30mm Ge windows found for ~US$12

[Ultrapurple]

Hi all

The Periodic Element Guy on eBay has some ~30mm diameter germanium windows for sale eBay auction: #273850457562 at about US$9.50 plus $2 post (to UK). I've just received one and have a couple more on order. The photo on the listing is reasonably representative of the sample I received. They're used and a little weathered, with one or two small chips at the edge, but may have potential.

The downside? I think the coating is MWIR. Certainly, my LWIR Therm-App has a tough job seeing through the one I've got, although it does transmit a bit.

[Fraser]

Thanks for the info. I had my eye on those as well. It is a pity that they are not LWIR though.

Regarding new periodic table Germanium ‘discs’ that are available on eBay. I spoke to the sellers of these and they are ‘just’ polished or unpolished crystal wafers with no AR coating. Their optical performance is therefore very variable with high transmission losses.

It is better to buy ex fire fighting camera Germanium windows when they appear on eBay, as they occasionally do. I thought the used ones detailed by the OP were likely ex fire fighting camera windows due to their condition. It appears not to be the case though.

Fraser

[Ultrapurple]

Like Fraser said.

The windows do look very much like the firefighting camera type, which is why I bought the first one.

The seller seems to be helpful and is very responsive. He says he has some other Ge windows coming in soon, though apparently they're a little smaller. I invited him to have a nose around EEVblog to see if any of our tech talk piques his interest.

[Ultrapurple]

Photo of both sides of my Ge window. Biscuit for scale.

If you don't have Bourbon biscuits in your country then please be aware they contain no whisky. I would have measured the biscuit to give a scale factor but unfortunately before I thought of that I ate it.

[Fraser]

The picture is interesting as both sides of the Germanium window are damaged. Normally it is just the outside face that suffers so these have either been poorly stored (scrap bin) or were used in an application other than a thermal camera, such as a laser test bench.

Fraser

[Fraser]

Hmmmm, I just took a look at other Germanium windows from Israel.

To my eye, the damage present on them is not natural and looks like decommissioning deliberate damage or the parts have spent a long time outside exposed to harsh weather.
I suspect these Germanium disks may be part of military equipment that has been officially scrapped. The disks may not be windows and could be spectral filters or even attenuators. The condition of some that I saw suggests that they are basically scrap metal now, as advertised.

Fraser

[Ultrapurple]

Thanks Fraser

I'm vaguely surprised at the idea of bandpass filters but I suppose I shouldn't be, on the basis that it may make sense for a military thermal imager to blot out everything except, say, the characteristic spectral wavelengths emitted by burning JP-8 or rocket fuel or whatever. If you can cut down the wideband background noise you can significantly improve the effective sensitivity of the system. And it's not rocket surgery to realise that having a filter element you can switch in and out at will may make an imaging system that bit more versatile.

I wonder what each of these little discs cost the original purchaser. I suspect it was slightly more than US$10.

It will be interesting to see what arrives in the post! I expect it to take two weeks or so for the carrier pigeon to strut its stuff but I'll definitely report back when they're in my hands.

I'll let the bird go back unless anyone had any good recipes for pigeon pie.

[ArsenioDev]

I think Ultrapurple is onto something with the bandpass for jet emissions filter thought, I know most, if not all heat seekers these days have them in front of the FPA to do better countermeasure rejection. Might snag a few of these for the lulz, see how they do

[Fraser]

I could not possibly comment on missile use 

I bought some ex military Germanium filters from Turkey that had come from the conflict zone in Iraq. They are in perfect condition, large diameter at around 50mm and definitely coated for spectral bandPASS. Sadly not much use as a general purpose window. They cost £14 each so no great loss.

Fraser

[Ultrapurple]

@Fraser

Without asking you to reveal any secrets, what sort of region do the filters pass? A bit in the middle of LWIR? A bit on the short edge of MW?

Of course if you have accurate figures that'd be interesting.

[Fraser]

I honestly do not know. Where military use is concerned, that information is classified.

Fraser

[Ultrapurple]

I have been in further communication with the seller. He has a variety of other LWIR windows of different sizes and shapes and was kind enough to send me a selection so I could check which pass LWIR. The first photo shows my yes/no results with a ruler to indicate size.

I understand that there are hundreds of the 18mm 1.3g windows available so I provided some simple test images of a warm resistor with and without the window in visible and LWIR.

Although I haven't arranged anything, if you see windows like these on eBay and want more than one or two it might be worth asking if you can get a small discount as an EEVblog member, mentioning that you learned about the windows here.

[Fraser]

 

[Ultrapurple]

I think Ultrapurple is onto something with the bandpass for jet emissions filter thought, I know most, if not all heat seekers these days have them in front of the FPA to do better countermeasure rejection.

This Wikipedia article may shed some light (!) on these questions, at least from an historical perspective.

[ArsenioDev]

I know entirely too much about how seekers work, blame younger me for getting fascinated by the AIM-9 Sidewinder and that leading to my modern love of missiles and seeker tech