[TheForge] Re: Subject: Didymium? I say "no way."

[Bruce Freeman]

Unlike Mike, I have NOT intensely researched
protective eyewear.  However, I have one pedantic
correction to his posting, and one genearl issue.

The pedantic correction is that the yellow "sodium
flare" has nothing to do with soda reacting with air. 
Merely heating sodium to flame temperature is
sufficient to produce the flare.  Okay - who cares!?

The other observation is what I consider an excessive
concern about IR radiation.  IR is translated to heat
when it encounters anything that absorbs it.   If that
thing then gets warm, it will retransmit IR.  Ordinary
glass absorbs most IR, and many plastics do as well. 
You can tell if a lens absorbs IR:  place it in front
of a heater and see if it gets hot.  Unfortunately,
that will not tell you what wavelengths it absorbs,
should that matter.  

The thing about IR is that it is the least likely
radiation to get you in trouble -- because you can
feel it.  If you're so "manly" that you refuse to look
away from the fire as your face cooks, then your
eyeballs will cook along with your face.  Remember the
reverse-square law:  Twice the distance from the heat
is 1/4 the heat intensity.  Pretty easy to avoid too
much IR.

None of this is to say that IR should be ignored. 
But, finding a clear material that would PASS IR has
been a significant problem in science.  Ordinary salt
(NaCl) works, as do the chemically -related salts,
such as potassium bromide (KBr).  Even those start
absorbing if they pick up water, so they have to be
kept dry.

Bottom line:  Don't be stupid and try to cook your
face and eyeballs, and you'll probably have no problem
with IR.

UV is another ball of wax.  Never look at an arc, even
briefly.  Even the reflection of an arc can be
damaging.

Bruce
NJ

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