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UltravioletPhotography

Buttercups show SWIR patterns that match UV!


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Andy Perrin

I found that a buttercup (exact species of Ranunculus to be determined) shows patterns in SWIR that match the ultraviolet pattern.

 

First the visible image, taken with the Sony A7S using BG38 2mm under an "energy saving" halogen light. WB was off PTFE.

F16 0.8" ISO50

post-94-0-31317700-1560133640.jpg

 

Next the UV image, same camera, using S8612 1.75mm + UG11 2mm and a Convy S2+ torch. WB again off PTFE.

F16 6" ISO1000

post-94-0-38384500-1560133797.jpg

 

Then the standard, featureless NIR rendition using a nameless Chinese IR1000 filter (obviously the exact cutoff is unknown since it's a no-name). Halogen again.

F16 2.5" ISO80

post-94-0-79974000-1560133850.jpg

 

Finally the SWIR results using the TriWave Ge-CMOS camera under halogen. 1500-1600nm Thorlabs bandpass filter

post-94-0-86665300-1560134152.jpg

post-94-0-41789900-1560134167.jpg

post-94-0-25773300-1560134179.jpg

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How interesting. I wonder if this the same compounds that are absorbing in the UV and the SWIR, or if it different things which are present in the same areas? Could there be more water in those parts of the flower, leading to the greater absorption in the SWIR?
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Andy Perrin
Someone posted a paper on here awhile back about showing that Raniculus's colors come from physical interference effects between layers, rather than pigments (mostly, anyway). I suspect that the same structural color thing might be going on here too. Sometimes interference effects repeat at multiples of the wavelength, like how the colors on oily water repeat themselves.
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  • 3 weeks later...

This is fascinating. The darker SWIR area matches the UV-absorbing area identically it seems. And there is also a slightly darker matching area in the NIR version.

 

Added:

This might be the paper you mentioned. It discusses the structural color properties of Buttercups.

https://www.ncbi.nlm...les/PMC5332578/

Paper also mentions that UV-absorbing areas prevent UV reflection which might damage reproductive cells (pollen).

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Andy Perrin

Andrea, yes, that's the paper.

 

So far there is no general way to predict the SWIR flower patterns from other regions of the spectrum. I have seen SWIR patterns that match UV as here, ones that match NIR (like morning glory), and ones that match neither (daisy fleabane).

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That is a very interesting observation about the SWIR patterns. Does make me want to dig into investigating it. Wonder what it could possibly mean?
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Andy Perrin

Well one thing is for sure: it has nothing to do with pollination, for the simple reason that pollinators cannot see it. Water absorbs in this band, which causes eyeballs to be black, so no light would make it to the retina. (Look at my selfie, for example.)

post-94-0-64152200-1562093355.jpg

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