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From: cmpbsdb@gitpyr.gatech.EDU (Don Barry)
Newsgroups: sci.astro
Subject: Re: Solar Filters
Message-ID: <2445@gitpyr.gatech.EDU>
Date: Tue, 21-Oct-86 00:11:11 EDT
Article-I.D.: gitpyr.2445
Posted: Tue Oct 21 00:11:11 1986
Date-Received: Wed, 22-Oct-86 06:03:09 EDT
References: <332@alliant.UUCP>
Organization: Georgia Institute of Technology
Lines: 31


I have run tests as well on Welding filters, and have found the 
#14 and #15 filters I have tested to be perfectly safe for direct 
viewing of the sun.  UV is no problem because of glass' inherent 
absorption of it, as well as the design of the filter (which is designed
to filter a color temperature hotter than that of the sun's surface!).
Infrared radiation is also taken care of (one reason why these are rather
thick), even though the vitreous humour of the eye is rather opaque to 
wavelengths much beyond 1000 nm.  

Although with the sun, safety must *always* be the rule, several common 
materials have a spectral density suitable for solar viewing.  exposed,
black-developed B&W film (say, from a leader, or custom-made) is safe if
the density is sufficient to make the sun's image of reasonable brightness.
   ****WARNING**** - the same is *not* true for color film, whose dyes exhibit
in some cases absorbances with dangerous valleys near the visible region in
danger zones to the eye.  B&W silver emulsions have a uniform absorbance 
which renders them rather safe.

If in doubt, don't do it.  Projections are always the safest way.  And
*Never* use a filter material on the back of a telescope eyepiece, even
those little filters that come with telescopes.  I have heard first-hand of
one such filter that *exploded* after several minutes viewing, fortunately
when no one was looking.  The heat in this area of the telescope is most intense.


-- 
Don Barry (Chemistry Dept)          CSnet: cmpbsdb%gitpyr.GTNET@gatech.CSNET
Georgia Institute of Technology    BITNET: CMPBSDB @ GITVM1
Atlanta, GA 30332      ARPA: cmpbsdb%gitpyr.GTNET%gatech.CSNET@csnet-relay.ARPA 
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