Evaluating the UV Bandpass of Older Lenses: a Complication?

[OlDoinyo]

We all lust after lenses that block as little UV as possible, and often contemplate many old and obscure optics in our quest. There all manner of twists and turns to this. Sometimes a manufacturer made a seemingly insignificant design tweak to a lens that caused one batch of a lens to transmit UV very differently from another. Sometimes what seems to be one lens model is actually several lenses from different manufacturers rebranded by a reseller to look similar, even though their UV characteristics may be anything but. And there yet another matter, the issue of lens yellowing and browning over time. Some lenses, though they may initially have been clear, will have their elements seem to turn yellow or even brown over time. In extreme cases, the lens's ability to be used even for visible photography becomes seriously compromised; however, it does not require anything so severe as that to cause trouble with ultraviolet transmission. Two lenses may appear identical to the eye, yet, if one has suffered worse yellowing than the other, that lens will also have substantially worse bandpass properties in UV.

 

The yellowing seems to be due to two possible reasons. The first has to do with the use of thoriated glass (glass containing the element thorium-232.) This was a common practice before 1980 to achieve certain refractive properties. The isotope mentioned is an alpha-emitter with a half-life of 14.05 billion years. There is little medical hazard from this glass, as it is only slightly radioactive and alpha particles are not especially penetrating: even a sheet of paper will typically stop many of them. However, alpha particles emitted in the depths of a glass element are energetic enough to create local electron-displacement defects known as F-centers, and these displaced electrons will often absorb light of certain wavelengths. If enough F-centers accumulate, the bandpass of the glass starts to be compromised, and mechanical strain may build up in the glass. In the most extreme cases, glass elements are reputed to have warped and fractured. Certain Asahi Takumars are particularly notorious for degradation related to thoriated glass, but there seems to be no comprehensive list of which lenses used it and which did not.

 

A second factor may (or may not) be the use of a glue called Canada Balsam to cement lens elements together; this was widely used before 1950 and perhaps somewhat thereafter until 1980. When fresh, the film is colorless, but aging does cause changes, and some of these changes may cause light absorption. Some claim that using Canada Balsam next to a thoriated glass element causes exacerbated issues, as the glue is slowly bombarded with alpha particles over time. I don't know if I entirely believe this--the glue film, if applied correctly, should be soap-bubble thin, and it would take a very intense absorber to stop many photons in so little thickness; but that is what some claim.

 

The good news is that these changes seem to be reversible by bleaching the lens with UV radiation. In some cases, leaving the lens uncapped on a sunny windowsill for a few days or weeks is said to have cleared the problem; others advocate disassembly of the lens and irradiating just the offending element(s). Some lens owners have used mercury lamps and LED lights to try to speed up the process. Most claim that the lenses, once bleached, remain clear until the problem redevelops (which may take many years.)

 

So next time you evaluate a lens, should you ask yourself if it contains thoriated glass, and whether it has been photobleached before testing?

[Alex H]

That is of course true, but:

 

1) How are we supposed to know if the lens has thoriated glass - not all of these lenses are documented. Manufacturers do not provide such data. If you think that presence of thoriated glass can indeed be detrimental to UV-transmission of the lenses in question, it would be good to start with the list of lenses that have it. Most lenses that I know of that have thoriated glass are fast normal lenses - their UV transmission may be affected by other factors too, such as use of other glass types with high refractive index, and thickness of glass elements. Do you have a reference to UV-transmission of the thoriated glass in general? Is there a list of thoriated glass types? (to be honest, I did not check it). And what about Lanthanium glass?

 

2) Canada Balsam transmits UV almost to 300nm, much better than most of the lenses that are not "dedicated". I mentioned it already several times in this forum, in particular in those discussions where it was suggested to disassemble and re-cement lenses with modern cement to increase UV transmission. I do not know what effect thoriated glass can have on Canada Balsam. It is a substance that can deteriorate when storage conditions are not optimal, I have few lenses like that, but even balsam with bubbles and separation in one of the lenses I tested did not affect UV transmission as far as I can tell. The lens performed on pair with Cassars and Focotars. Of course, it was not as sharp as it should have been, but that is another question.

[OlDoinyo]

Natural lanthanum consists of isotopes 138 and 139. the latter is stable; the former has a half-life of 1.02x10¹¹ years and then decays via (emissionless) neutron capture. Thus, there would seem to be no comparable issue with lanthanum-containing materials. Some lanthanum-containing glasses have been found to have trace thorium contamination, which probably accounts for some of the rumors; however, such contamination is insufficient to be of practical importance.

 

Here is the best-known list of lenses alleged to contain thoriated glass:

 

http://camerapedia.w...ioactive_lenses

 

I would bet that this list is not all-inclusive, however. If the front or rear element is thoriated, it is said that it can be detected by laying the lens end-down on a sheet of film in the dark and waiting a few days before development; a fogged outline indicates thoriated glass. This would not work if the offending element was inside the lens, however. Likewise testing with Geiger counters.

 

I agree with your skepticism about Canada Balsam, for the reasons stated above.

[Alex H]

I do have three or four lenses from that list (fast 1.4-1.7, normal 40-55 mm), but I never thought of trying them for UV in the first place. Not because of thoriated glass, but because of their complex design and thick lens elements made from glass with unknown properties.

[Bill De Jager]

Not all the radiation is alpha. The decay chain from thorium 232 includes alpha and beta emitters. There's also some gamma along the way, but it's usually omitted from diagrams of the decay chain due to being a smaller component at any stage where it appears.

 

It takes only 10 years for the thorium 232 decay chain to become fully populated when starting with refined thorium, so any lenses with this isotope are emitting some gamma by now. If I put my SMC Takumar 50/1.4 behind sheet metal, the Geiger counter still reads far above normal if it's close enough. Gamma is present.

 

https://web.archive....hain/Th232.html