more "practical" purposes, like modeling optics in military hardware, one could lay a piece of this film on the backside of a clear-molded lens, then insert it as an assembly.