Removing yellow tint with UV light from radioactive lenses: Sharing personal experiences

[Niels - NHSN]

Here's an account of my personal experiences, in case you are in a similar situation:

I put a few weekly hours in a local camera shop checking and cleaning up old film equipment the shop receives. A while back we got two very interesting lenses; a Canon 35mm f/2 S.S.C "Concave" and a Konica Hexanon AR 57mm f/1.2.

Both lenses are quite valuable, but both were also severely yellowed/browned to the point where light transmission must have been reduced by around 2 stops. Color photography would definitely also be very problematic.

Glass yellowing is a typical phenomenon in lenses made with glass containing Thorium and Lanthanum, as the case was for the lenses in question.

It is common advice on the internet to expose an affected lens to hours or days of sunlight as UV exposure is said to help reversing the yellow tint process.

As I am living in Scandinavia where prolonged sunshine is a rare event and hardly ever happens during the short winter days, I was thinking of using a dedicated UV light to achieve the same.

I asked in an unrelated thread here on photo.net and it was suggested using a UV-C light.
That UV-C light would be effective made perfect sense although I have no understanding of what is going on in the glass. UV-C effectively kills DNA in organic material, and I imagined it would provide more "punch" over the other more gentle UV light options.

I did try to search the internet on previous experiences with UV-C to clarify it's effect on radioactive glass and type of suited lamp, but there seemed to be little if anything. I also consulted some AI chat-bots to see if they could provide some input, but all I got was warnings against using UV-C due to the inherent dangers involved in exposure to that light spectrum.

I decided to make my own empirical experiment and got a UV-C light tube from Amazon.de.
I created a silver foil lined cardboard box with ventilation holes to keep direct UV-C light exposure contained inside the box - I had no idea of how hot such a tube (25W) would run in a small box, so I installed a temperature probe close to the lens location to make sure I didn't melt the lubricants. I did place the box in a room with an open window to allow the ozone produced by the light to escape.

Observations:

• The UV-C light did not produce much heat - the inside of the box barely rose 2-3 degrees Celsius above ambient temperature.
• It is very inconvenient to work with UV-C light due to the dangers to you, your pets or plants if exposed to direct light (indirect reflections are OK, but not mirrored reflections from silver foil).  Also it produces ozone which shouldn't be inhaled so you don't want to be in the same room during the process.
• Most importantly: after more than 48 hours of very close direct exposure to the UV-C light source there were absolutely NO signs of reduced yellowing! (I do believe that all organic material that may have lived inside the lenses was effectively dead).

 

I re-consulted the internet, where I noticed someone claiming good results using a simple white modern household LED light bulb, and someone else reported good results using a black light UV-A panel (commonly used at parties).

Without wanting to throw more money at the project than necessary, I tried with a very bright Philips LED bulb I had at hand which claimid to provide the equivalent light of a conventional 100W bulb.
Fast forward 2 days of very close exposure in the foil-lined box: No visible improvement - whatever rays emitted from this particular bulb did not assist the project - other white light lamps may be different.

Without much hope, nor a wish to buy a large Black-Light UV-A LED panel, I cheapened out and ordered a simple 11W black-light UV-A LED lightbulb.

Lo and behold, after less than half a day in the silver foil box, the yellowing was visibly reduced.

After two days, it may not have disappeared completely, but enough that I wouldn't notice it if I didn't know what to look for, and certainly useable for color with a tiny bit of warming if any at all.

The bulb claimed to emit UV light in the 385-400 nm spectrum, which is what I would recommend to look for based on my experiences - apparently you do not need a very powerful light either - which is convenient as you would want to avoid exposing the lens to high heat. 

 

Further readings on use of radioactive glass in lenses: https://lenslegend.com/radioactive-lenses/

Edited April 25 by Niels - NHSN

[AJG]

Thanks for this--I haven't actually run into this problem yet, but it is good to know what will actually work.

[Ken Katz]

Thanks.  My FD 35mm F2 concave lens is a long gone memory, but its still good that someone tested how to reduce the yellow "radioactive" affect.

Does the repaired lenses now transmit more light?

[John Seaman]

Good information Niels. It's good to get the facts from someone who has actually carried this out, not the usual internet speculation.

[Niels - NHSN]

1 hour ago, Ken Katz said:

Does the repaired lenses now transmit more light?

I regret not making more precise measurements prior to the treatment, but yes, both lenses now perform within 1/3 of a stop from what would be expected for a given aperture. 

[dan_fromm2]

Hmm.  I used a UV-B compact fluorescent on some yellowed LF lenses.  The light was dim, the lenses took weeks to clear but they cleared.

[Niels - NHSN]

1 hour ago, dan_fromm2 said:

Hmm.  I used a UV-B compact fluorescent on some yellowed LF lenses.  The light was dim, the lenses took weeks to clear but they cleared.

So maybe the effective spectrum for cleaning yellowing is that of UV-A as I observed?

[SCL]

Good job. A couple of years ago I went with the sunlight route. It took almost two weeks to completely clear, but was well worth it, although my patience was wearing thin. If there is a next time, I think I'll try your method!

[tom_halfhill]

I have an old pair of Pentax binoculars with yellowed glass. Maybe the same treatment will work. But I've always wondered if it's safe to hold this glass close to my eyes for long.

[John Seaman]

7 hours ago, tom_halfhill said:

I have an old pair of Pentax binoculars with yellowed glass.

So do I, a yellow or gold tint was added deliberately by Pentax to some binoculars,, presumably to help viewing in dim conditions.

[Niels - NHSN]

It helps to look through the lens towards a strongly illuminated white background to determine if the glass is actually yellowed, or if you are just seeing the reflection of yellow anti-reflex coating.

Looking through the lens, you will expect a healthy lens to display minimal discolouration compared to looking directly at the background.

My old Nikkor-S Auto 50mm f/1.4 looks yellowish when light reflects in its coating, but the actual transmission looks fairly neutral.

Below image shows the mentioned Canon 35mm lens after 6 hours under the UV-A light and again after 2 days.
Unfortunately I didn't think of taking a photo under the same conditions before I started. I should also have measured the before and after light transmission.... well, next time.

[Niels - NHSN]

9 hours ago, tom_halfhill said:

I have an old pair of Pentax binoculars with yellowed glass. Maybe the same treatment will work. But I've always wondered if it's safe to hold this glass close to my eyes for long.

I don't think you have to worry. 

I saw a video of a guy holding a geiger counter directly on the front element of one of the famous radioactive lenses: the Asahi Pentax Takumar 50/1.4.

I don't recall the exact numbers, but from memory of my calculation at the time; If you held the front element directly towards your body for about 150 hours, that part of the body would be exposed to the same radiation a flight attendant is exposed to for a year.

Keep in mind, the flight attendant is exposed to the radiation all over the body, from all sides. In the above example you would only be exposed on a very small spot.

Besides, radiation from a small source, like a lens, lowers drastically with distance. One or two centimeters away from the front element the instrument hardly measures any radiation. 
Behind the camera, the radiation from the lens would probably be close to indistinguishable from the natural background radiation.

[Bettendorf]

22 hours ago, Niels - NHSN said:

  I saw a video of a guy holding a geiger counter directly on the front element of one of the famous radioactive lenses: the Asahi Pentax Takumar 50/1.4.

 

 

When I held a Geiger counter on my Canon 35/2.0 concave, all the noise came from the rear element.

 

[Niels - NHSN]

8 hours ago, Bettendorf said:

 

When I held a Geiger counter on my Canon 35/2.0 concave, all the noise came from the rear element.

 

I don’t have a Geiger counter, unfortunately.

What was the radiation reading from the rear?

[Bettendorf]

55 minutes ago, Niels - NHSN said:

I don’t have a Geiger counter, unfortunately.

What was the radiation reading from the rear?

I don't either.  I brought the lens to work with me and took it to the x-ray guys in my department.

The reading he got at the rear was in millirems (or converted to it) and then plugged that in to an exposure chart to see how dangerous it was.  He said something like you would have to wear that camera and lens around your neck 24 hours a day for several months (or years?) before it would be a concern.

That was about 14 years ago, but I wrote down all the facts.  If I find where I put that I'll report back here.

[SCL]

I'm always amused reading questions on this topic. Most people receive more radiation walking about their daily activities than they get from their watches, camera lenses, or probably binoculars.