Projector heat filters

[P3D <StvnDJhnsn@xxxxxxx>]

>A theory that I have about the white fogging that has been seen on
>several TDC heat filters, is that the it is a result of a deposit
> from a vapour sticking to the hot or over heated surface of the filter.
> Over a period of time the surface contamination changes the properties
> of the glass at the surface helping it to become slightly ridged and
> pock marked.

The contamination on my heat filters was not slight.
There was a definite accumulation of a white powder
on the lenses, as well as on the metal holder.
It actually reminded me of the white residue left by
an old lead acid battery that goes bad.  My first thought
was that it was moisture related, but the projector is in
otherwise perfect shape, without a trace of rust.  The other
lens elements are unaffected.  So I would have to guess that
it has to do with the nature of the type of glass and its
composition.  Possibly a very acidic glass mixture used for
heat absorption?
There is a condition I've come across in my collecting of
glassware (I collect old darkroom graduates, my wife collects
depression glass) that I've heard referred to as 'diseased glass'
and has been attributed to some sort of fungus. But I've never seen
it to the extreme of my heat filters.  Usually it just leaves
the glass looking 'etched', or dulled.  I have one graduate I
was given that looks frosted, and no method I tried would remove it.
So I guess the only cure is to find a source for the appropriate
sized replacement glass.

-Steven D. Johnson


------------------------------