Spectralon fluorescence?

[Andrea B.]

Brief Summary so far -----

 

Spectralon is made from PTFE. When PTFE is pressed under heat, it fuses together to make the targets. So the targets contain no carrier or matrix for the PTFE. For fluorescent targets or grey reflective targets, the fluorescent material or the non-fluorescent, darkening material is added before compression. (Darkening material is XYZ, I think? Like what Jonathan used when making his own targets.) There can be manufacturing impurities. And according to what I have been reading, hydrocarbon contamination is the biggest cause of fluorescence in PTFE/Spectralon materials in the 300-400 nm range.

 

These links are references to PTFE, Spectralon and how it is made, PTFE/Spectralon properties and non-fluorescence of PTFE/Spectralon, hydrocarbon contamination.

 

LabSphere Coatings & Materials

See in particular the mid-page paragraph: Why painted cards, fabrics and paper may put you at a disadvantage.

 

LabSphere Optical Grade Spectralon Material

Clicking this link will download a PDF..

Excerpt: Spectralon exhibits absorbancies at 280 nm, then absorbs strongly from 5.4-8.0 mm (Ed.: not nano-meters).

Exceprt: The open structure of Spectralon causes both reflectance and transmittance, but not absorbance of light.

 

LabSphere Coatings & Materials Solutions Brochure

Clicking this link will download a PDF.

This brochure mentions three grades of Spectralon: Optical Grade, EPV Grade and Space Grade ("ultra-clean").

 

Reflectivity Spectra for Commonly Used Reflectors

Clicking this link will download a PDF.

Labsphere SRS-99 does not fluoresce.

Ocean Optics Spectralon (WS-1-LS) does not fluoresce.

Teflon matte tape from Ace Hardware does not fluoresce.

PTFE glossy tape does not fluoresce.

....and more results.

 

Photoluminescence Response of Acrylic (PMMA) and Polytetrafluroethylene (PTFE) to Ultraviolet Light

PTFE does not fluoresce under 130-250 nm UV light.

Photoluminescence observed from a plastic is not necessarily due to the bulk material itself, but

but can be caused by impurities, defects or additives in the bulk material, or surface contamination.

Many common contaminants, such as oil from fingerprints, can fluoresce.

Residue from the sandpaper could fluoresce.

 

Ultraviolet Reflectance of Microporous PTFE

White Paper by Porex Corporation

Clicking this link will download a PDF.

Very interesting. Learn a bit about reflectance, sintering, PTFE, UV degradation of plastics.

 

Thorlabs High-Reflectance PTFE Sheets

VERY INTERESTING!!

No mention of fluorescence here, but I wanted to point out this PTFE paper.

For (US)$60 you can get a sheet of PTFE paper sized 33cm x 33cm x .75cm (about one square foot?) which is 93% reflective between 250-400 nm. This might make for a very inexpensive, accurate white balance target for reflected UV photography?? You can get it with an adhesive backing and mount it on canvas, a board or something?

We would need to consider how robust PTFE paper is, but this is a heckuva lot cheaper than Spectralon standards, isn't it?

For (US)$1000 you can get a 12m (40ft) roll of this stuff. Geez, I wish this had been available back when I invested in those 5 Spectralon targets.

 

Thorlabs Integrating Spheres

Scroll down to: Ultraviolet and Blue Fluorescence Emitted by Integrating Spheres

However, it should be noted that integrating spheres coated with both PTFE and barium sulfate, which is an alternative coating with lower reflectance, emit low levels of ultraviolet (UV) and blue fluorescence when irradiated by UV light.

It is not the PTFE that fluoresces. The sources of the UV and blue fluorescence are hydrocarbons in the PTFE. Low levels of hydrocarbon impurities are present in the raw coating material, and pollution sources deposit additional hydrocarbon contaminants in the PTFE material of the integrating sphere during its use and storage.

The UV and blue fluorescence from PTFE is primarily excited by incident wavelengths in a 200 nm to 300 nm absorption band.

 

---

 

I would reasonably speculate that the age of our Spectralon targets and their exposure during use plays a role in the degree of post-manufacture contamination we might see in them. I have sanded mine a few times under running water as suggested. And I know Cadmium has also. That of course would not remove hydrocarbon contamination within the porous-packed Spectralon.

 

Of course, it is not clear what grade our Spectralon targets are. I'm not sure there were "grades" when I purchased mine years ago? However, I would reasonably surmise that mine are Optical Grade.

 

Also note that our Spectralon targets are packed in non-fluorescent bins.

 

---

 

I can only assume that for whatever reason my white Spectralon target shown above reflecting some violet light which passed through the BaaderU filter is less contaminated than Cadmium's?? Steve, do you think this might be the case. My targets were purchased new as calibrated targets from LabSphere. They have had a lot of field use over the years. Given that I use them for white balance and medium gray reflectivity, I don't think I will be replacing them any time soon! Too expensive.

 

---

 

How did this discussion get started in the first place???? :lol: It's been going on for years now!!!

Well, no matter. I ***LOVE*** these discussions. I really do. It is great education. Over the last two days I've reviewed material about plastics, how integrating spheres work, what hydrocarbons are, how fluorescence works and more.

 

If we were using Spectralon or PTFE to look for violet leaks from light, then that was NOT the proper method. We should always use a shiny spoon or ball bearings or similar to check for visible light leak when working on UV-induced Visible Fluorescence photography.

 

I must leave this now and try to update the UVIVF Sticky a bit.

[Cadmium]

The only way we are going to figure this out is get a perfectly new one and compare.

I rather doubt that a new one is going to look much different when illuminated with UV only light, and I doubt even more that it will look black.

Not going to happen, and I base that on the comparison between Spectralon and PTFE sheet which look to have the same or almost identical fluorescence brightness and color.

Labsphere hasn't returned my recent emails. So one of these days I will call them.

All the scientific papers I read about PTFE and Spectralon say that PTFE is 'low fluorescence', or not fluorescence, the material is not thought of as being fluorescent.

However, I did see a demonstration of a Spectralon Sphere lighting up with fluorescence from the inside...

I will paste a few links below, for what it is worth.

The thing is, whatever this is, we should figure out what it is, simply because if we are seeing this with something that isn't fluorescent, then what else are we seeing in UVIVF photos that isn't fluorescent either?

You know?

 

https://link.springer.com/article/10.1140/epjc/s10052-019-7152-2

 

https://www.researchgate.net/publication/259811063_Analysis_and_reduction_of_fluorescence_on_PTFE-coated_integrating_spheres_poster

 

Back to my point, how come PTFE fluoresces? No porosity there, solid material, easy slick surface to clean, that is why they use it on cookware, things don't stick to it.

Yet it fluoresces the same as my Spectralon? Hmm... That is just one reason I fail to be convinced that the spectralon is old or contaminated or has been smoking a pack a day...

Someone is going to have to get a new one. Don't be disappointed if it looks the same.

[aphalo]

I am arriving late... can we guess how much is the fluorescence yield? How much is the fluorescence emitted compared to the UV irradiance impinging on the target? We are getting better and better UV LEDs and maybe this is making visible to the camera a very minor fluorescence yield from the target. With my good PTFE slabs I see very little fluorescence unless I push exposure up when there is not any other source of fluorescence. It is unlikely that we can clean and decontaminate anything so that fluorescence is zero... So it would be good to know or at least guess what fluorescence yield is behind this images. This is what could help narrow down this question. The irradiance on the target will be extremely high, say 1 W of UV onto 1 cm2, that would be 10kW m-2 of UV. What is the sensitivity threshold of human dark adapted vision, guessing from my own eyes about 0.01 W m-2. So fluorescence yield of about 1e-6 or even less could be visible... Or maybe the filters have OD < 6 in the visible and some visible light is getting through...

Does this make sense to others? This is in part back-of-the-envelope guessing, but seems that maybe the explanation is in how strong the excitation is and how good our eyes and cameras are.

[Andrea B.]

Removed a small rantlet by myself. Sorry.

[Andrea B.]

May I kindly suggest everyone read through the Summary links I've provided before drawing any definitive conclusions. Get as much knowledge as you can from what is already written before we go off on any speculations or re-purchases.

 

 

(Especially do NOT spend any more money right now. You might need it if this pandemic does not come under control anytime soon.)

[dabateman]

May I kindly suggest everyone read through the links I've provided before drawing any definitive conclusions.

 

What why. That will squash crazy opinions. Like microbes that are eating our CD's have moved on to our PTFE targets as carbon sources.

 

I have a PTFE slab purchased from Ebay for under $10. Has served me well and I cut sections from it. I will have to look closely now at it.

[Andrea B.]

Cadmium, you are posting some of the same links that I have in the summary.

The explanation for the fluorescence in an integrating sphere is given in one of those links.

 

---

 

I have a call in to LabSphere. Things are a bit tough in the US just now, so I can't say when I will hear back from them. But I will certainly report to everyone when I do.

 

---

 

 

 

I sometimes wonder if anyone ever reads any of the summaries or stickies. I hope so. I put hours of work into them!

[Cadmium]

I am arriving late... can we guess how much is the fluorescence yield? How much is the fluorescence emitted compared to the UV irradiance impinging on the target? We are getting better and better UV LEDs and maybe this is making visible to the camera a very minor fluorescence yield from the target. With my good PTFE slabs I see very little fluorescence unless I push exposure up when there is not any other source of fluorescence. It is unlikely that we can clean and decontaminate anything so that fluorescence is zero... So it would be good to know or at least guess what fluorescence yield is behind this images. This is what could help narrow down this question. The irradiance on the target will be extremely high, say 1 W of UV onto 1 cm2, that would be 10kW m-2 of UV. What is the sensitivity threshold of human dark adapted vision, guessing from my own eyes about 0.01 W m-2. So fluorescence yield of about 1e-6 or even less could be visible... Or maybe the filters have OD < 6 in the visible and some visible light is getting through...

Does this make sense to others? This is in part back-of-the-envelope guessing, but seems that maybe the explanation is in how strong the excitation is and how good our eyes and cameras are.

 

Yes, it is definitely what I would call low fluorescence. I think Andrea's last example is quite similar to what I get.

Whatever it is, it is easy to see with my eyes (goggles of course), and not anywhere as bright as my psychedelic Sidney Coleman poster (which I don't actually own).

I have tried everything I can think of to make the fluorescence go away, except of course blocking the visible range.

The 'whatever it is' (AKA fluorescence) is definitely IN the visible range, with no other illumination besides UV-only light.

Given that, I think it is simply low fluorescence.

If anyone can think of any test I can try, and if I have the needed equipment to try it, please let me know and I will try it.

I am going to go look for a fluorescent Sidney Coleman poster on eBay now.

[Cadmium]

By the way, here is a UVIIRF 715nm shot, same Spectralon with PTFE, seems the same fluorescence above 700nm also.

 

White paint (left), Krylon 1602 paint (background), Spectralon black (above), Spectralon white (below), PTFE sheet (right).

Convoy UV light, RG715 filtered camera.

[Cadmium]

Cadmium, you are posting some of the same links that I have in the summary.

The explanation for the fluorescence in an integrating sphere is given in one of those links.

 

---

 

I have a call in to LabSphere. Things are a bit tough in the US just now, so I can't say when I will hear back from them. But I will certainly report to everyone when I do.

 

---

 

 

 

I sometimes wonder if anyone ever reads any of the summaries or stickies. I hope so. I put hours of work into them!

 

Sorry Andrea, just a few of the random links I save from the late night session...

Didn't mean to copy you or be redundant.

 

It all comes back to testing anyway, read what I can, but what I see is what I see...

I will buy a NEW Spectralon at some point just to make sure it is not from whatever else might be suggested, just to rule those ideas out... or in.

[Andrea B.]

Please don't buy anything until I can talk to Labsphere.

[Cadmium]

I think the SRS-99-020 that I have is about $450 (new), so I might take my time.

Yes, a phone call would be good. I didn't get what I was looking for from the emails.

Have you seen that fluorescence set on eBay? 112613415459

Pretty, huh? I want it... but... think I will hold off.

Cool, huh?

[Andrea B.]

Okie dokes, folks.....I talked to LabSphere this morning (Thur 10 Dec).

Told 'em who I was, how we use Spectralon/PTFE and about UVP & why we are asking.

I also learned a bit about their newer products.

 

Spectralon does not fluoresce. It does not absorb photons to cause release of light.

There really is nothing else to say about that. It's physics. You can't argue with physics.

 

So why are we seeing something? Answer: Spectralon is very, very, VERY reflective.

So whatever it is we are seeing in the photographs, it is *not* emitted visible fluorescent light.

 

AMENDMENT 25 December 2020: After reading through several tech papers, I learned that basic optical grade Spectralon retains certain manufacturing impurities which cause the fluorescence.

 

If anyone wants to further experiment, I would suggest some tight bandpass filtration on the lens (and the illumination??).

 

Let's experiment further, but please don't buy new Spectralon before trying better filtration. Too exensive!!

 

I can make an appointment to discuss this further with one of their R&D folks now that I have a contact in their front office.

 

P.S.#1 I passed along the suggestion that we could use a white fluorescence standard and told them about UV Innovations Target-Grey™. They were interested to learn about the need in photography and in art restoration efforts for such a thing.

 

P.S.#2 LabSphere does have some photographer customers. One photographer photographing insects, for example, uses Spectralon rectangles as background and for WB. I heard other interesting uses of Spectralon targets outside the usual laboratory settings:

• Spectralon squares to help in calibration of something to locate satellites from the Nevada desert (not quite sure how that works)
  
• Large Permaflect targets set up by the driverless car testers (not quite sure how that works either)
  

[Andy Perrin]

Andrea, it’s great to say you can’t argue with physics but that doesn’t mean our Spectralon (or rather Cadmium’s) isn’t fluorescencing due to either environmental contamination or degradation over time of the surface. Labsphere must mean “in factory perfect conditions.” I’m not so sure we have that here. I mean it seems like Steve of all people would know how to properly filter visible light!

[Andrea B.]

Andrea, it’s great to say you can’t argue with physics but that doesn’t mean our Spectralon (or rather Cadmium’s) isn’t fluorescencing due to either environmental contamination or degradation over time of the surface

 

Of course, your Spectralon may be carrying fluorescent contamination. I didn't say it didn't!

I was simply reporting on the Spectralon itself.

We covered the possibilities of fluorescent contamination above.

 

The suggestion to try tight bandpass filters is a good one because it may help us narrow down what we are seeing from whatever the source. If you are working with something which is 99% reflective, it is going to "catch" and turn back a lot of photons. :lol:

[OlDoinyo]

You could also try to capture a spectrum of the mystery light, with a slit and prism or diffraction grating.

[Cadmium]

1) It is not just me, "or rather Cadmium’s", you should include Andrea, and Jonathan, they both are getting similar results.

Andrea's photo shows the same as mine, and Jonathan said "And I see similar behavior to you too Steve (and Andrea)."

 

2) It is not contamination. Spectralon (white) fluoresces the same as PTFE sheet.

Sheet is not porous, you can cut it and the inside fluoresces the same as the outside, so it is not from contamination.

 

"Physics" according to who?! Labsphere?

If Spectralon is made out of PTFE, then it has the same built in default.

If their white version has nothing added, then it is basically the same as PTFE sheet other than the porosity to keep it from being a shiny surface.

The other monochrome gradations of Spectralon have whatever mixed into them to make them more and more gray, grayer, and black.

Let's say the white version does have something added to it other than just pure sintered PTFE.

Let's say they add something to make it whiter? Or could they add something to make PTFE not fluoresce? I would be skeptical of that.

That would be like adding baking powder to mud, it will always have the mud in it.

Physics according to Labsphere.

 

Send Labsphere a Convoy S2+ with a U-340 2mm on the front and let them wonder what they are seeing.

We will not do that, and they will not do that either.

 

I don't think Spectralon is any more reflective than PTFE sheet is. PTFE has a more shiny surface, Spectralon has a flat non shiny surface,

but I don't think either of them reflect more or less when the angle of light is such that the PTFE isn't reflecting the surface back at you.

If I shine my torch at either one, they both reflect the same.

 

So far, I am not convinced of anything that has been suggested by Labsphere.

 

"Whatever it is" we are seeing is probably fluorescence. I think.

So grab your Spectralon, and see if you think so to. Don't have any?

 

Andrea, thanks for calling Labsphere. Your answer from them is the same that I got in email.

[Andy Perrin]

Cadmium, I have no idea who you are addressing here, but there is no need to sound so upset with me. I have no Spectralon whatsoever, and all I meant was to give you credit for your own experiment! And I certainly didn't mean to exclude Andrea's or Jonathan's, it's just that's the one I was looking at because it was in the preceding post. The "our" in my original comment referred to UVP, not me personally.

[Cadmium]

Andy, No, I am in no way upset with you. I am just pointing out that it is not just me who is getting the fluorescence (or whatever it is).

[dabateman]

Andy,

I got the impression Cadmium is upset with Labsphere, not you.

I would be too if their answer was physics. My immediate reply would be "ok you're stupid."

Seriously, that is not an answer. If those are their words, then your hitting your head against a brick wall. It ain't going anywhere.

Max Planck was told not to study physics as there was nothing left to be discovered. We all know how that turned out.

If there answer is its reflecting some unseeable visible light so now we can see it. That's a great trick. How is it concentrating it? What is the pulse hold duration?

Cadmium may have found something. But Labsphere doesn't seem to be convincing me of anything useful.

[Cadmium]

I am not upset with Labsphere either. However, if they don't know what it is we are referring to, then I think they need a better answer.

The question remains unanswered.

[Andy Perrin]

Andy, No, I am in no way upset with you. I am just pointing out that it is not just me who is getting the fluorescence (or whatever it is).

I am not upset with Labsphere either.

(Got it.) And yes, it's not just your results, it's everyone's.

 

I did think Labsphere genuinely believes their product doesn't fluoresce. They suggest it's reflected light - how well have we controlled for stray light in these tests?

 

If it's not surface contamination, and the light is well-filtered (and I believe it is), and it's not reflected light, it's either fluorescence of unknown binder or florescence of Spectralon/PTFE (since they seem to behave the same way).

 

 

I found a paper, "Ultraviolet stability and contamination analysis of Spectralon diffuse reflectance material" which describes the following:

 

Could this impurity still be present inside the material and be causing fluorescence? That paper was in 1993, so it's been a while though.

 

ETA: dagnabbit, that was the same paper Jonathan found before! Sorry, I'm having trouble keeping all this straight. But it does seem like there may be some kind of thing inside the Spectralon that is glowing.

[Cadmium]

On a scale of 1-10, spectralon has a very low fluorescence (if that is what it is).

 

Take a read of these three posts.

First Stefano, then Andrea, Then Andy:

https://www.ultravio...dpost__p__40166

 

Stefano: "pretty much anything fluoresces under UV"

Andy: "...not much fluoresces? My experience has been more like Stefano describes."

 

I think the same.

This has been my experience. In total darkness, wander around with my U-340 2mm filtered 365nm torch, and I can't find anything that doesn't fluoresce some.

I keep saying this, but that is what I see.

Absolutely, in comparison to some other fluorescent things most things don't seem to fluoresce. However, separately from those very fluorescent things, I see seemingly non-fluorescent things fluorescing.

No light from something else that is fluorescing, put it in a box, alone, isolate it if you want...

Call it "low fluorescence" if you like.

So it is not surprising to me that Spectralon and PTFE fluoresce.

That being said, if it is not fluorescence that I see with the Spectralon, then it may also not be fluorescence that I see with all those other-everything things.

But so far, I can not rule out fluorescence, because that seems to be exactly what it is.

 

The beam. In the tests I have done there is always the torch beam on the Spectralon target.

You can't see that well defined in some of the shots I have done using aperture priority, but surely you see the torch beam on the target with my extreme multiple filter stack test.

So that beam is there, and it is the only illumination in the room, and how would any other illumination in the room somehow target itself on that Spectralon circle in the shape of a beam?

 

If it were a binder or such, then I think that would be something Labsphere would know about. Even if that were the case, then that would be fluorescence, part of the Spectralon, AKA the Spectralon.

I really can't think of anything else it can be other than fluorescence (low fluorescence).

"Reflected light"? What we see on the target is visual light, it isn't UV, it is not IR, we can see it with out eyes, even with the Amber glasses.

It is visual light, in the shape of the UV-only beam.

It is not contamination. It does it the same with Spectralon, PTFE sheet (highly impervious to water), and even with PTFE powder.

And there is no reflected light jumping around from somewhere, around corners, shaping itself into a beam, and playing tricks with me.

I would actually like to be wrong, I don't want this to be fluorescence.

Proof. There is no proof of it being anything except fluorescence.

I have been trying.

[Andy Perrin]

Cadmium, I think it's probably the chemical mentioned in my previous post (see that abstract I quoted). The hyrdrocarbons mentioned are PART of the Spectralon and probably present in commercial PTFE too? I suppose the pure PTFE could ALSO fluoresce.

 

Regarding the "low but non-zero" fluorescence issue:

 

I have a 405nm line laser (100mW) coming late this month, and I am planning to try LASER-INDUCED fluorescence. (I am not yet comfortable playing with UV lasers, even if I could find an affordable one, so this will be violet fluorescence rather than UV induced.) Allegedly, according to this paper, laser induced fluorescence brings out a lot of faint stuff that would otherwise be hard to photograph, simply due to the high fluxes involved.

[dabateman]

That paper was interesting. It hypothesizes that lubricant in the making of the disks is a contaminant, the source of large hydrocarbons. As vacuum or 95C for 1.5 hours seemed to remove the surface hydrocarbons which contained oxygen.

That actually doesn't rule out something growing in or on the material as a possible contaminant.

 

Cadmium, do you have a block of PTFE or teflon? If so image it and see if it has Similar features. Then place it on some aluminum foil on a cookie sheet and bake it in your oven at 350F for 2 hours.

The melting point of PTFE is over 600F, so shouldn't melt. But this will kill most things and increase the off gasing of hydrocarbons and dry up any water that might be there. Then reimage and see if better. 450F would be better, but thats up to you.