Ole,
that is indeed strange. I did the very same thing with several Apo-Lanthars, and the Geiger counter showed the fronts to be radioactive. The front group is also the one that shows the discoloration. In addition, the patent shows the front lens as the one with the unusual (for the time) combination of refractive index and Abbe number.
I'll check it again - and bring my Zeiss Planar as well (a very slight hint of warm tone discoloration might indicate a "special glass" element).
Here's my measurements of a 150 mm Apo-Lanthar. The front cell is very obviously tea colored. The measurements are one minute integrations with a Geiger counter. The background reading was 38 counts per minute. All of the measurements of the lens were made with a glass surface approximately 10 mm from the window of the Geiger counter.
Front surface of front cell: 13,990 counts. Rear (inner) surface of front cell: 886 counts. Inner surface of rear cell: 39 counts. Rear surface of rear cell: 25 counts.
The cross section diagram shows the five elements in two groups, with the first and last elements being positive. Clearly the front element is the radioactive one.
My guess is that all Apo-Lanthars use thorium glass. But there are lens models that have been redesigned to omit the thorium glass. Early Xenotars have thorium glass, late ones don't. The same for the 35 mm f1.4 Nikkor. This is based on radiation measurements I've made of a few samples. Color cast is only a clue and not definitive about the presence or absence of thorium glass.
And a link to a serial number table: Age of Voightlander Lenses at
http://www.photo.net/bboard/q-and-a-...?msg_id=004OCu.
The lens I measured has serial number 3,645,xxx -- the table says it is from 1954.
Michael, thanks for the measurements., which are in line with what I have seen. I am still interested in Ole's remeasurement though.
As far as changes in the lens formulation go, there is a claim in the Vadenecum that Voigtländer changed the formulation in 1956 to a non-radioactive one. The "youngest" Apo-Lanthar that I had access to was a 210mm from 1966 according to the age table (a 6,xxx,xxx number) and that was as radioactive as the ones from the early 1950's, with a reduced discoloration (but still visible), which I would attribute to the 12 years difference in age. So if there was a glass change, it would have to be after 1966.
Michaael and Arne,
Sorry I dropped out of the thread for a while, my home DSL has decided to act like dialup, soon to be replaced. How do I know it's not radioactive? Well, I'm a senior health physicist (radiationprotection) and I have plenty of radiation detectors just sitting around. It's clear to me that thorium has not been intentionally added.
Steve
Steve, the Thorium (as Thorium oxide) was put into the glass intentionally to influence the optical parameters (Abbe-number vs. refractive index). Lanthanum and Thorium glasses were a development from WWII; nowadays only the former are used, for obvious reasons. Thorium glasses were used in a variety of lenses sometime up to the late sixties/early seventies.
Btw, Michael has a nice web site on the first lenses to use thorium glasses, the Aero-Ektars:
http://home.earthlink.net/~michaelbr...aeroektar.html
Arne, there have been cases of manufacturers redesigning lenses to replace thorium glass. Of the several Schneider Xenotars and 35 mm f1.4 Nikkors that I have measured, early ones are radioactive, and late ones are not, so I believe that there was a design change. As far as I know, the manufacturers didn't disclose the design change.
Arne's article in View Camera magazine has Voigtländer LF lens production ending in 1972, so the 1966 Apo-Lanthar that Arne measured as being radioactive is fairly late production. Steve, is your lens more recent (serial number table link above)?
I'll check the serial when I get home and post the date.
Steve
Mine's 6939xxx, which would date it about 1968?
Steve
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