Radio – Active lenses

Figure  1,2,3 : Radio -active glass from the 20s, 30s, 40s.

I love to use old (vintage) camera lenses for my photography, but tend to stay away from the oldest ones.  Up through the sixties,  and (for some lens companies) – into the seventies and beyond – lenses were sometimes made with radioactive glass.  When this glass was marketed, the term “rare earths” was sometimes used to imply a glass additive or lens coating that contained thorium.

My readings of stuff on the internet (who knows whether that stuff is true or not?) – seem to imply that the thorium content of the old lenses is not a very large health concern.  Yet – one wonders how they define the term “not very large?”  Just as a matter of precaution,  I purchase old film camera lenses that I know or think have a manufacture date from the eighties and beyond.  My readings indicate that such lenses are less likely to contain “rare earths”.

In Figure 1 is shown a piece of glassware from the thirties, containing a uranium level which comprises two percent of the plate’s content.  Sorry for the horrid picture – blame it on the cell phone camera and my intentionally long distance from the subject (LOL).  BTW, it’s not my plate.  Reading about antique plates tells me that a two percent content is common, so most such plates will have that level of “rare earths”.  It’s quite common to find these items in antique stores and in secondhand goods stores.  I don’t know this for a fact, but I think that the two percent figure really refers to the ore that contains the active substance, which then would be reduced from that level.

Some folks say that the danger, if any, is equal to a few extra X-rays taken at the doctor’s office per year (or even less than that).  Is it more or less than the risk of second hand smoke?  I don’t really know.  More reading tells me that many people think antique plates do not constitute a large health concern, but again I take my precautions:  I didn’t use one for breakfast this morning.

The only foolproof way to determine whether or not a lens has thorium is to use a sensitive alpha-beta-gamma capable geiger-counter.  These can be had for a couple hundred bucks from online suppliers, and might not be a bad idea for an old lens enthusiast.  Ever wonder why a geiger-counter is called a geiger-counter?  Glad you asked.  It uses a tube invented by a guy named Geiger, that counts photons.  In particular, it counts the very energetic photons (tiny, tiny, tiny little globs of energy) – that emanate from radioactive substances, and that make up the “rays” known as alpha, beta, and gamma rays. Each little “click” you hear from the counter is a single photon hitting the tube. The meter on the front of the device indicates the cumulative amount of photons over a period of time. It’s still a matter of opinion whether or not the old lenses present any appreciable risks.  To each his own.

OK,  so why did they put the thorium in the glass, or in the coatings? My readings on this issue point to the glass/air interface, and how that interface reduces the amount of light that can pass through a lens.  Such a reduction of light can amount to as much as four percent per lens.  The thorium allows most of that lost light to pass through the lens.  One needs to consider that a “lens” is really a set of 5, 10, or 15 individual lenses, or elements, in groups inside of the body of the tube that we refer to as a “lens”.  So, 10×4 percent = 40 percent, which in the case of a ten element lens reduces the light transmittance by almost half.  This would produce poor low-light photography.

Apparently, newer lenses make use of glass and/or coatings that don’t need to be radioactive to work.  I’m for that.

Vaguely related article …

Note: the author is an amateur photographer, and does not possess a degree in medicine or nuclear physics.  All articles are his opinion, conjecture, or short night results.


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