Re: Feather tracks in lens coatings - any preventative?

Hello Dr. Kinch and Group,

Please refer to the texts below for reference and perspective.

I am glad you took my questions seriously.

It is curious, is it not, that "fungus" on optical surfaces seems
to occur only when optical coatings have been applied? Ordi-
nary glass, uncoated lenses etc do not seem to show the de-
scribed "feather tracks" thought to be characteristic of the fun-
gal growth on camera and microscope objectives and eye-
pieces.

I never had the opportunity to see a fungus infested lens. I as-
sume that the feather track pattern is similar to the growth of
beautiful icy structures which one can see in wintertime on the
inside of window panes. Such growth is also called dendritic
growth and is typical of many microcrystalline structures. Un-
fortunately I can't present images of such structures here. A
magnificent reference with images can be found in Physics To-
day, Nov. 2000, with the title "Diffusion Limited Aggregation:
A Model for Pattern Formation". The images presented there
are typical of the microscrystalline structures one can observe
under the microscope formed under suitable conditions by
most inorganic and organic compounds on the surface of glass.

Images of fungus as observed in biological specimens and
made visible under the microscope using verious methods of
preparation can be found in the standard work "Principles and
Practice of Infectious Diseases" by Mandell/Douglas/Bennet.
These images do not look like feather tracks.

It is possible that fungal growth provides the environment for
microcrystalline formations by the creation of organic acids
which, acting on the substratum of optical coatings leads to
the formation of dendritic growth.

It is also possible that the optical coating undergoes micro-
crystallinization over time, depending upon its compostion
and the exact mechanism of deposition.

It may be worth while to use scanning electron microscopy
to visualize the "fungal growth" on the surface of an affected
optical element. Such an observation would almost certainly
be able to differentiate between a microcristalline pattern and
a biological skeleton.

GR.

"Richard J Kinch" <kinch@xxxxxxxxxxx> wrote in message
news:Xns9AC5607286E9someconundrum@xxxxxxxxxxxxxxxxx
NoSpam writes:

Am I very wrong in suggesting, that nobody in this
thread has convincingly presented any evidence that
the "hoary frost" is actually a fungus, that is a sapro-
phytic plant?

Could it not be, that it is the formation of microcrystals
within the layers of coating which were applied to the
lenses?

That's a genuine question, and why I suggested microscopic examination and
the potassium hydroxide tests. Although I am equipped for this
exam/testing, I don't have any "fungus" specimens at the moment; the one
badly etched lens I had I disassembled and thoroughly cleaned before I got
interested in the subject (indeed, this was how I became interested)
(maybe
someone has a lens ruined by fungus they want to contribute?). The
thorough
cleaning was applied because I believed the lens was fogged by some
tenacious
contaminant, and it wasn't until I had applied every polar and non-polar
solvent and alkali in the lab supply cabinet to clean it off, that I took
a
microscopic look to see that the fogging was a very fine surface etch, in
a
pattern that suggested fungus growing in the edges of the lens elements
where
dust collected in this very old lens, and sparing the center of the glass.

While I haven't done any serious research, if you Google, say, "lens
fungus"
you just get a lot of lore about the subject, nothing like an academic
treatise, although such may exist. The casual evidence does strongly
suggest
actual fungus: required humidity, required presence of dust or
contaminants,
observed musty odors, macroscopic patterns resembling fungi, etc. And of
course it may be possible that there is both true fungus *and* some
physical
crystalization process in various cases.

Here's a Zeiss page on the topic, oddly following what I've been saying
here:


http://www.zeiss.com/c12567a8003b58b9/Contents-Frame/29cd78d56723da79c12571180042df52

Another idea that occured to me is whether the optically polished surfaces
might somehow promote certain fungi to "creep". Most places fungi grow in
nature
are rough and porous, and maybe the organism "goes wild" when it finds
something hard and smooth to crawl out on next to its food. Because you
get fungus on lenses when other nutrient-rich objects in the same
environment don't show it. This might explain the perception that
coatings
are feeding fungi, when they're not (and why would MgF, etc., feed
fungi?).
This sort of creeping behavior is a characteristic of fungi versus
ordinary plants.

.

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