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 |