Any suggestions on a good, that is simple and effective, way to replicate the external optical surfaces?

Ta Dave

Dave,
Have you tried Fine Molds out of Japan? Their parascope and episcope sets are great replicas made out of clear styrean. All you have to do is mask the sight off and paint them. I have no links at the moment but I do know that they make WW2,US,German and Japanese perascopes for most AFV.
Pete

Depending on the colour I want to get, I first paint with azure, silver, medium to dark green or dark grey. Once dried I give a coat of Tamiya Smoke (X-19) and I'm quite happy with the results.

For which types of armor? Modern with laser reflective coating? They tend to have a reflective appearance, much like some mirror sunglasses. There are some types of mylar that are in that color. Cut to fit and glue in place.

For WW2 AFVs, I have several of the Fine Molds sets Pete mentions. I've got a set for a Tiger, Pz III/IV and a US set for Shermans, Stuarts and Hellcats. These types are just a set of replacement parts that are molded in clear plastic. When you paint them, the glass surface remains unpainted giving it a clear see-through look.

I juse dark blue and than a coat of microscale micro crystal klear.
For larger surfaces (like on the M-109 ) i use speciaal plastic sheet minus 1mm clear sheet, i cut it in shape and then I paint the inside ( not the sheet) in black or dark blue.
on the observasion sight of the M-109 i put a clear half pill ( those pills (cilinders) that contain the medicine and break apart inside your body ).
i't gives an effect of a lens reflection.

80a2