[TheForge] Re: glass IR filter

[Mike Porter]

Bruce,
You can block  up to 85% of IR with two 1/4" thick pieces of window glass 
and an air gap between them, for as long as it takes them to heat up and 
start radiating IR themselves; that's true. You will only have the 
equivalent IR protection of a #2 ANSI filter while you're waiting for them 
to heat up. On the other hand, I wouldn't personally sneer at that amount of 
protection for short periods. Some folks might consider the half-inch of 
glass before you even add the color filter a little heavy, while others not. 
Would I be willing to use such a system at times? Sure. Would I recommend it 
to others? No. This is another one of those choices people must make for 
themselves. But, their decision should be an informed one.
Mike P.

----- Original Message ----- 
From: "Mike Porter" <michael.a.porter at comcast.net>
To: "Sponsored by ABANA" <theforge at mailman.qth.net>
Sent: Thursday, February 09, 2006 8:53 PM
Subject: Re: [TheForge] Re: Subject: Didymium? I say "no way."


> Bruce,
> Any light in sufficient amounts will burn your eyes. It is true that the 
> smaller the wave lengths, the greater the amount of energy they carry. 
> This is why UV is more harmful than IR. However, they both have the same 
> main hazard--they are invisible. You don't react to either UV or IR. There 
> is no pupil contraction to either one of them, no blink reflex, no urge to 
> look away, as there is with too much visible light. This has every bit as 
> much to do with  physical impact as the square of the distance.
>
> Also, only far range IR is felt as heat. Near range IR is not felt at all, 
> just as green light isn't felt. So, if it packs so little energy compared 
> to every other light band, what is the big deal? That there is so very 
> much of it whenever anything is heated past 1000 degrees. The amount of 
> energy that super heated objects put out as IR is huge before you ever see 
> the faintest glow of visible incandescence. As they brighten into the 
> visible range, the IR multiplies as well.
>
> Lots of people can't just look away from the heat source. How do you braze 
> or gas weld without looking at the heat source? How do you pound 
> incandescent hot iron without looking at it? I'm not saying everyone 
> should rush out and get protection from IR. That's your individual choice. 
> I was originally answering a question asked by someone who does want that 
> protection.
> Mike P.
> ----- Original Message ----- 
> From: "Bruce Freeman" <freemab222 at yahoo.com>
> To: "theforge" <theforge at mailman.qth.net>
> Sent: Thursday, February 09, 2006 6:45 PM
> Subject: [TheForge] Re: Subject: Didymium? I say "no way."
>
>
>> 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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