They are actually called periscopes. They are solid blocks of laminated glass or lexan plastic. The most modern, such as those on modern tanks and armored vehicles like the M2A2 Bradley or M1A2 tank also have a anti-laser coating that gives them a reddish or irridescent look.
To replicate older, non anti-laser coated vision blocks, I simply paint them black and overcoat with a couple of coats of gloss. For newer vision blocks with anti-laser coating, I paint black then overcoat with a thinned coat of clear red (stop light red). I thin it enough that you can still see the black through it. Then I again overcoat with a couple coats of gloss. This gives the impression of the anti-laser coating since you can see the black through the red. Also, at different angles, the red appears irridesent , or black. Just like the real thing. I have also heard of people using mylar from a balloon that is close to the red anti-laser coating as well. I have never tried this as I think my method looks good enough for me.
Hope that helps.
Gino P. Quintiliani - Field Artillery - The KING of BATTLE!!!
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