[zvon]

Precisely calibrated or not, the measurements in the original post are still quite useful for comparing relative emissions at various points and nonetheless give a good general sense of the exposure levels in various configurations. No one is publishing a journal article here.

Likewise, background radiation levels are going to have a negligible effect. I live in a relatively background high radiation area y US standards (Uranium bearing limestone causing high radon levels). Here, the typical background dose is less than 3 mSv per year. By comparison, 40.22uSVvper hour equates to over 352mSv/year, so less than 1% of measured values. Variation around the world is well under an order of magnitude, so given the numbers we are dealing with, even if it is using an average value, calibration using the background radiation is not going to skew the measured values nearly enough so that dangerous levels from the lens are obscured.

As to zvon's comment regarding alpha emissions below, for external exposure, they are virtually irrelevant when considering the safety of these lenses, since they do not even penetrate the skin. Beta radiation would be useful to know, but Gamma (produced secondarily in Thorium decay) is the main one to be concerned about, since it can penetrate a metal camera body.

Gammas are the main concern as they do penetrate a lot farther than alphas or betas, yes. But these gammas generated are relatively low in energy. While geiger counter does "scream" (there are may videos on youtube that measured what would a lay person say a lot) it is indeed low energy gamma and low in count, so even when you get some low dose of radiation from that lens, it is highly localized. I think that the biggest side effect from this radioactive element is not endangering your health while using it, but yellowing of the glue that was used to make that rear element and combination of radioactivity.

[butangmucat]

Gammas are the main concern as they do penetrate a lot farther than alphas or betas, yes. But these gammas generated are relatively low in energy. While geiger counter does "scream" (there are may videos on youtube that measured what would a lay person say a lot) it is indeed low energy gamma and low in count, so even when you get some low dose of radiation from that lens, it is highly localized. I think that the biggest side effect from this radioactive element is not endangering your health while using it, but yellowing of the glue that was used to make that rear element and combination of radioactivity.

I fixed the yellowing of my copy with the Ikea lamp method. Works well.

[zvon]

I fixed the yellowing of my copy with the Ikea lamp method. Works well.

so did I, but I was reffering to the technology of using thorium oxide lenses in general. At the time it was the best possible optical solution to the problem, but now, we have fluoride glass. Not causing any kind of yellowing over time, better optical characteristics, and no radioactivity exposure and health risks to those who handle it in production, and probably many more other, maybe financial benefits.

[butangmucat]

so did I, but I was reffering to the technology of using thorium oxide lenses in general. At the time it was the best possible optical solution to the problem, but now, we have fluoride glass. Not causing any kind of yellowing over time, better optical characteristics, and no radioactivity exposure and health risks to those who handle it in production, and probably many more other, maybe financial benefits.

I am not 100% sure, but isn't fluoride glass relatively soft and more affected by temperature? It seems many modern lenses lack a hard-stop infinity for this reason. I do MF all the time so I do value hard-stop infinity.