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Re: [Rollei] Rolleiflex T and Lanthanum Glass

- ----- Original Message ----- 
From: "Fox, Robert" <RFox  >
To: <rollei  
Sent: Tuesday, January 20, 2004 9:32 AM
Subject: [Rollei] Rolleiflex T and Lanthanum Glass

> I was reading a description of Rolleiflexes (see link),
and in the
> section on page 36 it states that the Tessar introduced on
the T
> contains lanthanum glass for "even better resolution and
> correction than previous Tessars."
> http://www.foto.no/rolleiflex/Kapittel_24.pdf
> Do later Planars or Xenotars contain lanathanum glass?
Would lanthanum
> glass still release residual radioactive particles? If so,
how would I
> know?
> R.J.

  You would have to find the particular lens prescriptions
to find out exactly what kind of glass they are made of.
  Lanthanum glass is not particularly radioactive. The
radioactive lenses made in the past used glass with both
Lanthanum and Thorium in them. Thorium is radioactive. The
Thorium was added delibrately to modify the glass constants,
it was not an impurity as is sometimes stated. I have
somewhere the Bureau of Standards patent by G.W.Morey,
et.al. detailing the formulas for several of the rare earth
glasses later developed commercially by Eastman Kodak. The
glasses range from purely Lanthanum and Purly Thorium to a
mixture of the two and the use of other rare earths.
  The advantages of these glasses are relativly low
dispersion for the average index of refraction, and high
average index. This makes possible shallower curves, with
less spherical and other aberrations, and the possibility of
more complete chromatic correction. Even better correction
can be had by the use of a single aspherical surface. The
Zeiss Index shows examples of  experimental Tessars with
aspherical surfaces from as early as the nineteen-teens,
with practically no spherical aberration or offense against
sine condition. They were impractical to make commercially
then but would not be now.

- ---
Richard Knoppow
Los Angeles, CA, USA