Re: Optics - Environmental Sealing


Hi John,
A very good book on the subject of mounting optics is "Opto-
Mechanical Systems Design" by Yoder. It covers the field pretty well
and can give you a starting point for mounting almost any kind of
optical component.

In your particular case, mounting a glass window in a plastic cell
and expecting it to hold a seal over the temperature range you quoted
might present a problem, as you suspect. If you can't reduce the
temperature range, you need to either look at reducing the thermal
differential expansion (why are you using plastic?) or look at an
elastic seal that won't degrade over time. Titanium is a pretty good
thermal match for optical glass, and RTV might work as a peripheral
elastic seal, if you can calculate the required thickness which will
deform properly under thermal expansion and yet won't induce too much
hoop stress in the window. Alternately, you could use a stainless
steel cell with a face sealing O-ring. If the ring rolls in its
groove, you might maintain a fair seal for a while.
You could even make a face seal from RTV. I would guess that if
the RTV were 1.5mm - 2.0mm thick (and very uniform) and no wider than
10 mm, and bonded to a cell less expansive than plastic, it would have
a negligible effect on the glass. But that is just a guess.
If you think water vapor will get inside the system, you could
either add a renewable desiccant cartridge to the system or heat the
optical elements to a point just above the dew point with low-wattage
heaters. Both methods work. In any case, it is best to avoid a lot
of air exchange. Air comes with lots of extra components which can be
surprisingly corrosive to optical coatings, and constantly renewing
them won't make your system better.

Wade Kelman

.

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