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Icicles in 980nm and LWIR

Infrared LWIR Multispectral
9 replies to this topic

#1 Andy Perrin

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Posted 20 December 2020 - 03:37

Here's the side of the building next to me in 980nm and also in LWIR using the thermal camera. The NIR water absorption peak is 976nm, resulting in dark water. But not as dark as SWIR!

The LWIR pic was taken after dark, some hours after the 980nm, so a few icicles fell in the interim.

Equipment
Thorlabs 980BP10 bandpass filter
EL-Nikkor 80mm/5.6 metal
FLIR E60 and tripod

Processing
Contrast adjustment in Photoshop and PhotoNinja for the 980nm pics
Assembly of the pano for the LWIR using Panorama Stitcher (a mac program).

Photos
Attached Image: _DSC8100 small.jpg Attached Image: building next door comparison 1.jpg

Attached Image: _DSC8103 small.jpg Attached Image: building next door comparison 2.jpg

Here's the full wall:
Attached Image: building next door thermal curved.jpg

Edited by Andy Perrin, 20 December 2020 - 03:39.


#2 Stefano

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Posted 20 December 2020 - 03:48

Interesting. Snow isn't dark at 980 nm as expected, but you can see some darkening where it is wet. Water appears dark in LWIR, but these are not reflected LWIR images, they are purely emitted LWIR since these were taken in the dark. Water does absorb LWIR a lot, but LWIR images are hard to interpret sometimes.

Very nice Andy. You are exploring water again, and I like that. In the future you can explore the other peaks, it would be interesting to see the results.

#3 Andy Perrin

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Posted 20 December 2020 - 03:55

Snow and ice don't absorb much at 980nm, as previously shown in other threads, so I wasn't expecting them to be dark. Liquid water IS dark at 980nm, but only if you have enough thickness/depth. On top of that, snow that is dry scatters light very efficiently due to the air gaps, so it's usually light colored at 980 and 1500nm for that reason. All that said, the icicles on the topmost 980nm photo did come out very dark. I think they had liquid water running down the outside.

In LWIR, the snow and ice are probably dark mostly because they're colder than the surroundings, which are emitting (relatively) more light in this case.

Edited by Andy Perrin, 20 December 2020 - 03:55.


#4 Bernard Foot

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Posted 20 December 2020 - 10:28

Interesting Andy. I'll have to try capturing the 976nm peak using my LP1000 filter (which is effectively a 1000nm bandpass filter because the sensor sensitivity descends rapidly to 0 at about 1100nm).

I'm wondering about the box in the window. Looks like an air-conditioner, but I guess it could be a heat exchanger. Either way, it must be cold in there with the window open!
Bernard Foot

#5 Stefano

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Posted 20 December 2020 - 10:54

If your filter starts at 1000 nm you may miss the peak. It's the same as Andy missing the 1450 nm peak with his 1500 nm longpass filter. But water is still dark there, and it came out blue in your tri-color IR images, so it will surely appear dark, but not as dark as with a bandpass filter nailing the peak.

I have a 970-980 nm IR LED, I didn't attach it to an heatsink yet (who knows when I will...) and I tested it running it at low power and water appeared quite dark, definitely not black but dark. I have some images, although I don't know about posting them in this topic.

#6 Bernard Foot

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Posted 20 December 2020 - 11:55

View PostStefano, on 20 December 2020 - 10:54, said:

If your filter starts at 1000 nm you may miss the peak.

It has 33% transmission at 970nm and 38% at 980nm, so I may get something.
Bernard Foot

#7 Bernard Foot

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Posted 20 December 2020 - 14:43

Well, here's the shot comparing water at about 750nm and 1000nm. The glass complained about having to hold water rather than its usual liquid, but needs must ...

Also here is an IR tri-colour shot of a basin of water from some time ago. The blue colouration illustrates the higher absorption through the 1000 nm filter (the red channel) compared to 750 nm (blue) and 850 nm (green).

Attached Image: Split Personality.JPG

Attached Image: IRFC 013 LoRes.jpg
Bernard Foot

#8 Cadmium

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Posted 20 December 2020 - 15:07

Andy, Very nice comparisons. :smile:

#9 Andy Perrin

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Posted 20 December 2020 - 19:40

View PostBernard Foot, on 20 December 2020 - 10:28, said:

Interesting Andy. I'll have to try capturing the 976nm peak using my LP1000 filter (which is effectively a 1000nm bandpass filter because the sensor sensitivity descends rapidly to 0 at about 1100nm).

I'm wondering about the box in the window. Looks like an air-conditioner, but I guess it could be a heat exchanger. Either way, it must be cold in there with the window open!
You can see the 976 peak well with the 1000nm bandpass.

It's definitely a window air conditioner. I have one in my window also. I forget that in England they don't use them much! I don't bother to take mine out for the winter because it's too much trouble to install/uninstall/risk dropping it on someone below.

#10 Andrea B.

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Posted 21 December 2020 - 07:42

Viewed with interest.

Those icicles look fierce.
Andrea G. Blum
Often found hanging out with flowers & bees.