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Iris birefringence (polarization, visible and IR)

Infrared
22 replies to this topic

#1 Andy Perrin

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Posted 23 January 2021 - 02:53

After reading the Petapixel article dabateman linked, I had to run off and try it immediately, so here you go. Beauty is in the eye of the beholder, but the eye you behold is mine.

Equipment
Linear polaroid film taped over a ring light (Nikon macro speedlight SB-29),
Sony A7S modified,
BG38 2mm + Tiffen 2E for visible light photos + Tiffen circular polarizer,
Micro-Nikkor 55mm/2.8 (old manual version).

Flash at ~5 degrees to camera line of sight
Attached Image: _DSC0027 PS UVP.jpg

Another shot
Attached Image: _DSC0035 UVP.jpg

Flash at 45 degrees to camera line of sight
Attached Image: _DSC0020 PS adjust.jpg

UPDATE: I tried again using a Tiffen 12 + polarizer, followed by the usual Aerochrome-style channel subtraction and swapping. Got this result. I think you can't see the effect as well in IR, although that may be because the polarizer doesn't work quite as well there? This is mostly red and 700nm IR I think, so not very deep in NIR.
Attached Image: _DSC0826 RAW aerochrome UVP.jpg

UPDATE2: I took another visible light (BG38 2mm + Tiffen polarizer on the camera) image from a profile view.
Attached Image: _DSC1093_res UVP.jpg

Edited by Andy Perrin, 25 January 2021 - 20:43.


#2 Stefano

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Posted 23 January 2021 - 03:04

Nice. I didn’t read the article, just gave it a quick look, but this technique makes for good eyes. I wouldn’t try UV for safety reasons (even if it might look cool), but I think you could try this in IR, maybe it gives a nice effect. Eyes in IR are already “soft”, as in general IR softens almost everything. I have blue eyes in BGR IR, so imagine how they would look like adding this effect too.

#3 colinbm

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Posted 23 January 2021 - 05:21

Scary.....what are the differences please ?

#4 Andy Perrin

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Posted 23 January 2021 - 05:28

Only the angle I held the flash at, colin. The top two images are with the flash almost in line with the camera (so ~5 degrees from the camera's line of sight or so) and they are quite similar to an ordinary photo of my eye. (Colors are a bit brighter but pattern is the same.) The bottom one is with the flash at 45 degrees or so to the camera.

#5 dabateman

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Posted 23 January 2021 - 08:00

I haven't tried it yet. But what I find interesting are the 3 brown spots in your eye near right bottom area. Also one small one in left center edge. I wonder if thats melanin?
I may have to try this. I have been told my bluish eyes change to grey or green depending on the shirt I wear.
I wonder if I can capture that.

#6 Bernard Foot

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Posted 23 January 2021 - 09:25

Really neat photos. They got me to read the article - I'd ignored it at first as I assumed it was an academic treatise, but quite the opposite - very pleasant to read.

Did you try rotating the polarisers? Would that make any difference anyway?
Bernard Foot

#7 UlfW

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Posted 23 January 2021 - 12:25

Very interesting. Eyes are always interesting and these images even more so.

I also have a personal interest in birefringence.
I was involved in developing a measurement tool to analyse the birefringence in CDs and developed some of the hardware in the instrument and tools to adjust it optically in the end of last century.

Low birefringence was important mainly for disc writing optical drives as too much birefringence attenuated the optical signal making it difficult both to read and write the CDs.
The resulting instrument was working very well, but the guys at the sales department were never able to grasp the technical background of birefringence and had big problems promoting it.
Ulf Wilhelmson
Curious and trying to see the invisible.

#8 Andy Perrin

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Posted 23 January 2021 - 14:57

Bernard, you need cross polarized light. All you get from rotating the polarizers is bringing annoying reflections back. And you lose the colors.

Ulf, that's very cool! Sorry sales didn't understand!

Edited by Andy Perrin, 23 January 2021 - 15:49.


#9 Andy Perrin

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Posted 24 January 2021 - 04:12

View PostStefano, on 23 January 2021 - 03:04, said:

Nice. I didn’t read the article, just gave it a quick look, but this technique makes for good eyes. I wouldn’t try UV for safety reasons (even if it might look cool), but I think you could try this in IR, maybe it gives a nice effect. Eyes in IR are already “soft”, as in general IR softens almost everything. I have blue eyes in BGR IR, so imagine how they would look like adding this effect too.
I tried one in Red-Green-IR (Tiffen 12 + polarizer) which I have added up on the main post at the end.

#10 Stefano

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Posted 24 January 2021 - 04:25

There isn’t a rainbow effect as you stated, but the colors are nice. You have a NIR polarizer, but you need two for this.

#11 Andy Perrin

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Posted 24 January 2021 - 04:43

Stefano, it's worse than that -- my NIR polarizer doesn't work in visible light (it's 780-1250nm), so a photo like this that's on the border is just totally out of the question, even if I had another one.

Also, the rainbow effect is there, but it's not as obvious. You can still see that kind of diamond-shaped feature that looks blue in visible light.

Edited by Andy Perrin, 24 January 2021 - 04:50.


#12 Stefano

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Posted 24 January 2021 - 04:48

Yes, now that you pointed it out, I see it. It is pink/magenta.

A UV/VIS/IR polarizer would be pretty handy, but I guess it is expensive.

#13 dabateman

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Posted 24 January 2021 - 06:17

Also Andy what would an IR rainbow look like? Mostly the same to us right. Only of it was spread out linearly, could we cut it up with filters and know what bands we see.
So I have no doubt you see an IR rainbow, its just all red to us. Camera sensor also don't help as it all goes to white with equal absorption above 800nm.


#14 Andy Perrin

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Posted 24 January 2021 - 06:29

David, this is TIffen 12 so you're probably only seeing the 700's as far as IR goes. The rest is red and green light. I would think a rainbow would be pretty similar in that range, since we're nowhere near the >800nm where you lose all colors. My best guess (as mentioned above) is just that my polarizer isn't working very well in the NIR range.

#15 Stefano

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Posted 24 January 2021 - 06:30

David, if you mean an IR rainbow done with the tri-color IR technique (with filters, like Bernard did), then it will look exactly like normal, especially if the filters have some overlap. The same occurs in UV. It is like the sky, with the tri-color technique it is always blue.

Camera sensors can discriminate between 700-800 nm and 800+ nm, very approximately. If you white balance an IR image, the shorter wavelengths will appear yellow and the longer blue, in a very similar way to UV (if you don't consider green at 340 nm and below). That's also why the sky has a yellow tone in both typical UV and IR photos.

Now I realize you meant something different, but still what I said above is valid (at least, I believe this).

Edited by Stefano, 24 January 2021 - 06:44.


#16 microbat52

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Posted 25 January 2021 - 10:12

The third one looks really cool!!
Where these cross polarised 90 degrees?





#17 Andy Perrin

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Posted 25 January 2021 - 17:03

Microbat, nominally 90 degrees but it’s really hard to keep the orientation exact when you are holding the flash up with one hand, trying to keep your head steady, and trying to snap the photo with the other hand! So probably not exactly.

#18 dabateman

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Posted 25 January 2021 - 19:11

Andy an alternative is to light your face with a bright screen of your phone. Just go to a white page and blast the brightness.
The screen of your phone should be polarized and you can angle it around as you take the shots with the camera.

#19 Stefano

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Posted 25 January 2021 - 19:17

View Postdabateman, on 25 January 2021 - 19:11, said:

The screen of your phone should be polarized and you can angle it around as you take the shots with the camera.
If it is an LCD display it will be polarized, but OLEDs aren't.

#20 Andy Perrin

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Posted 25 January 2021 - 20:30

Dabateman, that is WAAAY too dim to get a sharp photo (you would have to expose for a long time or use a too-high ISO). If you haven't tried taking a photo of your eye before, it's not easy if you have insufficient light. Eyes move fast, they blink, etc. Flash is really necessary at macro scale. Also, the depth of field is quite small, so you have to keep your eye/head very still which is almost impossible for more than a fraction of a second.

Stefano, my iPhone 12 Pro Max is OLED and while it's not as polarized as an LCD, it is actually still somewhat polarized. Not enough to help with photography even notwithstanding the above issues.
--

I just added a profile view in visible light to the original post.

Edited by Andy Perrin, 25 January 2021 - 20:44.