Re: Non-visible-light/IR focusing lenses-Si, Ge, ZiSe?
From: Harvey Rutt (xh.ruttx_at_x.ecs.soton.ac.uk)
Date: 01/28/05
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Date: Fri, 28 Jan 2005 10:31:12 -0000
"stevelu" <sp_lussier@yahoo-dot-com.no-spam.invalid> wrote in message
news:41e82c15$1_1@Usenet.com...
> 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.
I would agree with everyone else that the orange part is probably ZnSe.
Ge or Si would also be the most likely candidate for the lenses, *except*
for one small detail, :
"small chip is broken off one; the broken surface is rippled like a
broken glass surface but opaquely shiny like a metal."
Ge & Si dont usually break like that, you usually get flat or stepped break
surfaces.
This sounds more like a conchoidal break surface you get on a chalcogenide
glass like Amtir.
That is suifficiently uncommon I wouldnt normally think of ot as a
candidate, but the break does sound like it.
Other ways to tell Ge/Si/Amtir.
Density already mentioned. You can tell Si from Ge just by picking it up,
difference is so large.
Resistivity. Optical Ge is typically 5-10 ohm cm, Si considerably higher,
Amtir insulating.
If you have seen them, you can tell them apart; Si is paler than Ge, Amtir
different again & in very thin chip edges you can see some red.
IR spectrum gives it instantly of course.
If we knew more about the optical design, the choice of meniscus radus would
tell you the index.
Harvey
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