The ZnSe Window that almost brought tears to my eyes !

[Fraser]

I recently purchased some FLIR parts and accessories from a chap and he also found a large ZnSe window for me. Now I only paid 20 Euro's for the window so you cannot expect much for that. The window is approx 90mm diameter and 12mm thick ! I have no idea what it was used for, or what I will do with it, but I bought it anyway 

The box of parts arrived today and the seller had been very generous in adding extra useful FLIR parts, namely, a lot of FLIR lens cases and a nice promotional FLIR LED torch. All good so far but as I unwrapped the ZnSe window my heart sank.

The window looked to be in pretty good condition considering what I paid for it. Someone had done the unthinkable to it though....... it had its dimensions written onto its AR coated face in permanent marker !
It almost brought tears to my eyes to see it. I quite expected that pen ink to have leeched into the AR coating, permanently scaring the window. The window was dirty so would need a good clean and I would then establish if any damage had been done by the pen.

I very carefully cleaned the window with a mixture of IPA and water. The writing began to lift off of the surface. After more careful cleaning the pen marks were completely removed, but the important AR coating was undamaged underneath.  Phew !

I attach some before and after pictures.

Never try this at home, it can end in tears ! Writing on a lens AR coating would just never occur to me. Now graphite pencil on the opaque sides.... that I have seen. Even painted part numbers on the side.

Now what the heck do I do with a large, thick and heavy ZnSe window ?

Fraser

[Fraser]

More pics

[Spirit532]

Time to make a blast-proof container for one of your cameras 

[Fraser]

Yes 12mm thickness is a pretty substantial window !

I wonder whether this is a cabinet inspection window, as found in a high energy electrical cabinet.

It will be interesting to see what the Windows transmission is like. I have a Black Body thermal source so can determine the transmission pretty simply by seeing how much error the window introduces into readings. No time at the moment but I will check it later.

Fraser

[Fraser]

I just took a look at ZnSe windows from optical suppliers.

Wow they can be expensive. A 70mm diameter x 3mm thickness is £440 

Imagine what a 90mm x 12mm costs.

https://www.crystran.co.uk/windows/zinc-selenide-windows/zinc-selenide-circular-windows

Another supplier wants almost 1000 Euros GeForce a 70mm x 6mm !

http://eksmaoptics.com/optical-components/uv-and-ir-optics/zinc-selenide-znse-windows/

Even Edmunds Optics do not have such a large window shown. A 75mm x 6mm costs around $1700.

https://www.edmundoptics.com/optics/windows-diffusers/ultraviolet-uv-infrared-ir-windows/zinc-selenide-znse-windows/

Fraser

[MadTux]

Evaporate a semireflective gold mirror onto it and  build a massive friggin CO2-Laser? 

[T3sl4co1l]

Is that single crystal?

Tim

[Fraser]

I have just tested the ZnSe window for transmission loss.

A non AR coated ZnSe window has a transmission figure of around 60% or so. A window that has the correct wavelength AR coating provides around 85% transmission. Remember, this window is far thicker than normally stated in transmission tests.

Test Method:

I let my Black Body thermal source stabilise for an 90 minutes at 40C and took a picture of it with my E60. The reading n teh E60 was spot on 40C

I then placed the ZnSe window in front of the Black Body emission plate and measured the emission plate through the window using the E60. The measurement was 34C through the window.

Transmission calculation:

34/40 X 100 = 85% transmission with 40C reference. Or put another way, 15% attenuation with 40C reference.

Pretty much exactly what was expected. My lens has the correct AR coating for a LW camera.

Pictures attached

Fraser

[Myself]

For some reason I had it in my head that the ambient temperature, or the temperature of the window itself, would need to be figured into such a calculation.

Here's my logic: The portion that's not transmitted, what happens to it? It must be absorbed or reflected, meaning that the window is partially acting as an emissive or reflective object. If it's emissive, then it's emitting its own temperature. And if it's reflective, then it's reflecting the room behind you. Either way, the temperature of the window or the environment must contribute to the inaccuracy, and be calculated back out, no?

[Bill W]

For some reason I had it in my head that the ambient temperature, or the temperature of the window itself, would need to be figured into such a calculation.

Here's my logic: The portion that's not transmitted, what happens to it? It must be absorbed or reflected, meaning that the window is partially acting as an emissive or reflective object. If it's emissive, then it's emitting its own temperature. And if it's reflective, then it's reflecting the room behind you. Either way, the temperature of the window or the environment must contribute to the inaccuracy, and be calculated back out, no?

Yes, after all consider the case of a house brick at ambient, looks 23°C and is not 23/40 transmissive. 

Even so not too bad.  I suspect a furnace window, as high voltage ones would be thin germanium (earthed metal)

Bill

[IwuzBornanerd]

I have just tested the ZnSe window for transmission loss.
...
Transmission calculation:

34/40 X 100 = 85% transmission with 40C reference. Or put another way, 15% attenuation with 40C reference.
...
Fraser

So 0C would be total loss through the window?   Shouldn't the numbers be in Kelvin?  If we are concerned with the percent of the total amount of energy that is transmitted, the reference should be zero energy.  No??  Maybe we also need to take into account the minimum energy detectable by the sensor, but the device calibration may remove that concern.

I admit I don't know how these matters are normally dealt with on thermal equipment, but I have been trying to rationalize this sort of thing as I think about how to deal with a "long" lens with tight aperture. 

[mahony]

I suppose the transmission calculation should be done in Radiance, not in Temperature.
A very simplistic approach ignoring the windows own temperature and emission figures, as well as reflections from surroundings would be something more like this:

Radiance for 8-14µm assuming 98% emissivity of the BB source @40°C = 64.23 W/m²sr
Radiance for 8-14µm assuming 98% emissivity of the BB source @34°C = 59.00 W/m²sr

59/64.23 ~= 0.9185 or roughly 92%

By the way: a good AR coating should lead to way more than 90% transmission of the window - the main contribution to losses would then be attenuation within the material. I have got no attenuation figures at hand for ZnSe atm but attached is a transmission figure for a AR coated 5 mm thick ZnSze window from ThorLabs: https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=3981

But anyway a very nice catch 

[Ultrapurple]

Back to the original point - permanent marker - I well remember seeing a large stall at the Friedrichahafen amateur radio rally with all sorts of lovely optical components like lenses, windows, first-surface mirrors and so on, all with their prices boldly written across them in permanent marker. You could tell which of the visitors knew what they were looking at by the state of their faces...

I guess the vendor must have sold a few things to people who didn't realise how bad the desecration really was. I guess that the magic marker lived up to its name, by removing 99% of the value of everything it touched!

[Fraser]

Thanks to all for the comments.

You are absolutely correct, the calculation IS more complex than I showed. I was looking for a transmission figure to see just how bad such a thick window would be and also establish whether it was coated with a suitable AR coating.

The ITC guidance paper that I followed for this test was this one

http://flir.custhelp.com/ci/fattach/get/9908/0/session/L2F2LzEvdGltZS8xNTEwNzQzODMwL3NpZC9mVSU3RWR0U2FSYjZDUDVKYWxXUEtSSnVPN21DdVl1djNIaE5oUjdCNW1vWHM1cnFyZ1djVU44aFRacDF5WVNGdEU1enppSWZCUG1PNzZQMjdFQ013WkRpUFRJRzhkZkM2QkRreUFqaVd0WUNfbWVHem0xaFVkWERVQSUyMSUyMQ==/filename/Tech+Pub+-+Easy+Window+Transmission+Measurement.pdf

Fraser

[Fraser]

I will repeat the test at different source temperatures for interest. I can go as low as as 15 Degrees as my Black Body is Peltier based.

My calculation is simplistic, no doubt about it 

I will try using the Emissivity setting on the camera as detailed in the ITC guide and see what results I get.
I do not normally need to measure transmission through my auxiliary lenses as the manufacturer kindly provides that information. This is a learning exercise for me but I do not need very high accuracy as the window will never see service in a measurement critical scenario. It is a good learning exercise though.

More test results shortly 

Fraser

[Fraser]

I have just found this guide that may provide useful insight into the situation. No time to read it at the moment.

http://www.grupoalava.com/repositorio/97d2/pdf/5436/2/cordex-instruments----ir-window-transmission-guidebook.pdf?d=1

Fraser

[Fraser]

I have just managed to read the guide I referenced.

Hmmmm thermal inspection windows are not something I have ever needed to use and I am glad. They can really mess up your measurements ! As stated in various guides on these windows, they are used in preventative maintenance and absolute measurement accuracy is not expected. Relative measurement accuracy is, however, important. From what I have read, an inspection window needs to be thermally profiled and a correction table produced for each window. The same thermal camera (not just same model) should be used to take periodic measurements and even then a level of uncertainty exists.

I have never worked in high energy electrical compartments but thermography in this area appears fraught with issues. The Emissivity of the electrical connectors, that tend to be shiny with resulting low Emissivity, is a nightmare !

In my test case, I was using a Black Body thermal source with an emission plate designed to provide an accurate thermal source with little ambient temperature or reflected thermal energy influence on its output. Once I added the ZnSe window between the camera and the Black Body, I did introduce the spectre of self radiation from the window through absorbed energy and reflected energy from the surrounding environment. There appears to be an AR coating on the windows faces so the amount of reflected energy may be minimal. The temperature of the window was at ambient and this figure was entered into the camera. If the ARccoating is less than perfect, my reflected body heat from my face would skew the readings. Some hot spots are visible in the second image. These are not normally present on the Black Body plate. I avoided measuring within this hot spots in my test.

It is clear that profiling the transmission characteristics of a thermal inspection window is not a simple 'single point' test and calculation, but then I did not expect such. There are many variables effecting the measured temperature of a target when viewed through such a window. As shown in the guide, it is possible to produce a correction plot to improve accuracy.

My only use for such a window is in an external weatherproof camera housing used to observe wildlife so exact target temperature is not normally a requirement ! However, transmission losses through the window and its likely lower surface temperature than the target are concerns !

Automotive thermal camera lens protection Germanium windows are normally heated with a heating element around their periphery.

Heating a 90mm x 12mm ZnSe window would be an 'interesting' challenge ! Not one that I will be undertaking. I bought this window because it was interesting to me and at 20 Euros it was hard to resist. As originally stated, I have no plans for its use in any projects that I have in the pipeline.

Fraser

[TheSteve]

Now what the heck do I do with a large, thick and heavy ZnSe window ?

Fraser

IR port hole on your submarine!

[Fraser]

 

No thanks.... Peter Madsen built his own submarine (Nautilus) and appears to have 'lost the plot' as a result?

Fraser

[CatalinaWOW]

What is interesting is that there was someone who was knowledgeable enough to recognize and identify ZnSe but still wrote on the coating surface.  Maybe they were so experienced that they knew that this marker could be removed with appropriate cleaning.  There might even be a sad story of other brands discovered which could not be removed.

[Fraser]

The seller provided me with all manner of FLIR bits and pieces, including about a dozen lens cases, so he appears to have been involved in the industry in some way. He sold me a couple of the remote control units for FLIR PM series cameras and they are getting to be as rare as hens teeth these days. To have one is rare, but two... that is pretty amazing. They usually get sold with the cameras or thrown away when separated from their camera. I now own four of them 

Fraser

[Neomys Sapiens]

Time to make a blast-proof container for one of your cameras 

YESSS! Especially as this would make sense only if one is going to blow something up subsequently! EXPLOSIONS!
Big, beautiful Explosions!
Muahahaha!

[mahony]

I also read quickly over the document link by Fraser. Thanks for that, seems to quite reasonable.

If you are going to use the window I personally would not heat this thing. The lower the temperature of the window, the less additional emission you will get and therefore the least errors. So I would rate cool it than heat.

[Fraser]

Mahoney,

I agree. I am not thinking too clearly at the moment due to ill health, but having thought about it, the heating on automotive cameras is likely more about avoiding icing in Winter. The heater is controlled by an external mechanical thermostat in its supply rail. A bit like heated wing mirrors.

Fraser

[mahony]

Then get well soon!

[Fraser]

Thanks

It is just my M.E. (Chronic Fatigue Sydrome) .... it really messes with the brain and clear thinking

It will pass though. Sadly I cannot do much fun repair or experimentation at the moment as my thought processes are very 'woolly'.

Fraser