Some interesting things about the James Webb telescope's IR cams

[Ben321]

First, they use some very different definitions of wavelength ranges compared to definitions that I'm used to. In general, NIR is about 700nm to 1000nm. SWIR is about 1000nm to 3000nm. MWIR is about 3000nm to 6000nm. And LWIR is everything longer than 6000nm. But in the James Webb telescope, they have the NIRCam (near infrared camera), which covers 600nm to 5000nm. Apparently the scientists who designed the instrument and named it consider it to be a "near infrared" instrument, but it covers a huge range of wavelengths from orange visible light, all the way to MWIR.

Then there's the MIRI (mid infrared instrument), which I assumed at first was designed just for MWIR (3000nm to 6000nm), but actually it covers a huge range of wavelengths from 5000nm to 28000nm. That actually misses most of the MWIR band, and instead starts near the upper edge of MWIR, and goes up to well above what most LWIR thermal imaging cameras can see (which is usually about 14000nm).

Why is their naming for these space exploration IR cameras so different from the standard naming conventions for IR cameras? Or am I just missing something?

[Fraser]

Ben321,

I totally understand your confusion over these definitions…. You are not alone !

When it comes to SWIR, MWIR and LWIR I have seen various definitions of the wavelengths that they cover. If you look at the 1980’s AGEMA cooled thermal cameras and check out their lenses…. They clearly state “SW” for SWIR on them. This is because, at the time, AGEMA considered what we now know as MWIR (3um to 5um) to be Shortwave IR ! That was soon changed to the common definitions of SWIR, MWIR and LWIR in later camera series though.

I suspect it is a little like the RF spectrum where the boundaries between HF, VHF and UHF have varied over the years. Where the JW telescope is concerned, we might well be seeing a scientists view of the spectrum that differs to the more common definitions that we use. In truth, provided the wavelength boundaries are detailed in the same documents that contain the band identity, all is well. They could call the MWIR band the Unicorn band if they liked, so long as they also detailed the wavelengths that the Unicorn band encompasses.

You raise a very good point though. Have the E.M band boundaries become more fluid these days ?

Fraser

[Fraser]

Just reading the NASA spec sheet for the JW Telescope, I do believe we are seeing a scientists view of the IR spectrum coverage.

https://webb.nasa.gov/content/about/faqs/facts.html

https://webb.nasa.gov/content/observatory/instruments/nircam.html

https://webb.nasa.gov/content/observatory/instruments/nirspec.html

https://webb.nasa.gov/content/observatory/instruments/miri.html

https://webb.nasa.gov/content/observatory/instruments/fgs.html

Note we do not see the bands detailed as SWIR, MWIR or LWIR in that specification. They refer to the covered bands as “Near-Infrared” and “Mid-Infrared” They also avoid the word “band’ and use “Region” instead. I believe this is terminology specific to the JW telescope project to delineate the Infrared sensitive instrument packages capabilities within the project. The exact wavelength coverage is also provided to avoid any confusion. This appears to follow my “Unicorn” band example where a project split up spectrum coverage for a particular project into bands of their chosen definition and then detail what those bands actually cover. This avoids complications where an instruments sensitivity may cross traditional band boundaries yet not cover the complete associated band.

I believe I have a camera that demonstrates exactly this situation….. my FLIR SC4000 cooled science camera is a special version that covers the wavelengths 1.5 to 5um. What do you call that camera in terms of band coverage ? I call it a SWIR/MWIR broadband camera. It does not cover the whole SWIR band though so such a definition could be misleading if the full wavelength specifications were not also provided.

Note that FLIR still call the SC4000 broadband (1.5um - 5um) model a MWIR camera.

https://assets.tequipment.net/assets/1/26/Documents/HS_Brochure.pdf

Fraser

[Vipitis]

You will find different ranges and naming schemes on Wikipedia as well. One that is easy to search by is band names https://en.wikipedia.org/wiki/Photometric_system

LWIR is equal to N-band, so if you look up 'ground based N-band observations using uncooled microbolometers' on Google scholar you actually find results of groups using consumer cameras or modified cores.

[IR_Geek]

It's very sad that there is not a universal naming convention, but it really comes about because of the multiple fields that utilize the EO/IR spectrum.   CIE recomends a FAR UV, UV-C, UV-B, UV-A, VIS, IR-A, IR-B, and IR-C;    Astrophysics has Extreme UV, Lya, FarUV, MidUV, NearUV, VIS, NIR, Mid-IR and Far Infrared (FIR);  Atmospheric Science has UV-C, UV-B, UV-A, VIS, NIR, IR, Thermal Infrared, and Far Infrared;  Military is Vacuum UV, UV-C, UV-B, Solar Blind UV, UV-A, VIS, NIR, SWIR, MWIR, LWIR, and FIR    That's just the four groups I can remember off the top of my head.       My Avatar picture makes sense now      As you can imagine, almost none of them define the spectral bands the same way ... even when the name is identical.

From what I can tell, most IR camera vendors lean toward the military convention at least MW and LW.

Side note:  Inframetrics did something similar to Agema.   Their 760's were name SW, LW, and BB for their cameras.  However the SW was nominal 3-5um, LW was nominal 8-12um, and the BB was a nominal 3-12um

[Fraser]

IR_Geek,

Very interesting, Thank you 

Regarding Inframetrics.. yes I has seem “SW” and “LW” on the cases of my Inframetrics lenses. To find a Broad-band version was great as I could use it on any thermal camera I wished I have a few that provide both the SW and LW transmission figures, indicating that they are Broad-band versions.

Fraser

[Bill W]

For earth-bound uses of course the water absorption factors then to split the bands up as well.

Bill

[DaJMasta]

For earth-bound uses of course the water absorption factors then to split the bands up as well.

Bill

This.  When looking as an astronomer through vacuum, you've got largely unimpeded radiation through the whole spectrum, so there's no need for the in-atmosphere distinction between our SWIR/MWIR bands and the MWIR/LWIR bands, both of which are pretty clearly delineated by high absorption points in the atmosphere.

For an astronomer, the entire IR band is basically just red shift anyways.

[Ben321]

For earth-bound uses of course the water absorption factors then to split the bands up as well.

Bill

This.  When looking as an astronomer through vacuum, you've got largely unimpeded radiation through the whole spectrum, so there's no need for the in-atmosphere distinction between our SWIR/MWIR bands and the MWIR/LWIR bands, both of which are pretty clearly delineated by high absorption points in the atmosphere.

For an astronomer, the entire IR band is basically just red shift anyways.

The standard NIR/SWIR/MWIR/LWIR bands make sense (even when in space), due to the properties of those wavelengths. NIR is mostly invisible (more so as it gets closer to SWIR) and it is absorbed differently than visible light (you can see through some black inks with an NIR modified video camera). It can also see through certain types of plastic that appear opaque in the visible part of the spectrum (like that black plastic cover over the NIR LED in your TV remote). In the next longer IR band, SWIR is associated with some very interesting phenomenon, including air glow, which is a type of phosphorescence (the atmosphere absorbs some light from the sun in the day, and slowly loses that energy at night by emitting SWIR radiation). I've read that the military has been experimenting with SWIR cameras for night vision (using SWIR camera's detecting air-glow as a non-thermal-imager alternative to conventional NV scopes, for use in absolute darkness like when it's cloudy so no star-light reaches the ground). SWIR can also go through things that even NIR can't go through like layers of paint (art examiners can use SWIR cameras to see if there's an older painting hidden below the current painting).  SWIR and NIR (like visible light) are mostly reflected from objects, rather than being emitted by those objects. However, this changes in MWIR. In the MWIR band, some of the imaging of objects is the result of energy reflected from those objects, but also at temperatures just slightly above room temperature objects will begin to emit MWIR light. I think some air-glow also occurs in the MWIR band. In the LWIR band, almost all of the radiation is emitted from objects, not reflected. LWIR and MWIR can't pass through ordinary glass. Imaging in these wavelengths requires special materials to be used for the optics (these are more exotic substances like germanium). LWIR (and possibly MWIR) can pass though some commonly available materials though, at least as long as the piece of material in question is thin enough (including some objects that are opaque to visible light). That's how the Boston marathon bomber was caught by the police, even though he was hiding under a visually opaque tarp.

So yes, those different IR bands do have distinct properties that make them well suited to be placed in those bands. While obviously those uses for those bands I described are done with earth-based cameras, not for space viewing, I'm guessing that various objects in space may (like the different objects on earth I described) have optical behaviors that differ in similar ways to the earth-based objects at those wavelenghts. For example a cloud of gas in space (a nebula) depending on what the exact chemical composition of it is (hydrogen, helium, something else, some combination of these), would likely appear differently when viewed in NIR, SWIR, MWIR, or LWIR bands. So I think it still makes sense to keep those standard 4 bands, unless there's a compelling reason to use the astronomy 3-band system (NIR, MIR, FIR). Before reading about the JWT, I had always assumed that NIR in space based cameras corresponded to NIR as used on earth-based cameras, MIR was the same as MWIR, and FIR was the same as LWIR. But as it turns out.

But for some reason, NIR in space is not the same as NIR on earth (space variant of NIR imaging actually also covering SWIR and even some MWIR). And MIR covering the rest of MWIR and LWIR, and wavelengths longer than any LWIR camera on earth could detect (no earth-based LWIR cameras are designed to see such long wavelengths of IR). Then there's the FIR (far infrared) band. While I don't think cameras on the JWT can detect radiation in this band, I remember reading a while ago about another IR space telescope that could see FIR. If I remember right that was like anything over 50000nm or 100000nm. So basically FIR is the upper part of the IR spectrum just before getting to the highest frequencies of microwave radiation (mm-wave or THz band).

[Vipitis]

What dictates the partition in astronomy is absorption (and transmission) lines of different elements. See the wikipedia article. There are harmonics to most emission lines which are distinguish by giving them alpha, beta, ... suffixes

It's simply the best to give a measurement of wavelength in å, nm, μm, cm... Or a frequency in Hz