@plesa,
First I should make it clear that I am not an optical expert
I have always worked with expensive Germanium optics and was told that they were the best for Thermal Camera lenses. I am aware that some industrial and military thermal cameras use a combination of Germanium and ZnSe lens elements.
Some years ago I discovered ZnSe as a lens that works at thermal camera wavelengths. When I discovered that document presenters use a No.2 bi-convex singlet close-up lens to achieve close focus for a standard Sony camera module, I had the idea of using a ZnSe lens in the same way for
my thermal cameras. ZnSe has served me very well ever since. It has the advantage of being relatively cheap and available thanks to their use in CO2 cutting lasers. The Focal Lengths available were also perfect for my PCB inspection needs. ZnSe is pretty soft though and needs to be treated with care to avoid scratching. I have had a hard job finding any Germanium lenses with which to experiment so tried GaAs and that worked OK. GaAs has higher transmission losses though.
I am ALWAYS on the look-out for Germanium lenses and actively hunt e*ay. It is interesting that most auxiliary lenses seem to be based on Germanium so I have assumed that Germanium is still considered the lens material of choice. As I say, I am no expert in optics or optical material for thermal cameras. The little Germanium lenses that Junktronix has sent to me will be good to experiment with and compare to ZnSe. I have also purchased some Plano concave ZnSe lenses with which to experiment as they would be needed for building a non-image inverting telephoto lens. Finding a nice large, long focal length Germanium or ZnSe Plano convex lens for such a telephoto lens has proved challenging however.
I welcome anyone's insight into the complex world of optics and building auxiliary lenses for our cameras. I tend to build by experiment as I have to work with the specification of lenses that I can lay my hands on at a sensible price. It would be great to offer this forum a cheap way of achieving at least X2 optical magnification for the E4. The lenses will always be the challenge. I have just bought a nice mini lathe so can form the barrel with relative ease.
Playing with optics can be both challenging and fun