MC Rokkor-SI 28mm f/2.5 "radioactive" repair attem

[devinw]

I got this lens for only $20 and it's super clean except it has the classic yellowing that can happen with one or more of the elements which are doped with Thallium. I've heard of people exposing the whole lens to UV light to clear it up, but I decided I'd go ahead and just take all the elements out and try to expose them to a high power UV lamp we have at my work.

Here's the lens. See the yellowing!:


Rearmost element removed... Little/no yellowing:


Front element. No yellowing:


Left: Inner lens cell #1. This clearly contains the yellowed element. Right: Inner lens cell #2 (includes aperture assembly):


This inner lens cell contains 2 elements. 1 is removeable, the other appears to have the retaining ring potted in place:


And here we have the culprit! It appears to be a doublet. I hope that the information online about the doped optics yellowing is not incorrect and that we don't have a cemented doublet in which the cement has yellowed. If this is the case, I fear nothing can be done:




Next up I will try to treat the yellow element with a chamber which we had constructed for Ozone cleaning, as it has a wide UV spectrum. I tried the UV Arc lamp for 240 seconds and didn't see any change, but it has a fairly narrow wavelength band (it is for UV curing adhesives).

I'll report back if I make any progress, and reassemble!

[devinw]

Success! I went back to the UV lamp and hit it for longer and it finally cleared up. After about 10 minutes of exposure, I noticed what appeared to be a spiderlike pattern in the outer edge of the adhesive in the cemented doublet, so I stopped. I'd say the lens is probably 85% clearer than it was, but I couldn't go any more and risk it.

I also had to clean a few surfaces of some of the elements due to them having quite a bit of what appeared to be lubrication film (common on old lenses). I used optics grade Methanol and clean swabs and blew them off with a deionizing air gun.

UV lighting the yellowed lens:


The improved lens:


And all put back together!:



And a couple of quick snaps with it. It's quite sharp!:




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[miran]

Congratulations! Job well done. Lens looks good.

[memetph]

Congrats and thank you very much . I have this lens and I have tried already a UV treatment without success . So I use it as it is , loosing 0.7 ev and adjusting the WB . Now I know what I have to do.

Thanks from Poland to Portland !!

[Teemō]

It does appear to be the glass that yellows rather than the cement, though I believe both can occur. I don't believe the mechanism for the yellowing is specifically understood, but supposedly caused by disruption of chemical bonds (between the different materials in the glass?) as a result of gamma ray emission and decay of the radioactive products within the glass.

[devinw]

[devinw]

[Boris_Akunin]

[devinw]

That's interesting, Boris.. I think we have the "standard filter" in there which is ~350-500, but I'll go check. If that's the case, it sounds like I could have benefited from a bit deeper UV (lower wavelength).

[Teemō]

[devinw]

[Teemō]

[devinw]

Yeah, that Canada balsam is fully organic, so I could definitely see that getting screwed up by things especially like lubricants which may have petroleum products. It would be interesting to find out what they were actually doing back then.

Another interesting thing I noticed is that several of the lenses have a black coating on the outer diameter, presumably for scatter resistance? Whatever it is, it was not in great shape after 50 years. It didn't come off but was definitely quite soluble to the Methanol I was using to clean the lenses. Care had to be taken near the edges.

[Lightshow]

The black paint(?) reduces unwanted internal reflections and scattering and helps increase contrast.

[Boris_Akunin]

[Teemō]

That's really interesting. Did it affect the 28mm F2, on the inside?

[Boris_Akunin]

[Teemō]

[Lightshow]

Plastic and paint can be damaged by UV, so I would recommend that you protect what you can from exposure, ideally removing the effected element(s) and treating them away from the rest of the lens.

[devinw]

I would guess actually the metal outer parts are black anodized aluminum. Bleaching of the organic dies is very common due to laser light of certain wavelengths or UV light. There are a couple of places that do a special "optical black" anodize in the US, but it's a niche market.

I have parts laying around here at work which were exposed to only years of red (640nm) laser light and even those have turned a beautiful "copper" color, but were black to begin with. I also have some which have had the anodize bleach all the way to silver just like Boris' pictures.

[Teemō]

[RichA]

[Lightshow]

[Kira]

Hi to everyone,

Well done Dewinw. That's a very nice job.

I have the same issue with the same lens. I started to disassemble it but I am stuck with the inner cell.

[devinw]