In article , whunter <whunterjr@xxxxxxxx> writes
Pictures I have seen of these lenses suggest that the 'yellowing' is
remarkably homogeneous. Does your experience concur?
Yes - cutting a section through the lens shows that the discoloration is
homogenous throughout the glass, not restricted to the surface.
Your direct
observation of the lens in question is evidence, not speculation.
That's my point.
I
have never owned this lens. With uniform discoloration in a relatively
short time period, I would suspect a slowly developing chemical
discoloration from the rare earth or contaminants more than coloration
induced by radiation.
That is not the discoloration that the topic addresses - and rare earth
compounds *are* radioactive sources! The term "rare earth" refers
directly to the Lanthanide series of elements.
Just because the glass contains a trace quantity
of a long lived radionuclide does not automatically link ionizing
radiation to the color change.
Thorium-228 half life is under two years!
With this information, I am more
intrigued about the history and chemistry of the specific glass which
was used at that time. My general statement re the stability of glass
to ionizing radiation is fact. I have no scientific experience with
glass containing radionuclides but have seen many pieces of antique
Heisey glassware ('Marigold' as I recall) which utilized uranium as a
coloring agent. The radiation damage from the trace U-235 requires
decades to develop. It is manifested by physical degradation of the
glass which is not uniform. I suspect the uranium salts were either
not mixed well or were phase segregated in the molten glass.
But these types of glass use much more than a trace of radioactive
material - as can be confirmed by checking them with a counter.
Also, I am not aware of ANY organic compounds used for coatings - the
coating fabrication, being a plasma deposoition process, tends to
preclude most organics. I have specified diamond coating on some
optics, which technically is an organic material but not an organic
compound, and most coating materials are generally MgF or similar
materials.
Diamonds, inorganic (NOT organic) carbon, are remarkably resistant to
radiation, but not the technology of 1970 +/-.
Diamond, and indeed any pure form of carbon, is certainly a special case
and not typical of the era but the definition of organic is "carbon
containing", so I specifically mentioned this in case any readers
considered diamond to be so.
Vapor deposition of
fluorides, are certainly the standard of today.
Always has been - it is almost impossible to obtain the uniformity of
and control the thickness by any other technique.
Likewise there are
organic based coatings available, but I lack detailed knowledge of
these products and their precise use in optics. My 'varnish' analogy
was dangerous because optical coatings are extremely thin films in
relation to a 'coat of varnish'. Do you know the exact technology
which was utilized to coat the lenses which have 'yellowed'?
As far as I am aware there is no special technique related to the
coating of yellowed lenses - the yellow is in the volume of the glass.
Certainly radioactive materials have been used in coatings as well, most
notably caesium oxide and thorium oxide, but I do not believe this to be
the case with these optics.
Published
tables identifying MC as purple, black, yellow, etc., contributes
nothing to the quest for understanding. Was it vapor deposition or
solvent based technology?
The most common multicoating of commercial optics was, and still is, a
series of layers of magnesium fluoride, zirconium oxide and aluminium
oxide vapour deposited, usually from a plasma, on the lens.
More complex coatings have been developed for specialist requirements
and to improve hardness, diamond and boron phosphide etc. but these are
not typical commercial optics coatings and certainly were not available
at the time.
Improvements to pre-treating the optic surface to ensure adhesion of the
coating certainly has been made, but none of this relies on an organic
layer between the coating and the lens - indeed the whole principle of
coating forbids such a layer.
The aspect of coating the most people know of is the thin layer of
material on the lens which ensures that the reflection from the surface
of the coating and the surface of the lens are 180deg out of phase at
the nominal wavelength, thus ensuring that the two reflections cancel
each other by wave interference. What appears to be less well known is
that the reflections from the lens and the coating must have the same
amplitude - and that places restrictions on the refractive index, and
hence the choice of the material, used in the coating. You can't just
put down an organic glue layer to stick the coating to the lens - it is
the interfaces between the lens/coating and coating/air which determine
the amplitude of the reflections, and hence the efficiency of the
coating in the elimination of those reflections.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's pissed.
Python Philosophers
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