From: John De Armond
Subject: Re: Glass for viewing Eclipse?
Date: Sat, 17 Jul 1999 13:20:16 EDT
Newsgroups: rec.crafts.glass

The Crafty Owl wrote:
> 
> Thank you all for your advice. I really appreciate it.
> I was not planning to make these to sell - just for myself and friends
> to usa and as souvineers of a sort for us.
> 
> I will now see if I can get the #14 welder's goggle glass, and if not I
> will just use the mylar film viewers (which seem opaque unless you are
> looking at the sun) which are being sold by the Royal Observatory here.

I've followed this thread and watched things get out of control, as
they tend to do sometime when amateurs get involved with safety
advice.

Quick review of the hazards of watching an eclipse.  

1) 	IR radiation
2)	visible radiation
3)	UV radiation.

Naturally, one can see only the visible radiation or light.  Because
of its shorter wavelength, relatively more UV is refracted around
the image of the moon than visible light and the problem is
compounded because the darkness has one's irises wide open.  Though
present in lesser quantities, potentially harmful IR radiation is
also present.  Industrial hygienists are just now realizing the
hazards of IR to vision.  Long term exposure is known to damage the
surface of the cornea and the damage is of the type that can't be
repaired at the present time.

A parallel protection situation exists in the welding field and
means have been devised to completely protect the welder from
harmful radiation many times more intense than that coming from the
sun.  The dark lens many people are familiar with stop essentially
all UV and attenuate visible light according to the density.  It
does a poor job of blocking IR.  Fortunately one can now buy a very
cheap IR filter.  This is nothing more than a piece of polycarb
plastic vacuum deposited with a thin film of 24ct gold.  This filter
is designed to fit in front of the visible light filter in the
welding hood and blocks essentially 100% of the IR.

The #14 filter is far too dark for this application.  You'll see
little more than a faint glow of the corona through this lens.  The
only time I've ever seen a #14 lens used was in conjunction with
either ebeam or laser welding of high temperature superalloys and in
some foundry operations.  Very unlikely your local welding supply
store will have one that dark.  The standard stick and TIG lens is a
#10.  Some welders who are particularly sensitive to glare
(incipient cataract victims, for instance) will go to a #12.  With a
#12 I can see nothing but the arc and none of the work so #10 it is
for me.

During our last total eclipse, I took some excellent photos using a
Meade tabletop telescope with a 35mm camera adapter (kodacolor 100
using the camera's automatic metering.)  I taped a sandwich
consisting of a plate of #10 welding filter glass and the gold
filter across the objective lens.  IR filtering is necessary for
photography because many films are extra sensitive to IR which gives
the photo a blue pale. The magnification was sufficient to be able
to see the streamers (sorry, not much of an astronomy nut so terms
are approximate) emitting from the corona. 

One last note.  Lately I've seen some really cheap welding hoods,
usually supplied in the cheapo welding kits made in china and sold
at flea markets.  These hoods use plastic filters.  While they block
visible light as well as any other lens, they do practically nothing
to attenuate UV.  When you buy the filter, make sure you get a glass
one.

From: John De Armond
Subject: Re: Glass for viewing Eclipse?
Date: Tue, 20 Jul 1999 04:22:20 EDT
Newsgroups: rec.crafts.glass

Ric Rokosz wrote:
> 
> Neon John (johngdnospam@bellsouth.net) wrote:
> 
> : The #14 filter is far too dark for this application.  You'll see
> : little more than a faint glow of the corona through this lens.  The
> : only time I've ever seen a #14 lens used was in conjunction with
> : either ebeam or laser welding of high temperature superalloys and in
> : some foundry operations.  Very unlikely your local welding supply
> : store will have one that dark.  The standard stick and TIG lens is a
> : #10.  Some welders who are particularly sensitive to glare
> : (incipient cataract victims, for instance) will go to a #12.  With a
> : #12 I can see nothing but the arc and none of the work so #10 it is
> : for me.
> 
> I realy hope you're not serious about using a #10!!

Quite serious.

> I trust the media in Europe will make numerous announcements just prior
> to the event as to what to use to safely view the sun.

And I'm sure it will be at least as silly as some of the stuff I've
seen in this thread.

> You have to realize that there are people walking around today with full
> sight who on the night of Aug 11 will either be blind or have eye
> damage to the extent they will over time become blind or will have damage
> to the extent they will be considered legally blind.
> Seek out professional advise ,be it from a physics or astronomy dept at a
> university or from an eye doctor.
> 
> Sad very very sad...

yes, sad.  You just got advice from a professional and were too
ignorant to recognize it.  Before retirement I was a
health-physicist and owned a company which provided professional
services to clients all over the world.  Health-physics is the
subspecialty of the industrial hygiene profession that deals with
radiation safety.  "radiation" covers radio to gamma rays. (Hint:
that includes light.)  I am also a certified NDT inspector and in
the distant past, was a certified welder.  I'm curious on what basis
- credentials or otherwise - you criticize my advice.  Other than
what you read in the newspaper, of course.

On a personal level, I've now watched 4 eclipses in my life, the
last two through moderately powerful amateur telescopes.  All
observations use #10 welding filters.  The first two in a hood and
the last two with the filter taped over the objective of the scope.
(do not use the filter with the eyepiece.  The energy density at the
focal point will likely crack it.)  Other than a bit of
farsightedness that comes with age, my eyes are just fine, thank
you.

I find your advice to consult a doctor particularly humorous.  My
brother, who is a doctor, received a sum total of 4 HOURS of
radiation safety instruction in medical school!  My experience with
other doctors, mostly in the capacity of an expert witness helping
them to bail their asses out of malpractice lawsuits, is that while
most consider themselves experts on everything, their actual
knowledge is a bit less grand.  Your run-of-the-mill ophthalmologist
will be expert in damages caused by overexposure to light but will
be woefully uneducated regarding what level of light it takes to
cause that damage.  

As a homework assignment, why don't you research the energy flux of
light from the sun across the spectrum vs that of, say, a 300 amp
TIG arc at arm's length or a multi-kilowatt CO2 laser welding
machine and report back to us.  Yeah, you'll have to do a bit of
actual math but the results will be enlightening.  

John