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

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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.    :phew:   My Avatar picture makes sense now  :-DD    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

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