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Removing thorium yellowing with a UV light
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PostPosted: Thu Jan 14, 2021 4:02 pm    Post subject: Removing thorium yellowing with a UV light Reply with quote
So I come back here once again with another question. I know this might be a little bit offtopic but I don't know where else I could possibly ask. I want to get myself a UV light for two reasons. One reason is that I could potentially use it to remove thorium yellowing if I ever come across a lens that has it, second reason is that I do full spectrum photography and I would like to use such a light on flowers to make them look all weird. Mostly the latter, in fact. Now when I look up UV lights on eBay, I get tons and tons of listings of shady looking flashlights. Obviously as I'm buying something that is potentially harmful, I want to make sure that I'm not buying something that's different from what it claims to be. Many laser pointers on eBay for example are rated safe yet if you looked in them you could easily go blind. Does anyone have any experience with buying UV lights on eBay? Should I just go for it and buy whatever? Am I worrying for no good reason? I'm looking for something not more expensive than 20 USD.


PostPosted: Thu Jan 14, 2021 4:45 pm    Post subject: Re: Removing thorium yellowing with a UV light Reply with quote
Fandyus wrote:
So I come back here once again with another question. I know this might be a little bit offtopic but I don't know where else I could possibly ask. I want to get myself a UV light for two reasons. One reason is that I could potentially use it to remove thorium yellowing if I ever come across a lens that has it, second reason is that I do full spectrum photography and I would like to use such a light on flowers to make them look all weird. Mostly the latter, in fact. Now when I look up UV lights on eBay, I get tons and tons of listings of shady looking flashlights. Obviously as I'm buying something that is potentially harmful, I want to make sure that I'm not buying something that's different from what it claims to be. Many laser pointers on eBay for example are rated safe yet if you looked in them you could easily go blind. Does anyone have any experience with buying UV lights on eBay? Should I just go for it and buy whatever? Am I worrying for no good reason? I'm looking for something not more expensive than 20 USD.


I've brought both UV flashlights & a replacement bulb for a normal light fitting that produces UV.
Both work OK, but I'd recommend getting UV rated safety glasses as well & still don't look directly at it.

Actual wavelength for UV makes a big difference to how harmful it is, & will also affect your photography significantly. Sorry I no longer have a record of the wavelength for my UV torches. Germicidal UV is more energetic than the 'black light' variety sold cheaply.

My 51 LED torch has proved quite useful for this sort of shooting & only cost £6 when I got it back in 2014.


PostPosted: Thu Jan 14, 2021 8:39 pm    Post subject: Reply with quote
This topic has been discussed here many times -- use the forum search or Google with site:mflenses.com switch.

It has never been proven that uv light is necessary as some lenses have cleared using lighting without any uv.

Also, check yellowing using sun shining through lens onto white paper -- if image circle is not yellowed, you are seeing yellow lens coating not thorium yellowing...


PostPosted: Sat Jan 16, 2021 4:49 am    Post subject: Reply with quote
Many of the thoriated lenses are so radioactive that you probably don't want to have them around. I just had to send back a beautiful Rank Taylor Hobson 4" f/2 Cooke M=1 Copy Lens due to high radioactivity. Some folks will say that the radioactivity is only Beta, so just keep from getting too close to your cornea (don't use radioactive lenses as loupes!!) but indeed the breakdown chain of thorium includes Gamma rays as well as Beta particle emission. As an example, the lens I sent back read 0.3mR/hr at ~1" from the metal case, and 0.5mR/hr ~1" from the glass. The case measurement is Gamma, while glass measurement is Gamma + Beta. While this is not immediately dangerous, I'm not so interested in getting dosed if I can help it.


PostPosted: Sat Jan 16, 2021 3:29 pm    Post subject: Reply with quote
Ray Parkhurst wrote:
Many of the thoriated lenses are so radioactive that you probably don't want to have them around. I just had to send back a beautiful Rank Taylor Hobson 4" f/2 Cooke M=1 Copy Lens due to high radioactivity. Some folks will say that the radioactivity is only Beta, so just keep from getting too close to your cornea (don't use radioactive lenses as loupes!!) but indeed the breakdown chain of thorium includes Gamma rays as well as Beta particle emission. As an example, the lens I sent back read 0.3mR/hr at ~1" from the metal case, and 0.5mR/hr ~1" from the glass. The case measurement is Gamma, while glass measurement is Gamma + Beta. While this is not immediately dangerous, I'm not so interested in getting dosed if I can help it.


AFAIK all thoriated lenses are radioactive as the thorium is the element that makes them radioactive.
Very few are active enough to be an issue is direct contact is kept to less than a few hours.
Anyone who tells you It's safe because it's a beta emitter is wrong on two counts:

1/ Thorium is an Alpha emitter.
2/ Beta is more penetrating than alpha, externally alpha is not an issue the layer of dead skin cells all over your body is enough to stop alpha particals.

Alpha is more ionising than beta so if it gets inside you alpha is a more serious risk (about 20x higher).


PostPosted: Sat Jan 16, 2021 5:02 pm    Post subject: Reply with quote
So don't eat your thoriated glass!


PostPosted: Sat Jan 16, 2021 5:48 pm    Post subject: Reply with quote
DConvert wrote:

AFAIK all thoriated lenses are radioactive as the thorium is the element that makes them radioactive.


This is not quite true. Yes, all thoriated lenses are radioactive, but Thorium is not the only radioactive element within them. The Thorium decay chain is dominated by the long half-life of naturally-occuring Th-232 decaying to Ra-228 through Alpha emission. The Radium half-life is only 5.7 years, but because it emits Beta particles, the Radium within the bulk of the lens elements can be detected through its emission. This makes an interesting situation for these lenses...they get more (detectably) radioactive vs time because the Radium content is always increasing. I would assume it is the Radium that gives the lenses their yellow color, but it may be the Lead that the Radium eventually becomes.

I'm not sure why I wrote Beta and Gamma in my response above. Just replace Beta with Alpha and Gamma with Beta and that's what I meant to say...


PostPosted: Sun Jan 17, 2021 7:45 pm    Post subject: Reply with quote
Ray Parkhurst wrote:
DConvert wrote:

AFAIK all thoriated lenses are radioactive as the thorium is the element that makes them radioactive.


This is not quite true. Yes, all thoriated lenses are radioactive, but Thorium is not the only radioactive element within them. The Thorium decay chain is dominated by the long half-life of naturally-occuring Th-232 decaying to Ra-228 through Alpha emission. The Radium half-life is only 5.7 years, but because it emits Beta particles, the Radium within the bulk of the lens elements can be detected through its emission. This makes an interesting situation for these lenses...they get more (detectably) radioactive vs time because the Radium content is always increasing. I would assume it is the Radium that gives the lenses their yellow color, but it may be the Lead that the Radium eventually becomes.

I'm not sure why I wrote Beta and Gamma in my response above. Just replace Beta with Alpha and Gamma with Beta and that's what I meant to say...


Daughter elements are also radioactive yes, but the presence of thorium will make it radioactive, before any of the daughter nuclides have a chance to form. So the thorium is what makes it radioactive.
It's activity increasing afterwards is fairly irrelevant, if you re-read what I wrote you'll see I didn't claim thorium was the only radioactive element present.
I guess you do have a valid point about the increased activity once radium starts to form. All the daughter nuclides have considerably shorter half lives than Th-232 (which has a half life over 14 billion years!) Ra-228's 5.7 year half life is the longest of the remainder of the series most of the others being in hours, minutes or seconds & polonium-212 is in nano seconds.
Very much shorter than the Po-210 isotope we see building up at work.

I've often wondered about the yellow colouring. If it was down to an element forming in the decay series there's no reason why UV would have any effect. I suspect an electronic effect is much more likely. Disassociated electrons give rise to yellow colours quite often in organic compounds (especially in aromatics & conjugated poly-alkenes etc.) UV could then initiate another reaction that binds these electrons (beta particles).


PostPosted: Mon Jan 18, 2021 5:03 am    Post subject: Reply with quote
Yellowing is the result of something called color centers. Google that if you want to dive into it.

UV light is not required to clear color centers. Visible spectrum light is all that is needed.


PostPosted: Mon Jan 18, 2021 11:24 am    Post subject: Reply with quote
Mike Deep wrote:
Yellowing is the result of something called color centers. Google that if you want to dive into it.

UV light is not required to clear color centers. Visible spectrum light is all that is needed.


Thanks, so as I suspected an electron effect, but likely caused by ionisation of the glass due to atomic impacts from the alpha/beta particles.
I doubt red light would be energetic enough to clear yellowing in lenses, blue might just work but most sources suggest UV, which is more energetic.


PostPosted: Mon Jan 18, 2021 2:06 pm    Post subject: Reply with quote
Mike Deep wrote:
Yellowing is the result of something called color centers. Google that if you want to dive into it.

UV light is not required to clear color centers. Visible spectrum light is all that is needed.


Thanks for reminding me as I did not remember about color centers. I'm not sure how light can release electrons trapped inside an insulator, but if it works, it works. How long does it take? If I had a yellowed lens, and shine a flashlight through it, does it magically clear? Does it build up a static surface charge as it clears?


PostPosted: Mon Jan 18, 2021 4:51 pm    Post subject: Reply with quote
Mike Deep wrote:
Yellowing is the result of something called color centers. Google that if you want to dive into it.

UV light is not required to clear color centers. Visible spectrum light is all that is needed.


Happy Dog Thank you!

https://en.wikipedia.org/wiki/F-center
https://www.rp-photonics.com/color_centers.html


PostPosted: Mon Jan 18, 2021 6:56 pm    Post subject: Reply with quote
For UV: Also available 365nm wavelemgth

https://www.ebay.com/itm/372794791463


PostPosted: Mon Jan 18, 2021 11:21 pm    Post subject: Reply with quote
I have put yellowed lenses on a window ledge for a few days and it pretty much cleared the yellowing. But I had read that UV light was best method, so when I saw this UV light box at a radio ham fair / swap meet for £5 I bought it. Apparently it was made to cure some kind of resin used in electronic components ?

I have to say, I think it works faster than sunlight. 24 to 36 hours in the box cleared a couple of badly yellowed Takumars completely.
The lamp tubes are easily available, I don't know exactly what spec' they are and the box is buried in my garage at the moment. It's not a thing I use often, those lenses in the box were done in 2019 and they are still clear. I don't store my lenses in darkness though.

The box is overkill, but it was something that came along at the right price and works, a simple one lamp box would be very easy to make.



PostPosted: Tue Jan 19, 2021 10:56 am    Post subject: Reply with quote
Lloydy wrote:
I have put yellowed lenses on a window ledge for a few days and it pretty much cleared the yellowing. But I had read that UV light was best method, so when I saw this UV light box at a radio ham fair / swap meet for £5 I bought it. Apparently it was made to cure some kind of resin used in electronic components ?

I have to say, I think it works faster than sunlight. 24 to 36 hours in the box cleared a couple of badly yellowed Takumars completely.
The lamp tubes are easily available, I don't know exactly what spec' they are and the box is buried in my garage at the moment. It's not a thing I use often, those lenses in the box were done in 2019 and they are still clear. I don't store my lenses in darkness though.

The box is overkill, but it was something that came along at the right price and works, a simple one lamp box would be very easy to make.


A friend of mine has built a box similar to that to make circuit boards. He draws the circuits he want with a pen and when the UV is turned on everything else on that layer is removed.
I should ask if I could borrow it to clear a Pancolar, Takumar and Topcor!


PostPosted: Wed Jan 20, 2021 1:48 am    Post subject: Reply with quote
Ray Parkhurst wrote:
How long does it take? If I had a yellowed lens, and shine a flashlight through it, does it magically clear? Does it build up a static surface charge as it clears?


I imagine it depends on the lens and the light intensity. My Takumars, Yashinon and Fujinon cleared up significantly in about a week under a decently bright LED desk lamp--one with a flexible neck that allowed me to position it against the end of the lens. I also removed the back lens cap, placed the lens on a mirror and flipped the lens after several days.

There's no static charge.


PostPosted: Wed Jan 20, 2021 1:58 am    Post subject: Reply with quote
Mike Deep wrote:
UV light is not required to clear color centers. Visible spectrum light is all that is needed.


Interesting theory. What evidence do you have to support it? My thoriated lenses don't agree with it at all.


PostPosted: Wed Jan 20, 2021 6:27 am    Post subject: Reply with quote
Unless you filter it out, sunlight contains UV, so it is likely that he effect you are seeing when using visible light is from the small amount of UV present.

You can buy UV led torches very cheap in a variety of wavelengths now, I expect those do the job quite well.

Does anyone happen to know what wavelength(s) of UV are required?


PostPosted: Wed Jan 20, 2021 7:23 am    Post subject: Reply with quote
I wouldn't spend a fortune on an LED to clean your lenses up.

You might be better off just buying something useful that contains some cheap LED's, like a magnifying stand or a 'helping hand'. I have a small desktop magnifier that came with a couple of cheap LED's built in that work a treat.

Out of boredom, I wanted to see if the UV contained in weak winter light could do anything. I live at 56 degrees North. It was actually working, even though a fair lump of the dose must have been from scattered UV and not direct. But god was it a slow process.

Winter. 56 Degrees North,


I eventually cracked, and put the magnifying stand over it. The colour centres were completely gone in a day or so.

And as always. A reminder - Open lenses in direct sunlight make serious fire-starters if the light gets focused. In just positioning the above one day I managed to laser a trough along my lens cap. Happened in a split second. Original Asahi rear cap too!

Last edited by Sciolist on Wed Jan 20, 2021 7:45 am; edited 2 times in total


PostPosted: Wed Jan 20, 2021 7:35 am    Post subject: Reply with quote
danfromm wrote:
Mike Deep wrote:
UV light is not required to clear color centers. Visible spectrum light is all that is needed.


Interesting theory. What evidence do you have to support it? My thoriated lenses don't agree with it at all.


Did your thoriated lenses agree uv clears yellowing? Did you first try visible light without success?

Some type leds may leak uv from low pass filter...


PostPosted: Wed Jan 20, 2021 8:35 am    Post subject: Reply with quote
iangreenhalgh1 wrote:
Unless you filter it out, sunlight contains UV, so it is likely that he effect you are seeing when using visible light is from the small amount of UV present.

You can buy UV led torches very cheap in a variety of wavelengths now, I expect those do the job quite well.

Does anyone happen to know what wavelength(s) of UV are required?


I don't know but it 's unlikely to be short wavelengths, a glass blocks these increasingly effectively.
I would imagine over 350nm will work, which might be just within the visual range for young eyes.


PostPosted: Wed Jan 20, 2021 9:06 am    Post subject: Reply with quote
A 365nm LED torch is about 5 pounds or dollars, that would be my choice. Not much UV in the sunlight in winter up here in the north.


PostPosted: Wed Jan 20, 2021 11:09 am    Post subject: Reply with quote
Some data on UV lamps can be found here: https://www.dpreview.com/forums/thread/4320070


PostPosted: Wed Jan 20, 2021 4:49 pm    Post subject: Reply with quote
visualopsins wrote:
danfromm wrote:
Mike Deep wrote:
UV light is not required to clear color centers. Visible spectrum light is all that is needed.


Interesting theory. What evidence do you have to support it? My thoriated lenses don't agree with it at all.


Did your thoriated lenses agree uv clears yellowing? Did you first try visible light without success?

Some type leds may leak uv from low pass filter...


Fair questions. Yes -- I used a 20w compact fluorescent BLB lamp, the lenses were a 55/8 Repro Claron and three TTH tessar type process lenses -- and yes.


PostPosted: Wed Jan 20, 2021 5:25 pm    Post subject: Reply with quote
I must be lucky, I don't have a single yellowed lens. I'm trying to think of what lenses I have that are known to have thoriated glass. I have a 'zebra' Pancolar 1.8/50 and it's not yellowed at all. I have a few other CZJ 'zebra' lenses and none of them have either. Now I've said that, I'll probably go and pull some random lens out of one of my boxes and discover it has yellowed.
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