Thorium glass?

[koraks]

the warning it gives is that the yellowing does come back after UV clearing but I could see no mention of how long this re-yellowing takes.

As long as it did originally. A couple of decades.

 

[halfaman]

I had a Takumar 6x7 105 mm f/2.4 S-M-C with evident yellowing. I put the Ikea lamp for 2 weeks, some was removed but not completely. Another week more and there was no change.

It was not much, but I could noticed the yellowing when I changed the lens and put other without thorium.

 

[titrisol]

From what I read, probably Google AI, the warning it gives is that the yellowing does come back after UV clearing but I could see no mention of how long this re-yellowing takes. As no-one has mentioned this, it may be a wrong statement so can I ask, has anyone who has successfully cleared a thorium noticed this effect?

Thanks

pentaxuser

It takes a while, at least 10 yrs
I guess that if you take it outside form time to time it should stay clear

 

[blee1996]

I had a Takumar 6x7 105 mm f/2.4 S-M-C with evident yellowing. I put the Ikea lamp for 2 weeks, some was removed but not completely. Another week more and there was no change.

It was not much, but I could noticed the yellowing when I changed the lens and put other without thorium.

Same experience here: I bought the Ikea lamp, and one week later there is only small reduction in yellowness. I'm considering a dedicated UV flashlight, which could be used for UV photography as well.

 

[Petrochemist]

You can use regular light instead of sunlight. I'm not sure where the idea that it had to be UV light came from. I put my 50mm f1.4 SMC Takumar under an LED lamp for about a week and it is completely clear now.

It requires UV or short wavelength blue light to have sufficient energy to start the reaction. EM radiation is more energetic as the wavelength gets shorter. UV will probably be better than blue.

 

[Petrochemist]

Same experience here: I bought the Ikea lamp, and one week later there is only small reduction in yellowness. I'm considering a dedicated UV flashlight, which could be used for UV photography as well.

A UV flashlight can easily be used for UV induced fluorescence but reflected UV photography is MUCH harder needing an adapted camera & a carefully chosen lens.
It is theoretically possible with film but there's no way of telling if you're focused or judging exposure, until the film is processed...

 

[isaac7]

Let the yellow stay on as a filter for black and white film.

The problem is that it also darkens the glass. Some tests I’ve seen have been over a stop lost!

UV lights should work on lenses like the Takumar 50mm f1.4 since the affected element is the rear one. Some other lenses have the element inside. Lenses block UV so it won’t reach the inner elements unless you disassemble the lens. My IKEA lamp mostly cleared the yellow in my Tak in a couple of days but the last bit took the rest of the week. I had the light about an inch from the rear element.

 

[pentaxuser]

As long as it did originally. A couple of decades.

So 24hrs to maybe 1 week to clear and lasts 20 years. Sounds like a reasonable "bang for buck"

Thanks all who replied

pentaxuser

 

[DREW WILEY]

A few contradictions here. The fact that a variety of specifically UV LED options exists informs one these can be engineering for sake of UV transmission. But that stills leaves open the topic of FULL SPECTRUM LED lamps, which can look white at specified color temperatures (I use 5000K), but deliberately have a UV component to better simulate sunlight. The blue is actually intensified and shifted into the violet, with some of that reaching into the UV zone itself. No, you won't find these in home center junk bulb selections, where actual spectrograms are almost nonexistent. But there is an entire high-end full spectrum category which overlaps into LED color matching bulbs, which purposefully emit a small amount of UV too. And there's even more UV output in "health-related" LED bulbs designed to simulate sunlight in indoor winter spaces.

 

[koraks]

When pointed out you're wrong, there are a couple of possible responses:

1. Quietly shuffle off
2. "Gee, I didn't realize that"
3. Put up a charade referring to some rare and in the context irrelevant exception in an attempt to save face
4. Go full on delusional and actually believe the stuff you make up

3 & 4 can be entertaining, for sure, but tend to leave a lot of noise in a place like this.

 

[gbroadbridge]

From what I read, probably Google AI, the warning it gives is that the yellowing does come back after UV clearing but I could see no mention of how long this re-yellowing takes. As no-one has mentioned this, it may be a wrong statement so can I ask, has anyone who has successfully cleared a thorium noticed this effect?

Thanks

pentaxuser

I believe that the Farbe center's (yellowing) will come back if you leave the lens in the dark again.

Then you clear it again.

 

[brbo]

When pointed out you're wrong, there are a couple of possible responses:

1. Quietly shuffle off
2. "Gee, I didn't realize that"
3. Put up a charade referring to some rare and in the context irrelevant exception in an attempt to save face
4. Go full on delusional and actually believe the stuff you make up

3 & 4 can be entertaining, for sure, but tend to leave a lot of noise in a place like this.

This should be the opening post for every thread!

 

[Dan Fromm]

I believe that the Farbe center's (yellowing) will come back if you leave the lens in the dark again.

Then you clear it again.

Why do you believe this? Have you seen it happen?

 

[gbroadbridge]

Why do you believe this? Have you seen it happen?

Physics

 

[Dan Fromm]

Physics

Theory, not observation.

Do you know how long a lens with cleared thorium glass takes to yellow again? Name a lens, give a time.

 

[koraks]

Thorium‑232 has a half‑life of about 14 billion years, so it keeps emitting ionizing radiation essentially forever. That radiation continuously creates color centers in the glass, which means that even after you clear the yellowing with strong UV, the discoloration will inevitably return at the same rate as before.

Theory, not observation.

We can hypothesize that a brick dropped from a high building will land on the surface of the earth. That's just theory, not observation. It's also a physical reality.

 

[Patrick Robert James]

Just for a point of reference, I cleared a Pentax 50 in a I believe 3 days with some left over UV LEDs I used for an alt UV printer. IIRC it was pretty clear after two.

 

[Dan Fromm]

Thorium‑232 has a half‑life of about 14 billion years, so it keeps emitting ionizing radiation essentially forever. That radiation continuously creates color centers in the glass, which means that even after you clear the yellowing with strong UV, the discoloration will inevitably return at the same rate as before.


We can hypothesize that a brick dropped from a high building will land on the surface of the earth. That's just theory, not observation. It's also a physical reality.

I'll ask the question again. Have you measured how long it takes for a cleared lens with thorium glass takes to need clearing again? If so, which lens and how long?

I just pulled out a 30 cm f/9 Taylor Hobson Cooke Apotal that I cleared ~ 20 years ago under a UV-B compact fluorescent (not the best UV source for this purpose). It is very slightly yellow. Slightly. I don't have a spectrophotometer so I can't measure transmission by wave length. In my opinion it is still fit for general use.

The 232Th half life you quoted is true. But and however that has nothing to do with the rate at which it creates color centers in glass. Does anyone here know what that rate is, given Thorium concentration in the glass, and how quickly the yellowing becomes significant?

 

[koraks]

The 232Th half life you quoted is true. But and however that has nothing to do with the rate at which it creates color centers in glass.

Yes, it does. It's the cause.
The yellowing will become as significantly on a 'cleared' lens as it did on the new lens. In practice that boils down to the couple of decades that I mentioned before.

 

[DREW WILEY]

Was there even a constant amount of thorium in optical glass?, or did it differ case by case, and in relation to different glass types? Those kinds of questions might also be pertinent, but hard to answer unless one is themselves an optical engineer who has specific background.

I've seen a fair share of older thoriated lenses, but the only one I ever owned was stolen. Now the only kind of example I have left is an old Pentax MC Skylight filter which has appreciably ambered; but I don't want to UV experiment with that because I actually like its cumulative effect.

 

[koraks]

Those kinds of questions might also be pertinent

Not to the question how the rate of yellowing in a (back then) new lens compares to the rate of yellowing today, several decades into the game.

 

[DREW WILEY]

I guess someone will have to set up a quantitatively measurable experiment, with a relevant statistical sample of various lenses, and hope to live as long as a Galapagos tortoise in order to report back with the result.

 

[koraks]

That, or shift the old grey matter into gear.

 

[reddesert]

I have looked up technical references on radiation damage in glass a couple of times - one is linked in this thread https://www.photrio.com/forum/threads/yellowing-of-pentax-1970s-lens.217074/post-2948462
but unfortunately you need an institutional login like a university library to see the article - and the references I've found haven't given much detail on the mechanism of how the absorption of light undoes the damage to the glass structure. It seems likely that the absorption excites out-of-place atoms and they relax back into place, but the specifics aren't explained.

My guess is that since the color of the dislocations absorbs blue-ish light as well as UV, blue-ish light may result in enough absorptions to eventually repair the damage, but I have no empirical tests. I do have a yellowed Takumar lens and may someday try to expose it to different kinds of light each for some time and see if the rate of increase of optical transmission can be measured, but it is likely that with home equipment I would not be able to make measurements precise enough to be definitive.

 

[koraks]

My guess is that since the color of the dislocations absorbs blue-ish light as well as UV, blue-ish light may result in enough absorptions to eventually repair the damage

I think that's a reasonable guess. I'd add one more thought to it: photon energy. This correlates with wavelength and blue photos are thus less energetic than ones in the UV bands. There may be a threshold energy required to bump the stray electrons back into place and allow the matrix to relax. This likely depends on the material and perhaps on the type of F-center. I fed the question into AI (sketchy, of course) and it comes up with a range of 2.5-3.5eV, which encompasses the longer UVA wavelengths and the blue spectrum. It also alleges that UV is more efficient; this seems to make sense when taken at face value given the higher energy involved, which would make it more likely that an exposure event has the desired effect. I think it's a reasonable assumption that UV will work better, but that blue might get the job done provided sufficient exposure is given.