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UltravioletPhotography

Dark water and the NIR absorption peak


Andy Perrin

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Water has several absorption peaks in the infrared. The first of these is at 976nm, the second at 1205nm, and the third at 1453nm. The objective here will be to photograph things at the first of the three peaks. (Part of the third peak has been shown in my prior work in SWIR, although I have not yet tried photographing using a bandpass exactly at that peak, as we will do in this case.)

 

The filter is the Thorlabs 980nm bandpass with FWHM = 10nm, abbreviated (by me) as "980BP10" in the graph below. When reading this graph, note that the absorption coefficient axis is logarithmic.

post-94-0-45265400-1568741809.png

 

Here is a first image. Bear in mind that the "darkness" of the water depends quite a bit on how you process the image, so without some other photo with a different filter as a control, it's hard to tell how much difference it makes.

post-94-0-67509500-1568741917.jpg

 

This was just a quick-and-dirty first shot, and as you can see, I have a light leak around the edge of the filter to fix. I plan to add onto this thread in coming days.

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I don’t have it on me but it was something like F5 0.01” ISO3200. Shot hand-held with no trouble on my Sony.

 

ETA: It was F5 1/60" ISO3200.

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Even the RG1000 alone incorporates the 980 bump, 50% is about 975nm, and up from there, and has much darker water than does the RG850.

It does, but it also includes area not in the bump, which tends to wash out the dark areas (same any other leak). The reason it doesn't totally wash them out is just because the camera sensitivity goes down so fast in that region. Better to use a bandpass and get just the bump.

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Alcohol (specifically poitín, "Irish moonshine", 80 proof = 40%) on the left, and water on the right. Poitín is clear like water in visible light.

post-94-0-33407400-1568954768.jpg

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How interesting!! I didn't know about those peaks.... Thanks for the knowledge! :smile:

although anyway i don't have narrowband filters to try) But if i take 950nm IR filter and the camera "sees" hardly much more than 1000nm maybe I can to see this effect

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:grin:

I tried too. thanks for the new fun)))

 

post-237-0-64218600-1569063566.jpg

 

I should have poured more water) hurried

in a glass even less water, even less noticeable darkness. water on the left, alcohol on the right. The fly Drosophila prefer alcohol :smile: :bee:

 

post-237-0-02029900-1569063872.jpg

 

I wondered if I could use this property of water for landscape photography so clouds are water. i made a comparison of 950nm and 850 nm. Undoubtedly there is the difference, but it is small. and a giant increase in time of exposure.

 

post-237-0-66321000-1569064167.jpg

 

Andy Perrin,

is there a cloud effect with your filter? it should be more noticeable

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No major cloud effect and I would not expect there to be because clouds are usually ice crystals (not liquid water) and ice has a different spectrum. Also, I wonder if the scattering might outweigh the absorption anyway for clouds? Not sure.

 

My data on ice doesn't include this part of the spectrum but does note in the text that ice peaks are shifted to longer wavelength relative to water peaks. Here's a direct experiment:

 

Water alone and ice alone. The ice was taken directly from the freezer and photographed as fast as possible to prevent any significant film of water building up.

post-94-0-85773500-1569086753.jpg

 

Here is an ice cube in the water:

post-94-0-59856800-1569086812.jpg

 

Again, the exact darkness of the water is down to contrast adjustment, etc. but you can see a huge difference with the ice in the same photo.

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How interesting! the ice does not darken. Thanks for the illustration!

You are right about clouds, but not absolutely. High clouds are ice crystals. or when it's cold weather. but low clouds are liquid water drops in the warm season. perhaps they can darken in this range spectrum. at least it can clearly be seen at my picture above that the clouds are a few darker with the filter 950nm than with 850 nm. although it seems hardly worth while multiple increasing the exposure time with a filter 950nm to get such minor differences)

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With droplets, reflections will probably be significant. I am going to try using a polarizer if I can find one for IR and that may make them darker.
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