SF6 filter for IR camera

[agiorgitis]

Hi guys,

I'm trying to find an SF6 filter (band reject filter at 10.6μ) to simulate what the gas cameras do. I know that there will be noise because the filter and the sensor will not be cooled down, but I can't buy an expensive gas camera. 
At least I can try and check the results. High Sensitivity Mode is available at FLIR Thermal Studio, so I'll give it a go.

I see that CO2 safety glasses are able to cutoff at exactly the 10.6μ region I'm looking for (give or take), but I'm not sure if the glass/plastic will be transparent to the rest of the IR.

Are you aware of any other material I can buy for a low price?

Thanks!

[Ultrapurple]

Perhaps the easiest thing to do would be to buy or borrow a pair of these glasses and examine them with a LWIR camera. If you can see through them...

More seriously, I'd expect a narrow band filter to be pretty expensive because of the necessarily exotic material (possibly ZnSe or whatever magic stuff FLIR makes its LWIR-to-violet inspection windows from), plus many layers of carefully calculated and applied coatings to get the required response. I've never used a SF6 camera but from the little I've seen in videos I think there's a lot of real-time image processing going on too - it appears to me that frame-to-frame differencing is used to get proper contrast.

As ever, I'm sure someone here will know all about the subject and be able to give you chapter and verse.

[agiorgitis]

Ultrapurple, that's what I was thinking to do, buy a couple of CO2 goggles, but I want to avoid that cost if possible (they are at around £20-30).

About the processing, gas cameras have the HSM build in and they do it in real time. But I can do it offline in Thermal Studio.
So I'll record videos from different locations and then post-process them for possible leaks.

I idon't need too much detail in the videos, just to be able to locate the leaking part.

[Fraser]

Hmmmm, I am no expert on gas detection using a thermal imaging system but I always thought the gas detection cameras contained bandPASS filters and not bandSTOP types. That is to say, the camera is filtered so that it sees only the area of the spectrum in which the target gas is detectable. In the case of SF6 this is 10.6um. The filter is therefore a 10.6um bandPASS type with a bandwidth chosen to pass the whole SF6 spectrum response but not much more. The target scene is viewed with the normal energy levels present at 10.6um and the presence of SF6 creates the required contrast in the image in order to see the gas cloud effect on its surroundings. The gas contrasts to the background energy levels at 10.6um. The last thing you would want to do is mask the 10.6um area of the spectral band with a bandSTOP filter.

Also my understanding of safety glasses is that they are pretty broad spectrum bandSTOP filters to protect the eyes from higher energy visible and invisible sources that would otherwise cause damage to the retina. They are high loss and not intended to pass any significant energy at LWIR wavelengths ! Such safety glass material would effectively blind the LWIR thermal camera.

Maybe I have got totally the wrong idea about what you are trying to do so please do put me right on this if that is the case.

Fraser

[Fraser]

I just did a quick Google search on the FLIR cameras and also SF6 detection in general. I attach a link that may be useful.

https://www.drone-thermal-camera.com/optical-gas-imaging-with-thermal-camera/

Fraser

[Fraser]

Also, regarding high sensitivity modes in thermal cameras. These are normally a function of the Microbolometer bias settings in hardware and not a software image processing function. You effectively ‘tune’ the microbolomter for maximum performance in the specific scenario. This can lead to decreased dynamic range and issues with imaging stability but, like most things in life, nothing is for free it is a little like tuning a car engine.... you can get better performance but there are consequences to such. Generic thermal cameras can have what some manufacturers call ‘high sensitivity’ mode, but this is a very broad statement and the reality is often that the mode is the standard -20C to +120C, High contrast LUT, Image noise filter impact reduction or in some rare cases a true ‘tuned’ mode where image quality is played off against, and in favour of, sensitivity. In general, temperature stabilised microbolometers (Peltier stabilisation) perform better in true high sensitivity modes that involve tuning the bias on the microbolometer for peak sensitivity with lowest NeTD.

In general, a Stirling process cooled QWIP LWIR camera is used for SF6 detection as it provides the low noise floor and sensitivity required for such a task. If a SF6 gas detection camera could be built using an uncooled ‘generic’ camera, FLIR would have pursued such as SF6 is a gas that is infamous for its negative effect on the planet.

Fraser

[Fraser]

ThecFLIR GF306 datasheet tells you much about what is needed to image SF6. Look at the stated Spectral Range of 10.3-10.7um. Compared to a generic LWIR camera, it is tiny. Also note the stated NeTD of less than 15mK ! That is very low noise but needed for the task at hand and thanks to the cooled detector array combined with narrow bandwidth operation.

http://www.flirmedia.com/MMC/THG/Brochures/OGI_007/OGI_007_US.pdf

Fraser

[Fraser]

If you want to experiment with detecting SF6 gas, you will need to source a narrow passband 10.6um filter. Sadly they are specialist and not exactly cheap ! Almost $400 plus taxes etc from Edmund Optics and I could not find a 10.6um pb filter listed on the Thorlabs site.

Some sources are listed below.....

http://www.multi-ir.com/product/narrow-bandpass-106um-ir-coating-filter.html

https://www.spectral-systems.com/wp-content/uploads/2016/01/Standard-Infrared-Bandpass-Filter.pdf

https://www.edmundoptics.com/p/106m-cwl-125mm-diameter-15-fwhm-ir-bandpass-filter/2850/

[agiorgitis]

Thanks for the info Fraser!

I'm pretty sure that in a video I had seen that they use bandstop filters, that's why the gas clouds appear dark.
Also in another video a guy from Flir was saying that he was able to visualize gas (CO I think) with his T650 camera and Research software to add some software filters...

[Fraser]

The gas cloud is an attenuator of the 10.6um energy so will appear cooler than its surroundings. It acts like a 10.6um bandSTOP filter between the background and the camera. Of course it can be made to look light against a dark background through the simple Monochrome inversion mode in many cameras.

Gas concentration levels will be a factor in how well such shows up in a particular scenario. Sadly I cannot comment on the FLIR demo as I have not seen it.

[agiorgitis]

Gas concentration levels will be a factor in how well such shows up in a particular scenario. Sadly I cannot comment on the FLIR demo as I have not seen it.

Check on 30:15

[Fraser]

He clearly states that the CO leak was found using the hot air as the tracer element of the investigation and he was not able to actually image the CO gas. He was imaging an associated emission (heat) that would indicate that a CO leak might also be involved. Capturing multiple frames when tripod mounted can indeed provide an image set that can be layered or sequenced for analysis. I have ResearchIR 4 Max and it us a very sophisticated analysis tool. It can only work with the data that the camera has captured to file however. If the camera is not up to the task then the ResearchIR software cannot create the missing data during analysis. It can only make data more visible to the user if it is there to start with, hiding in the background.

ResearchIR 4 MAX is normally very expensive. Do you have it already ?  I do have brand new genuine FLIR ResearchIR 4 MAX USB HASP licences for sale at a great price but I would not feel happy selling it to you if I thought it would not do what you want in this scenario.

Fraser

[Max Planck]

Thanks for the info Fraser!

I'm pretty sure that in a video I had seen that they use bandstop filters, that's why the gas clouds appear dark.
Also in another video a guy from Flir was saying that he was able to visualize gas (CO I think) with his T650 camera and Research software to add some software filters...

Bandstop filters are used to build cameras that are blind to a gas. For example such filters are used to make MWIR cameras blind to CO2. As Fraser explained, we are talking bandpass filters here.
Now, choosing the best band is another story, especially if we are talking uncooled cameras. Several factors are to be taken into account: background temperature, gas temperature, etc. In your case, a filter matching exactly the SF6 band will not be the best solution, probably. Looking at the absorption spectrum:
http://vpl.astro.washington.edu/spectra/sf6pnnlimagesmicrons.htm

I would probably go with a 10-11 um filter or something close.

If looking for an off-the-shelf solution, I would probably go with;
https://www.spectrogon.com/wp-content/uploads/spectrogon/BP-10635-1000-nm.pdf

Max

[agiorgitis]

Fraser I'm using the demo version. I have 4-5 computers that I use, where I install demo software around.
Mostly I'm using Flir tools because I edit photos, or Thermal Studio (demo has just a watermark, and you can use all functions - can't save though).

But if you offer your license in a good price, and I manage to get some results with my camera and SF6, I wouldn't say no  (Or if you have a Thermal Studio license).
Just let me check first.

Max thanks for the info, your idea seems reasonable 
A 10-11μm filter would be better. I'll check if I can get one in a good price, somewhere...
 

[Bill W]

I have had a go with uncooled cameras (MDTD 40mK) to see if they might do cheap gas imaging.  You see a little but not a lot except for R123 etc refrigerants.
Ethylene, ammonia, SF6 just about viable especially if the gas is cooling a fair bit by expanding.
No go for Methane (or any '-ane' gases)

As the FLIR brochure that Fraser linked shows, one useful mode that can come for free is to remove the background (ie shutter calibration with the shutter open)

Bill

[agiorgitis]

I have had a go with uncooled cameras (MDTD 40mK) to see if they might do cheap gas imaging.  You see a little but not a lot except for R123 etc refrigerants.
Ethylene, ammonia, SF6 just about viable especially if the gas is cooling a fair bit by expanding.
No go for Methane (or any '-ane' gases)

As the FLIR brochure that Fraser linked shows, one useful mode that can come for free is to remove the background (ie shutter calibration with the shutter open)

Bill

Thanks for the info Bill!
So basically we're trying to indirectly see the gas by the changes it creates to its surroundings!

About the calibration, how do I do an open shutter calibration?
The guy on the video stated that on normal thermoscopy it's advised to NUC the camera with the lens cap on, but on gases you need a "scene NUC", which you do after fixing the camera to a tripod.

[Bill W]

Thanks for the info Bill!
So basically we're trying to indirectly see the gas by the changes it creates to its surroundings!

Not really, you are (for a cold gas that is opaque) seeing a cold blob in front of the scene.  Think of a lump of candyfloss.
If the gas is not opaque somewhere (eg methane) see straight through
For an opaque gas at ambient - just foggy as you do not see through it
For an opaque gas hot - hot blob.

Now you appreciate why the FLIR videos often have a sky background.

About the calibration, how do I do an open shutter calibration?
The guy on the video stated that on normal thermoscopy it's advised to NUC the camera with the lens cap on, but on gases you need a "scene NUC", which you do after fixing the camera to a tripod.

Just unplug the solenoid, or insert an in-line switch.  The camera will probably still think it has operated the shutter.

Bill

[Max Planck]

So basically we're trying to indirectly see the gas by the changes it creates to its surroundings!

Exactly.

If you have a gas leak in your camera filed of view then:
(A) the camera sees the background radiation in the areas without gas
(B) in the areas with gas the camera sees the background radiation attenuated by the gas + the radiation emitted by the gas + the radiation reflected by the gas; the last term is negligible 

The gas can by cooler or hotter than ambient.

To see the leak, |(A)-(B)| has to exceed NETD.
Putting aside cooled cameras with cold filters...
1. You need a thermal contrast (gas/background) and spectral contrast; the latter provided by a filter.
2. The longer is the path length travelled by the background radiation through the gas cloud, the better for detection purposes.
3. The filter (warm interference filter) strongly affects the camera operation. Its choice is the tricky part.

[bap2703]

Just passing by: don't forget using infrared absorbing gases, aka "greenhouse gases" is bad for the environment.

[Ultrapurple]

@bap2703 - precisely why one would want to use technological means such as a thermal camera to detect and thus be able to stop leaks!

[bap2703]

Sure if the native application is using one of these gases it's legit. I didn't think about that 
I thought about putting SF6 in a non-SF6 system just to use infrared detection.

[Fraser]

It is also worthy of mention that experimenting with some environmentally hazardous gases as part of filter testing could be deemed illegal in some countries. I have worked with refrigerant gases that clearly stated on the cylinder that a licence was needed to operate the cylinder and deliberate release of the contents was illegal.

I know this is a small scale experiment but care is still needed as breaking the rules can have consequences, such as an operator losing their licence.

Fraser

[Fraser]

https://www.refcom.org.uk/news/refcom-welcomes-successful-f-gas-prosecution/

Fraser

[gerdir]

I have build a SF6 (and methane) camera some years ago.
I still have a SF6 narrow band-pass filter for this purpose (spectrogon BP-10500-775).

You have no chance to build a (passive) gascam with a simple bolometer, if you want to see small amounts of the gases.
You definitley need a cooled FPA for this, with very low noise.

Please remember, that you want to detect the photons, that are emitted from the gas itself. You can measure this only, if enough photon will reach your pixel.
With such a narrow bandpass you limit the spectrum a lot, but your bolometer furthermore measure all photons from the broad range, that the bolometer is sensitive at (i.e. 7.5 to 14 µm) and the band pass filter itself in front of your camera is NOT cooled, so it "heat" the FPA normally a lot more than the few photons from the gas you search for.

You have to detect nearly nothing on top of a huge radiator. Not possible with a uncooled detector.

Don't waste your time on this. 
And PLEASE don't wast SF6 for such trials!
1kg of SF6 has an identical climate effect (Global warming potential) as 23500kg CO2 (23.5 metric tons of CO2!!).

[Max Planck]

An uncooled camera with a warm filter will not match a cooled system with a cold filter, no doubt about that, but I have to generally disagree with what you wrote. There are many variables to be taken into account here, but it is possible to detect gas leaks using uncooled cameras.
By the way, FLIR's newest methane camera is an uncooled one.
From memory, Konica-Minolta and Bertin are also offering uncooled systems dedicated for gas/chemicals detection.

Max