Andy Perrin Posted September 17, 2019 Share Posted September 17, 2019 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. 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. 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. Link to comment
Cadmium Posted September 17, 2019 Share Posted September 17, 2019 Andy, Looks interesting. What was the exposure time and setting? Link to comment
Andy Perrin Posted September 17, 2019 Author Share Posted September 17, 2019 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. Link to comment
Cadmium Posted September 17, 2019 Share Posted September 17, 2019 Oh that's fast!I did this once, but you can see how slow it was. The stack creates a kind of bandpass, but way less amplitude. Link to comment
Andy Perrin Posted September 17, 2019 Author Share Posted September 17, 2019 Hah, wow, yes, that is much slower. If you push the filter peak too far past the 976nm point, it actually should get lighter, according to the water graph. The next water peak is not until 1205nm. My filter is this one:https://www.thorlabs.com/thorproduct.cfm?partnumber=FB980-10 Link to comment
Cadmium Posted September 18, 2019 Share Posted September 18, 2019 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. Link to comment
Andy Perrin Posted September 18, 2019 Author Share Posted September 18, 2019 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. Link to comment
Andy Perrin Posted September 20, 2019 Author Share Posted September 20, 2019 Alcohol (specifically poitín, "Irish moonshine", 80 proof = 40%) on the left, and water on the right. Poitín is clear like water in visible light. Link to comment
ins13 Posted September 20, 2019 Share Posted September 20, 2019 How interesting!! I didn't know about those peaks.... Thanks for the knowledge! 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 Link to comment
Andy Perrin Posted September 20, 2019 Author Share Posted September 20, 2019 Yes you can get it (Cadmium did). Link to comment
Andy Perrin Posted September 20, 2019 Author Share Posted September 20, 2019 Well THAT'S disturbing. Link to comment
ins13 Posted September 21, 2019 Share Posted September 21, 2019 It's terrible and wonderful !!!! like an old bw horror movie Link to comment
Andy Perrin Posted September 21, 2019 Author Share Posted September 21, 2019 Hah, yes! I want to do a fountain next. Or a waterfall. This was a test for one of those. Link to comment
dabateman Posted September 21, 2019 Share Posted September 21, 2019 Such a waste of chocolate syrup.Psycho should have used this filter. Interesting, idea. Fountains may look odd. Link to comment
ins13 Posted September 21, 2019 Share Posted September 21, 2019 I tried too. thanks for the new fun))) I should have poured more water) hurriedin a glass even less water, even less noticeable darkness. water on the left, alcohol on the right. The fly Drosophila prefer alcohol 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. Andy Perrin,is there a cloud effect with your filter? it should be more noticeable Link to comment
Andy Perrin Posted September 21, 2019 Author Share Posted September 21, 2019 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. Here is an ice cube in the water: 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. Link to comment
ins13 Posted September 25, 2019 Share Posted September 25, 2019 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) Link to comment
Andy Perrin Posted September 25, 2019 Author Share Posted September 25, 2019 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. Link to comment
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