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Is there a fluorescent mineral that will allow one to distinguish between different UVA wavelengths

A recent discussion on Facebook evolved about using a mineral to determine if an LED is 365nm or 405nm. The result of the discussion was that it is pretty much a no-brainer. Very few minerals are really fluorescent using cheap 405nm flashlights. Ruby, sodalite, and maybe a few others - but the lights are usually awash in extra white light that specimens look terrible under them. 365nm on the other hand, do very well. Moral of the story: cheap lights = no good. Best to get a filtered 365nm LED light from a reputable manufacturer.

This is the discussion from Facebook, complete with pictures for comparison: (names of posters replaced with initials)


MC: Got a question from a friend. He's not a member here (he's one of those cynics who thinks FB is out to steal your soul, your bank account, and provide you with a lifetime supply of spam - refuses to join. But I think he sneaks around in the shadows (of his wife's account)).

Anyway - it's a good question so I thought I would throw it out for discussion:

"Is there a fluorescent mineral that will allow one to distinguish between different UVA wavelengths (e.g. 365 from 405 nm)?"

Longer version:

Back in the old days when all backlight lamps were fluorescent tubes, there were really only two UVA wavelengths one had to be concerned with: 352 nm and 368. What you got depended on the phosphor a manufacturer used. If you didn't know which one you had, there was a decisive way of finding out: the mineral agrellite. Under 352 it exhibited its characteristic lilac fluorescence: under 368, there was no fluorescence or at most a faint red.

Recently, I became interested in comparing the fluorescence of minerals under the traditional UVA wavelengths and the longer wavelengths like 405 nm that LED UVA flashlights can provide. So, I purchased (on the web) a LED flashlight that claimed to be 365 nm and one that claimed to be 405. I compared the fluorescence produced in with a good many longwave specimens. To my surprise (and annoyance), I observed very little difference.

Then it occurred to me I didn't actually know that the wavelengths produce by these lamps was what the vendors had claimed. Of course, it's hard to imagine a vendor on eBay would misrepresent a product. Still…. But how to find out, short of buying a spectrophotometer? What I want is a test mineral that will allow me to reliably distinguish 365 from 405 nm - like the agrellite allowed me to distinguish 352 from 365. But I do not know what mineral might be suitable. Does anyone else out there know?

MC: Short answer - you cannot reliably expect an Ebay seller to provide you with a true 365nm LED flashlight. Most are JUNK and they lie terribly. On top of that, even if the LEDs are 365nm their output is pitiful and they generate tons of white light. You really need to purchase a good 365nm light with a filter. 405nm - can't put a filter on those pieces of junk. What would be interesting to me (if I were so inclined to focus on the trivial) would be to see the various responses of minerals under 350nm 360nm 370nm 380nm 390nm with certified LEDs (they are available in all those wavelengths)

DW: What i think you said was to create a database with each mineral that reacts with various nm's and list how each reacts. Taking into account other factors of course. And finding a grouping of minerals that react predictably as a way a person can do what the OP wants done. Minerals have various chemistries, and certainly fluoresce differently depending on the chemicals that do that sort of thing. From what i understand the same fluorescent minerals might fluoresce in different colors, strengths, etc..

DW: calcite for example, fluoresces different colors depending on different chemicals.

MC: True - but I think he's looking for something like agrellite that can reliably respond to one wavelength and not the other.

BO: Cleavage rhomb calcite is a great one to use.

DB: I agree with Becky. Some clear terlingua type calcite seems to be most useful to me for this purpose. It definitely fluoresces a different color for me depending on whether I use the 405 laser or lights labeled as 390 or 375 (both filtered with woods glass) or 365 from a transilluminator. However, the exact colors may not be the same from sample to sample.

MC: I think everything is different with a 405nm laser. Those things make rocks glow that shouldn't. But the question is how to figure out if you have a 405nm LED vs a 365nm LED.

MC: I bet ruby responds better to 405nm than it does to 365nm - anybody got both lights and a ruby?

RP: I can test this later tonight.

MI: Rubies do typically glow brighter in the longer wavelengths. They glow under good 365nm but not as well as under the cheap 395-405 LED flashlights.

BL: Really Michael? My experience is that the rubies glow much brighter with my led flashlight than under my long wave display or handheld.

BL: I have an Inova x-5 UV flashlight that says it is 365 - 400 that really lights up Ruby

MC: That's the problem with the little LED flashlights. There's no telling what the LEDs are putting out. 365-400 means nothing. It's one or the other... But rubies respond better to the higher wavelengths so I would assume they'd be pretty good at 405nm

DB: Rubies glow wonderfully with a 405 no laser, and I have to assume that the 405 laser really is 405nm. However, not all rubies glow equally well. Perhaps a lab grown ruby, or if we were really looking for an indicator "mineral", a lab grown version would be more capable and reliable in achieving the purpose of determining the wavelength of the incident light.

MI: BL: Yes, you are correct! I got it backwards. Rubies do better towards the blue end of UV, hence their beauty even in daylight. Anything I type past 12:00A is off the clock, mentally.

MC: Does anybody have a 405nm flashlight AND a 365nm flashlight? A direct comparison would be nice. And yes, every mineral is going to have a slightly different response, but like in the example, agrellite is pretty reliably fluorescent under 350nm and not under 365nm. Would be nice to have a "control" mineral with that kind of response to tell whether you have a true 365nm LED vs junk 390/405nm.

MI: I have several that are 395+ and my 365er's. But in practice, a single reference mineral is hardly required, the cheap LED's are so bad they an easily be discerned with little difficulty. Visually, they emit far more purple light than even 405nm or lower nm UV. Good old Sterling material offers several choices that make it abundantly clear what wavelength of LW is being emitted. Here are three photos taken just now (not in great conditions since is daylight) for this post. When dust fluoresced a lot, I removed it in post. Left to right: #1 is a small piece of Sphalerite/Cleiophane #2, Sterling Hill Calcite, Willemite, and yellow-fluorescing Calcite #3 SH Calcite, Willemite with streaks of bright blue Cleiophane #4 SH Calcite/Willemite

Picture #1 is the 365nm LED flashlight and all described colors are clearly evident. Willemite glows green and sometimes very brightly under 365nm. The Cleiophane is brilliant blue. The yellow-fluorescing Calcite is always wispy but is really bright.

Picture #2 is an "Ultrafire WF-501B" UV flashlight. It's at least seven years old and the current model number has changed but is likely still junk. I beleive this light emits 385+nm UV along with the usual tons of purple that all cheap LED's produce. You can see that some of the Willemite is slightly green. Some of the dust and lint is also glowing (removed in the 365nm photo).

Picture #3 is a nameless, eight year old 41 LED light. 395-405nm. Great purple flashlight with incidental UV. The photo shows zero mineral reaction and very little reaction from even the dust and lint. Another easy way to tell...if you've paid less than at least few hundred for your 365nm LED ain't 365nm. I also have a couple of lights from TANK007, sold as 365nm. They produce a small amount of UV that may or may not be 365nm but they also emit a ton of brown/yellow light that makes for a very poor visual reaction on minerals.

MC: Excellent! The blue light on these is painful. I bet they are just purple LEDs marketed as 395nm. Remember, 8 years ago even true 395nm LEDs were very expensive.

MC: I wonder what would happen if you put a piece of Wood's Glass (or Hoya) in front of those flashlights. My bet is that all the light would be filtered out...

DB: I do have ultrafire LED lights. One is supposed to be 375 and the other is 390. The 375 is pretty good, but it had a lot of white light coming out of it as well. The 390 isn't very good for much, though rubies love it. Both have been adapted, though, and the original clear filter was replaced by woods glass. The 375 is pretty good now. It is very easy to see the wavelength difference between them on terlingua type calcite. Also,, I think, they are reasonably different on scapolite from Canada. They do get very hot, though. I bought the 375 as it was recommended by Axel.

MI: MC: I've done it as a hoot. Things improve with the purple mostly gone but they are still feeble emitters of useful UV.

MI: DB: That's why I mentioned how old my Ultrafire is. I've assumed that better LED's have become cheaper and the Ultrafire lights may be better by now but a real 365nm LED still creates far better responses with much less visible light. Scapolite is tough to use for this thread's original question since it glows very brightly regardless of which UV lamps it.

MC: MI: That's the key - the filter wipes out the blue/violet color and they emit little (if any) UVA. 8 years ago cheap "UV" LEDs were really terrible

MC: MI: Agree on the scapolite (wernerite) from Canada. My pieces here don't show any significant difference

ED: Unless you are buying reliably specified products, or have a spectrowhatever, you cannot be scientific about it. I feel Mark's pain.

GB: My probably faulty memory seems to remember Don Newsome doing something with response curves for minerals.

MC: Yeah - he did some testing with 350nm~ and other phosphors. Noted some minor difference in response with a piece of tugtupite.

DB: What I think Don Newsome wanted to do was create a standard color response, but he might also have done some investigating of stuff like this. I definitely think the topic has come up on this list before.

DN: The FMS has had two research committees working on a Fluorescent Color Standard (FCS) in the past. The first was in 1990, a prototype sample was made but it was not practical to manufacturer or inexpensive to make (see old UV Waves for more information). Then in 2013 another attempt was made to have the FMS Research Chapter (RC) work on the FCS concept again. However, the RC got bogged down with the mechanism that might be used to produce a FCS and a detailed plan or outline was never approved. I still think a FCS is possible, but whether, it will be inexpensive, consistant, or otherwise practical is another story. The bottom line is it would take a lot of time to define, develope, and determine if it was possible to manufacture, and who has the time to work on that project. I don't think I have right now.

MI: Why not Terlingua Calcite? 365nm LW is pink to orange, 254nm SW is purple to blue, 395-405nm crappy LED is (just tested) very pale green, tan, or yellow with NF matrix white.

CF: That was exactly what I was thinking of. I got my piece from Way to Cool.

MC: That's a good one - didn't realize it would do that. Problem is I don't have any lousy LEDs - nothin' but good ones here ;-)

DB: I don't think the point is that you need (or even want) crappy lights for this test. You want known different lights. Crappy is fine if you know what is coming out of it.

MC: That's the whole idea here - find a mineral which will reliably reveal if your flashlight is built with crappy LEDs or good ones. The junk from China is marketed as 365nm but in my experience it's all lies.

DB: I mentioned in a comment that you probably need a lab grown mineral to perform this task because any natural one is going to vary for sample to sample. I think if someone really wanted to make a "cheap" spectrophotometer substitute material, it would probably be easier to do using plastic. Could you make a set of plastic tiles that would fluoresce selectively to UV light? So maybe 5-10 tiles that react anywhere from MW to Blue, but each tile only reacts to stuff in a specific range. (It would be nice to have a single indicator, like a mood ring for LW lights, but multiple tiles would probably be easier to produce. not that anyone is going to make anything like this on purpose, but maybe some existing materials already do this by accident?)

MC: Great idea, kind of like Don Newsome's FCS project above. But where/what plastic material has this kind of selective fluorescence? Are you aware of any?

DN: Mark, Most (not all) calcite rhombs from Challenger Cave, Monterrey, Nuevo Leon, Mexico can be used to tell the difference between LW350 (peak at about 352 nm) and LW370 (peak at 368 nm) UV. Most of those rhombs will fluoresce a "straw" or salmon color under LW350, but a pink under LW370. So if you have one of those rhombs you can use it to tell the difference. There are photos of those calcites that Axel made on the UV SYSTEMS web site.

MC: Yep - sounds like Terlingua calcite is a good candidate. Now I gotta find some crappy 390/405nm LEDs (I've got some really good 365nm)

DB: Mark, don't you give away a tiny crappy LW led light as a freebie to your customers?

MC: Yes I do - but they are 365nm! (but put out a lot of white light)

MC: OK - I knew I had to have a crappy flashlight around here somewhere. Dug into my junk box and pulled out a Mastiff Z2 395nm flashlight ( The seller admits it's 395nm :-) (love honesty). So I proceeded to select some samples to test. Here are the white light pics, labeled with rock ID. UV pics in next two posts

MC: This is the UV pic - 365nm (reliably spec'd)

ED: I couldnt help but notice that there is an area of blue on the Terlingua under LW. Check if it glows pink under SW. I have seen this before, and it is very odd.

MC: Didn't notice that, but you're right. Rocks still in position - the blue remains in SW. MW, or LW and is phosphorescent.

MC: And this is the 395nm pic. The exposure for the 365nm pic was iso100, f18, 6 secs. I had to crank the exposure time way up on this one just to be able to see anything - iso100, f18, 20 secs - all other settings and RAW development remained the same.

(Natural pic for good measure)

MC: Conclusion - 395nm sucks (but we knew that). But a couple of minerals seem pretty reliable to use as indicators: Terlingua calcite as has been pointed out already, cerrusite - no response under 395nm. Aragonite - much dimmer response, and interestingly my local travertine. Another factor - phosphorescence! Both the calcite and aragonite phosphoresce nicely under 365nm and barely/to not at all under 395nm

MC: I would have to go with Terlingua Calcite as the best indicator - pink under 365, barely blue under 395. Cerrusite pretty good too - not fluorescent under 395


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