Non-visible-light/IR focusing lenses-Si, Ge, ZiSe?

From: stevelu (sp_lussier_at_yahoo-dot-com.no-spam.invalid)
Date: 01/14/05 

Date: 14 Jan 2005 14:31:17 -0600

Hello optics fans --

I picked up a large, surplus optical assembly a while back and am
finally getting serious about learning more about it. I'd especially
like help figuring out what the exotic lenses may be composed of.

Clearly a lab piece, with a heavy golden-toned tube, probably
aluminum. Looks aged but not ancient. It contains several lenses,
up to 5" in diameter, which appear to be mirrored but are actually
composed of a shiny, blackish substance opaque to visible light.
They are mostly meniscus, with faint purplish, reddish or bluish
sheens - which I presume are antireflection coatings.

Plate on the tube reads OSTI, which turns out to be a small division
of Telic optics (now part of Axsys) - a group that specializes in IR
optics. Faintly discernable on the tube is the name of a major
military contractor, so I assume we're talking defense rather than,
say, astronomical IR optics. No indication of focal length or such
stats that I can read, but some cryptic letters and numbers.

There is also a large flat 'filter' which is transparent and vivid
yellow in color. This was in the center of the assembly, between the
dark/mirror lenses.

>From digging around on the web I am speculating that the yellow
'filter' may be ZiSe. If so I'm curious about its properties when
used as an apparent filter. I gather that it is often employed as a
focuser of IR light. (I do realize that, if I'm correct, it is a
fairly toxic -- though fortunately also rather inert -- substance.)

And the dark lenses - silicon or germanium perhaps? How could I
tell? I know that they are not just mirror-coated glass because a
small chip is broken off one; the broken surface is rippled like a
broken glass surface but opaquely shiny like a metal.

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