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UltravioletPhotography

Snow in the SWIR redux


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I finally redid my pano today, thanks to the overcast weather. (Link to previous attempt) When the sun is out, my mere 10 bits of dynamic range are not enough to capture everything in the photo. It becomes like trying to photograph the moon AND the stars simultaneously -- pick one!

 

Even so, there was enough variation in the photo that I had some trouble stitching the bottom edge with all that black snow. The snow seemed much darker today, which is probably a sign of the thaw, since it was well above 0 deg C.

 

Details are the same as the last attempt except that I stitched in Hugin.

 

post-94-0-87525500-1608605660.jpg

 

Visible (today)

post-94-0-15872400-1608605682.jpg

 

Previous pano for comparison purposes. We have had additional snow + melting in the interim.

post-94-0-31963100-1608605714.jpg

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Andy, very nice, much better than your first attempt. Seeing those black stacks of snow in the street edges/corners is so nice and odd. The world really is a different place in SWIR.
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Here is the 980nm version.

post-94-0-10730400-1608592228.jpg

 

Repeating the above 1500nm version here for comparison.

post-94-0-52131500-1608592340.jpg

 

And the ~10000nm version (taken during daytime, so not strictly thermal IR, just LWIR). This was taken the day after the first storm, so not quite identical to the above two.

post-94-0-16476600-1608592573.jpg

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Andy the first post here seems to have the photos deleted. but the post above I see stuff so get an idea.

 

For that coal look need 1500nm, that 980 filter will not cut it.

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Weeeeird. I reuploaded the photos because they self-deleted earlier also. I wonder if our database problems aren't over yet. I will try to repair it now.

--

 

Yes, definitely you won't get black snow at 980nm. There is an additional water peak at 1205nm or so that I haven't tried yet. I wonder if you get gray snow?

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We had some yellow snow, but its mostly all melted now.

 

I see the photos now. Black snow is interesting. You seem to be at the edge of teasing out water vs ice vs snow with these bands. You could maybe try an odd test in a glass or plastic to get the layers.

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We had some yellow snow, but its mostly all melted now.

 

I see the photos now. Black snow is interesting. You seem to be at the edge of teasing out water vs ice vs snow with these bands. You could maybe try an odd test in a glass or plastic to get the layers.

Just don't eat the yellow kind.

 

With regard to your second comment, I would one day like to try a tri-color with 980nm, 1205nm, and 1450nm which are (in round numbers) the three water peaks. That might do the job. I don't have a 1205nm bandpass filter though.

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With regard to your second comment, I would one day like to try a tri-color with 980nm, 1205nm, and 1450nm which are (in round numbers) the three water peaks. That might do the job. I don't have a 1205nm bandpass filter though.

I'm so interested in this. For the 1205 nm peak a 1200 nm bandpass would work Ok, as that peak is also quite broad.

 

Would you use your TriWave for all shots, including the 980 nm one, to make images perfectly (or almost) stackable?

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Oh, it would have to be the TriWave for the reason you said. The resolution isn't the same on the other camera, and the sensor size is different, so there would need to be a lot of manipulation otherwise.
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Andy,

Its just as bad to eat the grey/black kind. Especially if yellow is next to it.

 

Funny how my desire to image water on the other end (185nm) is just as hard as on your end at 1500nm. Although, we may both be there now. I still need a good light.

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Sometimes SWIR reminds me of UVC. You need special lenses for UVC, and not all lenses are good in SWIR (most are, it seems). UVC and SWIR LEDs have similar efficiencies, power and prices. Skin is black in both bands, things start to absorb again in SWIR (and most things absorb UVC), the atmosphere is opaque to UVC and starts to be opaque in SWIR, and of course water absorbs well in both bands (especially below 200 nm, the absorption rises very quickly). Going far in both directions is difficult.

 

David, even a 10 nm decrement in wavelength would probably do a lot. Water started to appear dark at 185 nm, but by 165/170 nm it should be pitch black. Look here: https://en.m.wikipedia.org/wiki/File:Absorption_spectrum_of_liquid_water.png

 

Also, you may want to use distilled water or as pure of a water as you can get. Down there even tiny amounts of impurities probably absorb a lot.

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I have ordered filters at the following wavelengths:

1200nm

1250nm (it was a group of two in the eBay listing, so this is a “freebie”)

1450nm

 

All are 10 to 12nm bandpass.

 

All are originally from Thorlabs so should be correctly labeled. 1450 is the actual water absorption peak.

 

These should allow head-to-head comparisons after I normalize with some kind of white standard.

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Spectralon works well in SWIR too, so if you have some you can use that as your white standard.

 

The 1450 nm filter will be the most interesting. That’s the main wavelength that makes SWIR what it is. Black water is one of the main reasons why someone would want to reach that band.

 

I think it could become your standard SWIR filter if you are looking for water-related patterns or stains. Ice peaks higher, but water will be as dark as you can get it with your camera. The next stronger peak is at about 1900-1950 nm, outside your camera’s range.

 

Who knows, one day you may have a MWIR camera or a camera that can see in the 2000-3000 nm range. After about 3000 nm (this is a very arbitrary and broad cut-off) things start glowing, so darkness will be difficult to achieve. But you may catch the very strong ~2950 nm peak.

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  • 1 year later...
Daniel Csati

These narrow bandpass filters are dirty expensive, especially if a larger size is needed. They are interference filters, so a filter designed for lets say 500nm should transmit a lot at 1000nm, 1500nm.. too. Maybe a less expensive astronomy filter for 500nm (O-III line) would work nice for camera which can see 500nm with lower efficiency anyway.

There is a sharp water absorption around 940nm too so a "cheap" 500nm bandpass interference filter could - just about - work for two water absorption peaks simultanously. Not for the 1250nm tho. Anyone tried this before?

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Andy Perrin

Daniel, the Thorlabs filters are well-blocked at nearly every wavelength even in IR. If you find a cheap brand with a SWIR “hole” let us know!
 

The sharp water absorption is at 976nm or so, not 940nm, and I’ve already posted many pictures with a 980nm filter (10nm bandwidth) here on the board. 
 

Please see this thread below, and also Stefano’s page on water absorption! We have covered this topic extensively.

 

 

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Daniel Csati

I see, 12mm filters are still affordable, makes sense to get it from Thorlabs. 

My bad, I remembered for some reason 940nm, all these typical high power laser doide wavelengths.

Anyway, 980nm is even closer to the second order transmission. I plan to build a NIR-SWIR spectrometer in the future, maybe I can measure how suitable are these for larger sensors. 

 

I'll have a look at the 980nm photos, thanks for the reference

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