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Old 09-07-2017, 12:15 AM #1
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Default Any mid-IR sources besides blackbodies?

I've acquired a thermal imaging camera, so of course I've been going around imaging everything and observing how mid-IR at ~8-15 um behaves. I even found and purchased a diffraction grating designed for ~10 um light. What I'd love to do is come up with a relatively inexpensive way to show IR absorption bands, and how halogenated gases (e.g. R134a or SF6) contribute much more (on a molecule-by-molecule basis) to the greenhouse effect than other gases - by absorbing wavelengths right in the 8-15 um "window" that the atmosphere is otherwise transparent to.

I was wondering: are there any mid-IR sources besides ordinary thermal blackbodies and burning CO2 lasers operating at many watts? I would love 100 mW CO2 laser, but it seems those don't exist. Anything like an LED would be great, but I can't seem to find anything that is longer than 5 um, and even those tend to be quite expensive.

Does anybody have any ideas? All I can think of is taking a normal blackbody and putting overlapping materials with different IR transmission curves in front of each other (e.g. Si cuts off around 10 um, and then finding something else that transmits starting around 11 would give me a narrower wavelength range than normal. That would produce diffraction lines at different spots than say a similar setup for 12-13 um, etc, with all this against a cold background like a freezer wall. But it would be great if there's something kind of LED-like, or even better a mid-IR laser of reasonably low power.

The ultimate idea is to make and calibrate kind of a crude DIY spectroscope for IR.


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Old 09-07-2017, 01:16 AM #2
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Default Re: Any mid-IR sources besides blackbodies?

Quote:
Originally Posted by Stibnut View Post
I've acquired a thermal imaging camera, so of course I've been going around imaging everything and observing how mid-IR at ~8-15 um behaves. I even found and purchased a diffraction grating designed for ~10 um light. What I'd love to do is come up with a relatively inexpensive way to show IR absorption bands, and how halogenated gases (e.g. R134a or SF6) contribute much more (on a molecule-by-molecule basis) to the greenhouse effect than other gases - by absorbing wavelengths right in the 8-15 um "window" that the atmosphere is otherwise transparent to.

I was wondering: are there any mid-IR sources besides ordinary thermal blackbodies and burning CO2 lasers operating at many watts? I would love 100 mW CO2 laser, but it seems those don't exist. Anything like an LED would be great, but I can't seem to find anything that is longer than 5 um, and even those tend to be quite expensive.

Does anybody have any ideas? All I can think of is taking a normal blackbody and putting overlapping materials with different IR transmission curves in front of each other (e.g. Si cuts off around 10 um, and then finding something else that transmits starting around 11 would give me a narrower wavelength range than normal. That would produce diffraction lines at different spots than say a similar setup for 12-13 um, etc, with all this against a cold background like a freezer wall. But it would be great if there's something kind of LED-like, or even better a mid-IR laser of reasonably low power.

The ultimate idea is to make and calibrate kind of a crude DIY spectroscope for IR.
There is a physical limitation imposed by the semiconductors in LED and laser diodes that prevents electromagnetic radiation outside of certain ranges, so you're not going to be able to get anything like that to work.

Not sure what you're looking for aside from that, but I feel like gas tubes are really your only reasonable option whether they be lasers or some special lamp.

Also, it's bad forum etiquette not to post in the welcome section and introduce yourself first.
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Old 09-07-2017, 09:32 AM #3
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Default Re: Any mid-IR sources besides blackbodies?

The only thing I can suggest unfortunately is to get yourself a 20W CO2 tube and run it at low current. It is possible to get CO2 lasers in lower power levels, but not as low as 100mW. The lowest I've ever come across is 4W myself, but these along with the 10W's are specialist models that are much more expensive than what you can get a 40/50W setup at times.
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Old 09-08-2017, 01:59 AM #4
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Default Re: Any mid-IR sources besides blackbodies?

Anything room temperature will emit virtually ALL of its radiation in mid IR. LEDs wouldn't make the least bit of sense here, considering a blackbody source is more efficient, much cheaper, and probably more reliable.

Quote:
Originally Posted by Stibnut View Post
What I'd love to do is come up with a relatively inexpensive way to show IR absorption bands
Microbolometers do not differentiate between wavelengths. A thermal camera is therefore inherently incapable of doing this.

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Originally Posted by Stibnut View Post
I would love 100 mW CO2 laser, but it seems those don't exist.
Firstly, most CO2 lasers can generate 100mW. I've had my 60W tube down to 3mW before.

More importantly, NO YOU DON'T. I made the mistake once of just looking at the spot of a CO2 laser at low power with my thermal camera once, and the afterimage was burned into the sensor for days. Even something as benign as pointing the camera at an electric stove or campfire can permanently damage the sensor.

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Originally Posted by Stibnut View Post
The ultimate idea is to make and calibrate kind of a crude DIY spectroscope for IR.
Try a visible one first since they're a lot easier, and give you experience. If that build doesn't dissuade you, I'll be impressed.
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Last edited by Cyparagon; 09-08-2017 at 02:04 AM.
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Old 09-08-2017, 03:02 AM #5
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Default Re: Any mid-IR sources besides blackbodies?

There is a good July 2017 article on mid ir leds , if that helps, here: http://www.laserfocusworld.com/artic...99&bid=1859697
"The newest generation of IR scene generators—which project simulated images for testing IR cameras—use cascaded superlattice LED structures because of their higher performance compared to alloys or quantum wells. For example, the University of Iowa (Iowa City, IA) mid-IR LED group reported outputs of 25 mW when developing mid-IR LED devices based on InAs-GaSb superlattices for scene generator"

mid-IR quantum interband cascade superlattice light-emitting diodes -- not very high output power but...

"Terahertz Device Corporation (Salt Lake City, UT) offers quantum interband-cascaded superlattice LEDs, called QuiC SLEDs, over the 3-12 μm wavelength range." See: Terahertz | Mid-infrared LEDs - Terahertz Device Corporation


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Old 09-13-2017, 03:12 AM #6
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Default Re: Any mid-IR sources besides blackbodies?

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mid-IR LED group reported outputs of 25 mW when developing mid-IR LED devices


For comparison,
I ran the numbers, and my middle finger emits about 700mW of mid IR.
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Old 09-13-2017, 05:13 PM #7
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Default Re: Any mid-IR sources besides blackbodies?

Yeah, this situation is more or less what I was afraid of. Just figured I'd fish around for other ideas in case there were any.

I have several of those $7 EISCO diffraction-grating based spectroscopes, and I agree that it would make perfect sense to try making one of my own based on a reflective grating (such as I have for mid-IR) rather than the usual transmission-based ones. I have a variety of lower-power (1 to 200 mW) lasers ranging from 405 to 808 nm, plus the usual CFLs and whatnot. So that would be a good start.

The mid-IR sources that Encap listed unfortunately use a different definition of mid-IR than I do. I think of mid-IR as being roughly the 7.5-15 um IR window, while they seem to think of it as being roughly 4 um while calling the window wavelengths LWIR. I've seen a few LEDs and even some (way out my price range) diode lasers out there, and it might make a decent backup plan to have one side of a pressure tube contain one such LED and the other side an appropriate photodetector to show the IR absorption of, say, compressed CO2 gas at 5 bar in a cheap pressure chamber. It doesn't appear that ~10 um LEDs exist though.

What I am hoping for is something that would visually show people IR absorption bands in a low-budget way, without the extreme price of IR spectroscopy equipment. It would be really cool way to demonstrate a variety of properties from the vibrational spectrum of various bonds.

Cool, iiterally, because it would have to be done in a cold environment with relatively low background IR emissions. Luckily I have a run-of-the-mill 1 cu ft freezer that can hit -35 C, and there's always dry ice/acetone (or alcohol) for -78 C if that's not enough.

The idea behind a ~10 um LED is to catch some specific bands in a much more visible way. I already figured a CO2 laser was out of the question; good to know that even 100 mW would blow out a bolometer and ~1 mW might be better, not that this would be easy.

Finally sorry for not posting in the newbie forum, I'll go do that soon.

Last edited by Stibnut; 09-13-2017 at 05:15 PM.
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Old 09-16-2017, 01:01 AM #8
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Default Re: Any mid-IR sources besides blackbodies?

This thread didn't really catch my attention until you started talking about vibrational energies of various bonds. This takes me back to some chemistry courses I took some time ago where we would use IR spectroscopy to identify certain covalent bonds in organic molecules. The ones that absorbed were all polar molecules and the IR range was from 2.5 um to 25 um. Had to go back and look at some old text books, so thanks for the walk down memory lane.
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