Is an LWIR LED possible? - Page 1
Forum section: electronics / general

author: Ben321
author_rank: Frequent Contributor
timestamp: July 14, 2021, 02:03:10 am
content: You probably are wondering, why not just use any resistor to generate heat for an LWIR source. Well the problem is they would take time to heat up and cool down. I wondering if an LWIR LED could exist, made out of the right semiconductor materials. If it could exist, it would be a non-thermal source of LWIR, and as such wouldn't retain any heat. It would emit it, and any object near it that could absorb it would get hot from absorbing the LWIR, but when turned off, the LWIR LED could instantly be safe to touch without burning yourself, because it wouldn't actually get hot itself (something NOT true with a hot resistor). It would therefore need to be made out of materials that would have very little absorption of LWIR. I'm not sure exactly what purpose it could be used for, or even if it's possible. I know the principle of emission for visible light LEDs is that there's a difference in energy level between electrons in one material, and electrons in the other material, and when electrons move between the two semiconductor materials, the difference in energy levels for the electrons is emitted as visible light. Are there any semiconductor materials that exhibit LWIR emissive properties, by having the correct energy level differences between the 2 semiconductor materials? If so, are these semiconductor materials themselves transparent to LWIR (that's another important thing, as otherwise any LWIR photons emitted will simply be absorbed by the surrounding material, resulting in heating of the diode, and no emission of LWIR other than the LWIR emitted from the diode being hot).

author: Fraser
author_rank: Super Contributor
timestamp: July 14, 2021, 09:16:39 am
content: At LWIR you would normally be looking at a Laser Diode rather than a light emitting diode.Thorlabs sell such diodes.https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=6932

author: Fraser
author_rank: Super Contributor
timestamp: July 14, 2021, 09:18:54 am
content: I stand corrected, LWIR LED’s are available………https://www.thzdc.com/news/2019/6/5/85same1yo3mqfd1q4l01d88agsfviahttp://www.ece.sunysb.edu/~oe/project/SUNY%20LEDs.pdfFraser

author: Fraser
author_rank: Super Contributor
timestamp: July 14, 2021, 09:27:01 am
content: Ben321,Did you Google your question ? That is all I did and there is lots of information published on LWIR LED’shttps://www.boselec.com/product-category/infrared-light-emitting-diodes-ir-led/Fraser

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: July 14, 2021, 10:29:34 am
content: There are some MEMS IR sources that are basically tiny filament lamps which can be modulated at a few tens of hZhttps://www.axetris.com/en/irshttps://www.nova-ir.com/

author: fcb
author_rank: Super Contributor
timestamp: July 14, 2021, 10:49:47 am
content: Any idea how the cost of LWIR LED's compares with thermal sources??

author: Fraser
author_rank: Super Contributor
timestamp: July 14, 2021, 11:43:03 am
content: Mikeselectricstuff,I have looked at those as a possible “micro calibration reference” and there are filament based types in a similar miniature package. Very neat but last time I checked they are not easily obtained in small quantities due to small order limits and handling chargesI have some broadband thermal references from FTIR’s that are basically a resistive patch on a long ceramic substrate or the other type is an “open to air” filament. It would appear that these newer MWIR and LWIR thermal sources are designed for similar applications inside laboratory equipment. They also have their uses as active camouflage in military applications of course(thermal domain display panels)Fraser

author: Ultrapurple
author_rank: Super Contributor
timestamp: July 15, 2021, 08:26:50 am
content: I find it fascinating that for years there have been LWIR moving-image projectors out there, normally used by the military, which project thermal scenes so that thermal imaging users can familiarise themselves with the equipment, tactics etc.Historically I believe the refresh rate has been quite low due to thermal inertia but an array of (for example) LWIR / MWIR laser diodes could refresh arbitrarily fast. But it's very interesting to note from the links that in 2011 it was reported in the open literature that a 512 x 512 LWIR emitter array had been made; coupled with very simple optics, this would make a fairly capable projector. And I have no very-good reason to think that it couldn't be scalable - or at least possible to have multiple units to provide a wider field of view, just as we do with visible light projectors. I suspect that only a government-scale budget would be adequate for such a toy though.

author: Bill W
author_rank: Super Contributor
timestamp: July 15, 2021, 09:23:20 am
content: Quote from: Ultrapurple on July 15, 2021, 08:26:50 amWould tie in with some of the things I was doing at the time as we (e2v) had both the Argus thermal cameras an gas sensors in one office. They were using MEMS structures (e2v had bought out MICS Neuchatel) in place of light bulbs as MWIR sources for CH4 gas detectors at 4um. They operated around a few Hz, but that might have been to suit the existing light bulb software. I modified a few camera chassis into microscopes for them to aid testing the devices, using the back to back lens technique.That gas sensors business, Neuchatel included, is now sgx-sensortech with the same product lines as before.Bill

author: Ultrapurple
author_rank: Super Contributor
timestamp: July 15, 2021, 11:21:48 am
content: Very interesting, Bill. It simply hadn't occurred to me that DMD would probably work just as well at LWIR as they do at visible, given of course that different coatings would probably be required (and likewise an IR-compatible window material).I was thinking more along the lines of displaying images of potential threats (targets) than gas sensing, largely because I have no experience in that field. However you have spurred me on to look for a deliberate gas leak using different wavelength cameras. But as usual that will be down to available time, a commodity in preciously short supply.

author: Ben321
author_rank: Frequent Contributor
timestamp: July 15, 2021, 07:12:52 pm
content: Quote from: Fraser on July 14, 2021, 09:27:01 amNo. I first checked Digikey, as it seems that if a given type of electronic component exists, Digikey has it. It doesn't matter what company makes it, or what country the component is from, if it's an electronic component that exists, Digikey has it.And from Digikey, what I found were these 2 LEDs, that are 4300nm LEDs, which makes them MWIR LEDs, not LWIR LEDs.Here's their datasheets.https://www.akm.com/content/dam/documents/products/gas-sensing/infrared-led-element-for-ndir-gas-sensor/ak9700ad/ak9700ad-en-datasheet.pdfhttps://www.akm.com/content/dam/documents/products/gas-sensing/infrared-led-element-for-ndir-gas-sensor/ak9700ae/ak9700ae-en-datasheet.pdfDigikey didn't have any LEDs with longer wavelengths, so I wasn't sure if LWIR LEDs actually existed, but I figured if such an LED did exist and was just made by some very obscure company that Digikey just hasn't heard of yet, there is one place on the internet with people knowledgeable enough to tell me of its existence. And that place is the eevblog forum. So I went straight to here to post, instead of googling it.

author: Ben321
author_rank: Frequent Contributor
timestamp: July 15, 2021, 07:28:07 pm
content: Quote from: Ultrapurple on July 15, 2021, 08:26:50 amI don't think those are projectors. I think those are probably arrays of LWIR LEDs. It's basically a true LED monitor (not LED backlit LCD) that contains LWIR LEDs, instead of visible light LEDs. They probably just feed a video signal to it, and it displays the image with invisible LWIR light. Because the LEDs are based on a non-thermal process (other than the slight resistance found in every electronic component), very little heat is retained, so there's very little persistence of the image. I also remember reading about those LWIR image display panels being used by the military, but not for mission practice. I remember reading about them being used for active thermal camouflage (in effect cloaking your military assets from enemy military thermal goggles). This works, by mounting LWIR LED panels on the side of your military assets, and facing them in the direction the enemy will likely be coming, and then feeding a video signal to these panels that causes them to display (in the LWIR part of the spectrum) a pattern that accurately mimicks the thermal patterns found in the surrounding terrain.I found out about these years ago in a news article I read about cutting edge military technology, and although the article never mentioned how it worked, I always assumed it was LWIR LEDs. Just recently I remembered that article again, and wondered if by now LWIR LEDs had managed to make it out of the military and into the civilian electronics market. That's why I'm curious about such technology now.

author: bap2703
author_rank: Regular Contributor
timestamp: July 16, 2021, 07:28:00 pm
content: There are definitely LWIR scene projectors: DMD based or "reverse bolometer" based.They are used in labs to test all kind of devices.Want to operate your heat seeker hardware in the loop style? Here's your infrared projector.

author: Ben321
author_rank: Frequent Contributor
timestamp: July 16, 2021, 07:41:10 pm
content: Quote from: bap2703 on July 16, 2021, 07:28:00 pmSo does it just project an image onto a reflective screen? Kinda like those old slide projectors? Or is it itself a display screen with an array of LWIR LEDs?

author: Bill W
author_rank: Super Contributor
timestamp: July 16, 2021, 09:32:01 pm
content: Quote from: Ben321 on July 16, 2021, 07:41:10 pmNeither really, they use a curved mirror to place the image at infinity.Aim your search engine of choice at "infrared scene projector" (not available via digikey)Bill

author: Ben321
author_rank: Frequent Contributor
timestamp: July 17, 2021, 11:31:40 pm
content: Quote from: Bill W on July 16, 2021, 09:32:01 pmIt's projected in a way that it uses a curved mirror, but how is the image made in the first place? LWIR LEDs?

author: Fraser
author_rank: Super Contributor
timestamp: July 18, 2021, 12:46:50 pm
content: Not exactly what Ben321 is asking about but may still be of interest, ADAPTIV camouflage……..https://en.m.wikipedia.org/wiki/AdaptivIt uses Peltier modules as ‘pixels’ to create active camouflage that tries to deceive opposition thermal imaging target identification systems and observers. Multi spectral imaging systems make such deception increasingly difficult however.https://en.m.wikipedia.org/wiki/Multi-spectral_camouflage

author: Fraser
author_rank: Super Contributor
timestamp: July 18, 2021, 01:07:12 pm
content: A thermal projection system was mentioned earlier. This uses an active micro mirror array that reflects the thermal energy from a thermal source towards a thermal camera. The pixels of the micro mirror array create the ‘image’. The thermal energy source that illuminates the mirror array is static and not dynamic in nature and is often nothing more complex that a high emissivity resistor that sits in a ‘lamp housing’ that contains optics to collimating the thermal energy and illuminate the mirror array. This is little different to a conventional visible light slide projector. If you place your hand in front of a slide projector lens you feel the projected heat energy from the halogen lamp. A high emissivity resistor is used as it is broad spectrum and not limited by any vacuum enclosure such as found around a halogen lamp. That said, a halogen lamp is broad spectrum.I found a company that makes the micro mirror arrays for such applications. It looks like an old page though as they are using an elderly FSI Prism SP cooled thermal camera. Note that they say the mirror array may be actively cooled to lower its own thermal contribution to the scene.http://optronsystems.com/docs/irsp.htmlFSI/FLIR PRISM DS MWIR camera (a little different to the SP model but gives you an idea)https://www.x20.org/product/flir-prism-ds-infrared-camera/