Re: [Leica] CRACK!

["Dan Post" <dwpost@email.msn.com>]

Thermal shock can cause breakage at any extreme, even in glasses such as
Pyrex (tm) and other glasses with low coefficients of expansion. Your
windscreen is probably two pieces of glass bonded to a middle layer of
plastic which would keep the window from being broken since the two layers
bonded are thinner and dissipate heat better than one thick piece.
I have seen cases where extreme heat- a car sealed up in the sun, had the
side windows blown out from the thermal stress... these glasses, however,
are made with built in stresses that cause them to "pebble" when broken
rather than form sharp shards.
Optical glass is annealed very slowly to eliminate stresses that might cause
abberation and faults when the glass is ground. The only optical glass I
have seen break from heat stress was a condenser in an old slide projector
that did not have a heat proof glass shield, and it was a molded piece at
that. Most optical glass, crown and flint in particular, have a co-efficient
of expansion that keeps the optical path true with all environmental
temperatures you might encounter.
In summary- "Don't sweat the small stuff... and it's all small stuff!"
Dan

dwpost@msn.com