Radioactive Pentax and Takumar lenses.

Some observations on yellowing lenses. The measurements are done with a Victoreen 190 radiation meter.

Lens Serial product number radioactive measured value
super-takumar 1:2/55 1327216 no
super-takumar 1:1.8/55 885276 no
super-takumar 1:2/55 2007701 37102 yes 50 uSv/H
super-takumar 1:2/55 2556625 37103 yes 50 uSv/H
super-multi-coated takumar 1:1.8/55 5016612 37104 yes 50 uSv/H
super-multi-coated takumar 1:1.8/55 5296854 37104 yes 50 uSv/H
SMC takumar 1:1.8/55 7081998 37108 yes 50 uSv/H
super-multi-coated-takumar 1:1.4/50 4958706 37902 yes 65 uSv/H
SMC takumar 1:1.4/50 7780013 37908 yes 65 uSv/H
super-takumar 1:2/35 1258114 yes 35 uSv/H
super-multi-coated takumar 1:4.5/20 5403707 43952 yes 1.8 uSv/H
super-multi-coated takumar 1:3.5/28 8315160 43872 no
super-takumar 1:2.8/105 1707085 no
SMC PENTAX 1:1.4/50 1500029 yes 65 uSv/H
SMC PENTAX 1:1.2/50 no
SMC PENTAX 1:2/55 no
SMC PENTAX 1:1.8/55 no
auto mamiya/sekor sx 1:1.8 f=55mm 110157 yes 13 uSv/H

Some extra notes, by putting a peace of isolation material about 1mm thick between a lens and the probe, the measured radiation halved.
Various sources on the internet report that thorium is the radioactive element, which was added to the glass to get a higher refractive index (thoriumdioxide to be precise).

Another curious find, although most people think that all Takumar and Pentax 1:1.8 (and 1:2.0) 55mm lenses have the same optical design, is this unlikely because some of them have radioactive glass and others not.

Clearing some Yellowed Takumar 1:1.4 50mm lenses.

I took three lenses which showed a similar degree of yellowing

Two of them got the blacklight treatment, the other one was my reference yellow lens.

And after a week:

A closer look at the 'hot' Takumar 1:1.4/50mm.

I have taken apart this lens to test the individual groups in front of a Victoreen 190 radiation meter, and found that the yellow marked elements are radioactive. I did not split the cemented group, but one side was much 'hotter' than the other side.

Above left the three front elements, below right the cemented group (element 4 and 5), above right element 6 , and below left the last element (7). Apart from the front three elements, all other groups show some degre of yellowing.
Then I gave element 6 the blacklight treatment for a week.

So, this element did clear.
Then I gave element 7, which was not very yellow to start with, the same treatment, but did cover half of its surface.

And consequently, half of it cleared.

So, what did I learn with these tests.

I did have more radioactive lenses than I thought I had.
Most of them have yellowed to some degree.
Yellowing can be cleared by a blacklight.
It is the glass itself which turns yellow, I cannot rule out that the cement not turns yellow also, but find it unlikely.

What about the causes of the yellowing, and why does it clear with a blacklight.

Well, I am no expert in this field, but searching the internet I found one likely theory.
The radiation from the radioactive element 'kicks' electrons out of their place to another atom/molecule in the glass. There they form what is called 'color centers', and then we get the yellowed (or browned) lens element.
This electron can be 'kicked' out of the 'color centers' by heat or light of the right intensity/wavelength. And there we have an explanation of the blacklight treatment.
That was the very short version, for the long version we have the internet .

And as bonus, a youtube video .