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07-09-2018, 06:33 PM   #16
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QuoteOriginally posted by boriscleto Quote
Ikea LED lamps are said to work.

JANSJÖ LED work lamp - IKEA
...and I am one of the person's saying it. I have successfully cleared five lenses with no more that 18 hours per lens and minimal heat transfer in the process.


Steve

07-09-2018, 06:35 PM   #17
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QuoteOriginally posted by Not a Number Quote
Isn't it the rear element that is thoriated?
It depends on model and brand, but it is usually the rear or an element in the rear group. Knowing this makes it easier to apply a more "focused" cure.


Steve
07-09-2018, 06:50 PM - 2 Likes   #18
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QuoteOriginally posted by Prince Harbinger Quote
You are correct. However the oxidation forms on all of the elements.
That is theoretically true, but disassembly of an affected lens by a PF member (sorry, I don't remember who) showed clearly that the tint was limited to the thoriated element.

FWIW, the color is not due to oxidation in the chemical sense. The radiation creates what are called "Color Centers" within the glass where electrons are nudged to a higher energy level. The result is a modification of light absorption as well as refraction properties. The latter has implications in regards to optical performance. The intent of the various "decoloring" solutions is to bump those electrons back to their previous state by applying energy from an external source. Strangely, while UV light has been shown to be effective, LED sources such as the IKEA lamp are not particularly rich in UV and very well indeed. I guess one would have to talk with a materials physicist to know the full reasons why.


Steve
07-09-2018, 07:25 PM - 1 Like   #19
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QuoteOriginally posted by Prince Harbinger Quote
I had no idea I was doing it wrong. All of this time from being misinformed by a article I read. I followed you on Instagram. You have a lot of wonderful photographs. Thanks for the clarification. Everyone here has been very helpful.

---------- Post added 07-09-18 at 06:15 PM ----------



Thank you. Would you mind if I sent you some photos of some of the ones that I have? Maybe you could tell me a little bit about them?
Sure, I’d love to see what you have. You also might be interested in an article I wrote for Casual Photophile last year about the legendary Super Tak 50 f/1.4 8 element lens...

https://www.casualphotophile.com/2017/07/24/desert-island-cameras-pentax-wit...of-pentax-u-s/

07-09-2018, 08:58 PM   #20
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QuoteOriginally posted by stevebrot Quote
That is theoretically true, but disassembly of an affected lens by a PF member (sorry, I don't remember who) showed clearly that the tint was limited to the thoriated element.

FWIW, the color is not due to oxidation in the chemical sense. The radiation creates what are called "Color Centers" within the glass where electrons are nudged to a higher energy level. The result is a modification of light absorption as well as refraction properties. The latter has implications in regards to optical performance. The intent of the various "decoloring" solutions is to bump those electrons back to their previous state by applying energy from an external source. Strangely, while UV light has been shown to be effective, LED sources such as the IKEA lamp are not particularly rich in UV and very well indeed. I guess one would have to talk with a materials physicist to know the full reasons why.


Steve
Thanks for all of the great insight. I'm very puzzled how such a small UV lamp could do that. Another question that I have is could using a radioactive lens cause facial tumors to develop? Also any risk of damaging my mirrorless sensor from the radiation?
07-10-2018, 02:58 AM   #21
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QuoteOriginally posted by reh321 Quote
I don't understand difference which end you send light from; doesn't it go entire distance in any case??
Even uncoated glass blocks a majority of UV radiation which is what is needed to de-yellow thoriated elements. So the most effective and efficient manner is to shine the UV source from the rear where the thoriated element resides.
07-10-2018, 03:48 AM - 1 Like   #22
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I'm no nuclear physicist, but I read a few articles (with pictures of radiation counters) that showed that the amount of radiation is far too low to cause macular cancer, in particular with the camera between the eye and the lens. And indeed, as Alex645 posted, glass won't let UV light trough.
07-10-2018, 04:30 AM - 1 Like   #23
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acids do the trick

If your glass has no other coating than thorium added to the glass, you could technically use the following acids to solve the oxide: hcl, hno3, hf. I never had a chance to try this, but I know some basic chemistry. there is a product called armor etch - a mild HF solution, it is used to remove the coating from polycarbonate lenses, no need to mention it hurts glass coating ( truth is, I never managed to remove scratched coating from any glass lens with Armour Etch, it only remove coating from plastic lenses.


when you work with acids, glass must be removed or isolated because these acids react and substitute metals instantly and give out heat.


please try this only when everything else fails. al these acids are poisonus and require extreme care and professional protective gear. I doubt your lens worth the hassle.


I guess, a drop of armour etch could possibly take care of the problem. hf concentration in the product is low so is relatively safe to use. maybe even a highly concentrated vinegar or even lemons (acetic and citric acid) would do the job i have no idea but acids do the trick, I would try the mildest first.


messing with potent acids is not for faint-hearted.

07-10-2018, 04:32 AM   #24
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QuoteOriginally posted by Prince Harbinger Quote
Thanks for all of the great insight. I'm very puzzled how such a small UV lamp could do that. Another question that I have is could using a radioactive lens cause facial tumors to develop? Also any risk of damaging my mirrorless sensor from the radiation?
contrarily sunlight catalizes the oxidation. uv light wont help.
07-10-2018, 06:10 AM - 1 Like   #25
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I have cleared several yellowed lens with a compact fluorescent (CFL) UV bulb. Small lamp 3 to 4 inches above lens 1 day expose each end of lens. More if not fully cleared. No heating of the lens, heat rises.
07-10-2018, 06:29 AM   #26
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QuoteOriginally posted by reh321 Quote
I don't understand difference which end you send light from; doesn't it go entire distance in any case??
Exactly. I had both ends up on a random basis, and it work fine either way. I had best results from the sun vs. a lamp. My S-M-C Takumar 50/1.4 Cleaned up nicely.
07-10-2018, 06:31 AM   #27
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QuoteOriginally posted by Prince Harbinger Quote
Another question that I have is could using a radioactive lens cause facial tumors to develop? Also any risk of damaging my mirrorless sensor from the radiation?
No and no.

You would have to have close exposure to the lens for many hours a day. Thorium was used in microscope eyepieces and that did cause trouble for lab technicians...
07-10-2018, 07:52 AM - 2 Likes   #28
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QuoteOriginally posted by ddb Quote
If your glass has no other coating than thorium added to the glass, you could technically use the following acids to solve the oxide
Welcome to the Pentax Forums. Thanks for your thoughts. A few points of clarification:
  • The thorium is not a coating. It was added to the liquid glass forumlation when it was made to change is light refraction characteristics.
  • The discoloration is not oxidation. The color is internal to the glass and results from physical, not chemical, changes on the atomic level. Do a Web search for "glass radiation color centers".
  • Acid would not be a good solution.
There is a fair amount of strange stuff on the Web regarding the yellowing and how to tell a radioactive lens. One of the most persistent claims is that it is caused by changes to radioactive lens coatings. While many lenses will reflect a yellowish tint, that is not the same as the radiation-induced yellowing seen in thoriated glass. Efforts to remove yellow-reflecting lens coatings will not remove radiation and will damage the lens.


Steve

Last edited by stevebrot; 07-10-2018 at 08:00 AM.
07-10-2018, 11:01 AM - 1 Like   #29
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Do you have low E glass or double paned windows?
I de-yellowed a SMC 50mm f/1.4 last year. At first not much happened after leaving the lens in a south facing window for a few weeks in the late winter. I had forgotten that double paned windows with low E glass block almost all UV which is why I wasn't seeing much improvement. Regular soda glass blocks a surprisingly large amount of UV even a single pane single strength piece does. Laminated automotive safety glass blocks even more than almost all other glass people commonly come into contact with. So I waited until the summer and by leaving the lens outside with the rear element pointed towards the sun for a few days and the problem cleared up. I would just rotate the lens ever hour or so such that it would be mostly pointing towards the sun over the course of a couple of weekends. I wrapped the rest of the lens in foil so I wouldn't cook it out in the sun.
07-10-2018, 01:04 PM - 1 Like   #30
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QuoteOriginally posted by stevebrot Quote
Welcome to the Pentax Forums. Thanks for your thoughts. A few points of clarification:
  • The thorium is not a coating. It was added to the liquid glass formulation when it was made to change is light refraction characteristics.
  • The discoloration is not oxidation. The color is internal to the glass and results from physical, not chemical, changes on the atomic level. Do a Web search for "glass radiation color centers".
  • Acid would not be a good solution.
There is a fair amount of strange stuff on the Web regarding the yellowing and how to tell a radioactive lens. One of the most persistent claims is that it is caused by changes to radioactive lens coatings. While many lenses will reflect a yellowish tint, that is not the same as the radiation-induced yellowing seen in a thoriated glass. Efforts to remove yellow-reflecting lens coatings will not remove radiation and will damage the lens.






Steve
Thank you, Steve, I assumed it is a thoriated glass only, without any coating but I was not sure since I never tried to remove the coating from an old takumar lens. This fact should make the removal even easier since there is no need to worry about the coating. Thorium is a not a reactor fuel it is slightly radioactive, probably below average continental runoff consisting of radon etc. there is nothing to worry about, one can test it with Geiger apparatus if concerns are an issue.

if a yellow stain is on the surface of the lens it is likely Thorim oxide Th0, Th02 or any other form of Th2,4+ and therefore removable.
both oxidation stages are yellow so it matches the description.


I doubt he will succeed with UV experiment and permanently remove the issue but I would try different approaches, it can't hurt. If the problem was a fungus of yellow color, UV bleaching on both lens surfaces will probably do the job. I always test for fungus first and dip my lenses in a light solution of hydrogen peroxide, works better than UV


I never managed to damage any lenses, I even ran spectrophotometry test to test for the changed composition of my washes, never found anything worthy of concern. I regularly use moderate solutions of ammonia vinegar ethanol and hydrogen peroxide to restore old lenses and I never managed to damage anything, even when i wanted to

Last edited by ddb; 07-10-2018 at 01:50 PM.
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