Re: Thoriated Lenses

[William Kolb <William.Kolb@faa.dot.gov>]

Ron,

You may want to double-check those lenses.  I don't know of anyone who is
intentially using thorium these days.  Thorium was added to optical glass in the
past to increase the index of refraction and reduce dispersion. U.S. patent
2,466,392, for example, describes an optical glass with a refractive index of
1.66 to 1.69 and Abbe values of 50 to 53 using a mixture of 36 parts by weight
of boric oxide, 12 thorium oxide, 12 lanthanum oxide, 20 barium oxide, and 20
calcium oxide.  Kodak, Schott Glass, Corning Optical Glass and other
manufacturers produced several such glasses until the late 1980s for use in
military optical systems, consumer cameras, projectors, and aerial cameras (the
Aero-Ektar f2.5, 7-inch lens was one of the better known).  Up to thirty-percent
by weight of thorium oxide is permitted in optical glass, and lenses containing
as much as 28-percent thorium oxide were produced for many years by Kodak. In
high thorium content lenses, the glass darkens to a reddish-brown color over
time due to self-irradiation. Many 35 mm cameras made by Kodak and Pentax in the
1950s and 1960s, and some enlarger lenses (e.g., Nikon), also contained thorium
elements.  Tektronix Inc. sold the Elgeet 3" f1.9 Oscillo-Navitar lens to take
close-up oscilloscope photographs.  Some Bausch & Lomb microscope eyepieces were
reportedly manufactured with thorium glass but the practice was not widespread
and was discontinued in the 1960s after the error was discovered.  Microscope
objective lenses, however, did use thorium to a limited extent. Some American
Optical 50/0.80 oil immersion plan achromatic objectives (cat. no. 1016) used
thorium as did a number of Austrian made Reichert objectives, including the
fluorite 63/0.90 (no. 400514). 

If the lens was made after say 1985, you want to check for lanthanum.  This is
another rare earth used to obtain similar combinations of refraction and
dispersion. About 0.1-percent of all naturally occurring lanthanum contains La-
138, a mildly radioactive isotope that emits gamma and weak beta rays. A number
of Leica lenses have been patented with formulas containing lanthanum and Leitz
took pains to have a high lead content glass following such elements to protect
the film.  Lanthanum eyepieces for high-end amateur telescopes have become
popular recently but I haven't had a chance to check them for radiation.

bill
william.kolb@faa.dot.gov

_______________________________________________________________________________
Subject: Thoriated Lenses
From:    rgmorgan@lanl.gov at Internet
Date:    3/4/98  1:18 PM

Hi Folks,
I've noticed that most of the better camera lenses I've seen lately are
loaded with thorium.  Can anyone tell me why?  It occurs to me that the
thorium would increase the density and (surely?) the refractive index, but
I don't know enough about optics to know why that would be an advantage.
The only thing else that I could think of is that the thorium might boost
the temperatures in the casting process...again, why might that be an
advantage?
Thanks,
       **************************************
       Ron Morgan <rgmorgan@lanl.gov>
       Operational Health Physics (ESH-1)
       Los Alamos National Laboratory
       MS E-503, Los Alamos New Mexico, 87545 (USA)
       Phone (505) 665-7843
       Fax   (505) 667-1009
       Voice pager 104-1787

      mailto:rgmorgan@lanl.gov

       **************************************