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Doubts about UVIF

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#1 Bernard Foot

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Posted 16 November 2020 - 21:37

I'm keen to try some UVIF work over the coming dark months. I have dabbled, and have looked at other people's work, and there is something that worries me - is the image really showing fluorescence? The reason for this doubt is that often the colours in the UVIF image are similar to the natural, visible light colours.

Here is an example that shows what is troubling me. It is a white plastic tray with a colour design on it, photographed in visible and with UVIF. (For UVIF I used a Nemo torch, with a Baader U added for good measure. Camera was a standard visible-only model. WB was against the white leaf on the wallpaper behind the tray.)

Attached Image: IMG_7835 LoRes.jpg

Attached Image: IMG_7834 WB wallpaper white lea LoRes.jpg

The plastic surrounding lip and handle of the tray are dark and so not fluorescing very much, but the bottom with the design is clearly another material, which fluoresces well. But the striking thing is that the colours of the image are essentially the same as their visible light appearance. The chance that those dyes would fluoresce with the same colour as their visible transmission has got to be zero. So I assume that what we're seing is the fluorescing base just providing a white backlight to the coloured image and so we're just seeing those coloured dyes with visible light.

So this does not seem to me to really be a UVIF image - there is certainly some fluorescence there but most of the image is just a visible light image. And I think the same applies to many of the UVIF images I have tried or seen.

What is your opinion?
Bernard Foot

#2 nfoto

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Posted 16 November 2020 - 21:42

Whatever fluorescence the pattern has is probably drowned in the stronger reflected light from the substrate (the tray itself).

Perhaps narrower bandwidth used for the camera capturing the image could help?

#3 Bernard Foot

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Posted 16 November 2020 - 21:46

View Postnfoto, on 16 November 2020 - 21:42, said:

Perhaps narrower bandwidth used for the camera capturing the image could help?

Sorry - don't understand what you mean by this. Could you explain a bit more?
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#4 nfoto

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Posted 16 November 2020 - 21:49

Sorry about being unclear - it's late over here -- I was think about using say a blue, green, or red filter; alternatively, narrow bandpass filters within the visible spectrum to try to separate "noise" from "signal" so to speak.

#5 Stefano

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Posted 16 November 2020 - 22:09

I think that if you white balance the image on the blue tray, it will become very similar to the visible one.

I too noticed that UVIVF colors sometimes are very similar to VIS colors. The best example was a piece of newspaper, it was pinkish-brownish in color, and under UV it looked exactly like under white light. I don't have it anymore, but if I find something similar I may post images.

Sometimes infrared longpass filters emit a little red fluorescence. I saw this on a piece of black plastic which is transparent to IR, and Hoya R72 also has some deep red/far red fluorescence.

#6 dabateman

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Posted 16 November 2020 - 23:26

Bernard.
It is fluorescent. Its more of a physivs definition than guess. If you shine a UV light on it. That will excite something, moving it up an energy level. Upon relaxation, it will emit light as it comes down, and or heat.
You will know its fluorescent if its lower energy than input light, longer wavelength, more red like, or red shifted.
So input 365nm, output 450nm with the glow is fluorescence. Depending on what it is will output different low energy light. I like 365nm in and greater than 720nm out. But your lucky if its greater than 500nm.

405nm light in can actually be better at times, but will need a long pass 500nm filter to block.
Cyan (488nm input) is also good. Common wavelength in microscopy. Commonly see 488nm in and 515nm out in cell systems.

Edited by dabateman, 16 November 2020 - 23:29.


#7 Andy Perrin

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Posted 17 November 2020 - 03:16

UVIVF fluorescence IS visible light, so I would expect it would be partially absorbed by the dyes in exactly the same way as reflected visible light, even though it is emitted instead of reflected. That would give something similar to the same colors as reflected if you white balanced appropriately. That does NOT mean it isn't fluorescence.

It's very much the same situation as comparing light reflected off a paper book to a backlit laptop. The laptop is emitting the light through the LCD, and it can give almost the same colors effect as shining the light on the book. It doesn't matter that one is emitted from the rear and then absorbed, rather then reflected off the page with absorption.

Edited by Andy Perrin, 17 November 2020 - 03:17.


#8 Stefano

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Posted 17 November 2020 - 03:21

That means the blue fluorescence from the plastic peaks in the blue but has a large bandwidth and contains even red light. Kind of like a cool white LED.

#9 Andy Perrin

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Posted 17 November 2020 - 03:29

View PostStefano, on 17 November 2020 - 03:21, said:

That means the blue fluorescence from the plastic peaks in the blue but has a large bandwidth and contains even red light. Kind of like a cool white LED.
Possibly, but it's also possible there is contaminating reflected visible light. I suspect you are right though.

Edited by Andy Perrin, 17 November 2020 - 03:29.


#10 Cadmium

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Posted 17 November 2020 - 04:11

First, we assume your room is absolutely dark with no other lights except your Convoy torch.
You have a Convoy S2+ 365 Nichia LED, with a Baader U in front of it cutting any possible violet/blue visible light from illuminating your subject,
only UV light under 400nm is emitted from your torch.
Your camera is stock, but that doesn't mean that it can't see some UV below 400nm, if it does then it can see reflected UV,
so the lens should be filtered with some barrier filter that blocks UV and light below 420nm/435nm.
If no reflected UV off the subject is entering your lens, then you have pure visual UV.
The only possible weakness of your setup that I can think of would be reflected UV entering your camera,
so filter your lens, try Wratten 2E, or Zeiss T* + GG435 see if that changes anything.

Edited by Cadmium, 17 November 2020 - 04:15.


#11 Stefano

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Posted 17 November 2020 - 04:40

Steve, he used a Nemo torch, not a Convoy S2+. This doesn't change things of course, just a detail.

I don't know how reflected UV can affect the results. For good measure (as Bernard said about putting a Baader U on the torch), he may put a longpass filter as you suggested. Probably there wouldn't be a lot of difference, especially if the fluorescence is bright, but Bernard, if you can give it a try. This way we are sure your image is purely fluorescence.

#12 UlfW

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Posted 17 November 2020 - 05:06

Yes Steve has a valid point here. If the "Camera was a standard visible-only model." is marginally sensitive to UV there might be some UV-contamination in the picture.
I think the white area at the bottom of the tray is fluorescing, acting in a way as a secondary light source in the image.
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#13 Cadmium

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Posted 17 November 2020 - 05:38

Bernard, if your camera lens is not filtered, then you need to try that.
If your light source is not emitting anything above 400nm, and your lens is not seeing anything below 400nm, then there is no reflected light, except what Ulf mentions, but you first need to try a lens filter.
I suggest using a Wratten 2E, easy to find, cheap, good test. See if that changes anything.

#14 colinbm

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Posted 17 November 2020 - 08:26

Bernard I agree that blocking low 400s could be beneficial.
I am wondering at the WB point that you have chosen ?
I am intrigued at the different whites in the dish, the white exterior of the dish & the white margin with the writing.
I am wondering if the WB can be selected on the white with the writing ?
What looked the most white to your eye when taking the photo ?
It can be fun & artristic to try different WBs, too....

Edited by colinbm, 17 November 2020 - 08:27.


#15 Bernard Foot

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Posted 17 November 2020 - 22:54

Thanks for all your thoughts on this.

To the comments that this was indeed fluorescent, I'm not really disagreeing. What I was trying to say was that the white background clearly was fluorescence (with a broad spectrum, as mentioned in one pf the posts), but the colours from the image were not themselves fluorescing - rather we are seing them in visible light (produced by the fluorescing background). So what's concerning me is that we can't be sure whether the colours we see in UVIF shots are caused directly by fluorescence or are the result of being seen in visible light generated by fluorescence elsewhere.

In terms of WB, WB-ing on the white background of the tray gives a very blue cast. WB-ing on the dark surrounds of the tray again gives colours similar to visible.

Re. the 2 different plastics in the tray - although it is difficult to see a join, I am guessing that the bright background is some sort of a printed decal or inlay, made of a material different to the rest of the tray.

Re. the possibility of the camera having some residual Near-UV sensitivity, I don't have a filter to check this out. A UV/IR cut isn't going to provide a sharp enough cut. Wratten 2Es on ebay are not cheap at the moment (at least not by my definition of cheap) - about $50 for a 2" square gelatin filter. I guess I could just try pointing the UV torch into the camera lens to see how wll the camera can see the light source.

Re. was the room dark enough to ensure no visible light pollution? Yes - it was night, no lights on, surtains drawn, using the leg well of my desk as a cubby hole. And the exposure was short - 2 second, ISO 100, f/8 (that Nemo torch really is very bright).
Bernard Foot

#16 dabateman

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Posted 17 November 2020 - 23:04

Bernard,
They still make 2E filters. You might be able to get one from a camera store online. I got my 52mm Tiffen 2E from akEH for $5. B&H had them couple years ago at about $20.
You could also get a 2A filter, those also should be around.

A #3
https://www.bhphotov...ht_Yellow_3.htm
Can work too

The 2Es seem,expensive as special order:
https://www.bhphotov..._x_12_Pale.html

Edited by dabateman, 17 November 2020 - 23:11.


#17 Bernard Foot

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Posted 17 November 2020 - 23:09

Just tried pointing the camera at the Nemo torch. Same exposure as for the UVIF image - 2 secs (hand-held), ISO 100, f/8.

The camera can see the light coming out of the torch, but it is very weak and certainly not enough to form a non-fluorescent image by. (the dust specks show up well, though.)

Attached Image: IMG_7837 LoRes.jpg
Bernard Foot

#18 Bernard Foot

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Posted 17 November 2020 - 23:12

Thanks, David.

Yes, I should look around for non-Wratten 2Es. The challenge might be finding one in the UK or that has sensible shipping costs from the US.
Bernard Foot

#19 Cadmium

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Posted 18 November 2020 - 01:39

Bernard, even $50 is cheap, not as cheap as I might have guessed, but cheap compared to a Zeiss T* I think, (did you check the prices on those?) or whatever other filter.
The 2E is still the cheapest solution to the lens UV blocking solution that I can think of, and THAT is what you need to add to your equation if you want to eliminate that remaining question in your UVIVF shots.
Another is the old out of production Schott KV-418, it is perfect for most.
Here is a 52mm Zeiss T*, not that expensive as I had thought.
https://www.bhphotov...ASABEgKn9_D_BwE
You might want to stack that with something 420/435nm though... so.
Best lease expensive test is the 2E.

Your torch test photo above:
Your camera should not be seeing anything. Block UV from entering the lens.

Edited by Cadmium, 18 November 2020 - 01:42.


#20 Andy Perrin

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Posted 18 November 2020 - 07:08

View Postdabateman, on 17 November 2020 - 23:04, said:

Bernard,
They still make 2E filters. You might be able to get one from a camera store online. I got my 52mm Tiffen 2E from akEH for $5. B&H had them couple years ago at about $20.
You could also get a 2A filter, those also should be around.

A #3
https://www.bhphotov...ht_Yellow_3.htm
Can work too

The 2Es seem,expensive as special order:
https://www.bhphotov..._x_12_Pale.html

I had some questions -
1) What is the spectrum of the 2E exactly?
2) Does a 2E from, say Tiffen have the same spectrum as a 2E from Kodak/Wratten? What exactly does "2E" mean?
3) Does the 2E offer any kind of infrared blocking? (Could we be seeing any UVIIF in these photos?)