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    Re: Polarising Filters
    From: Michael Daly
    Date: 2007 Nov 27, 01:37 -0500

    Ken Muldrew wrote:
    
    > Sure, but that happens any time your eyes are exposed to the sun (whether
    > you are looking at it or not).
    
    You're right - I should have added that this is only significant when
    you are looking through a telescope (binocs, or your sextant's 'scope),
    as that increases the light grasp compared to the naked eye.  Barring
    internal stops, the 'scope will take light on the area of the objective
    lens and concentrate the light on the cornea in an area more or less the
    same as the exit pupil of the 'scope.  This works out to be equal to the
    magnification of the 'scope - so if you have three times magnification,
    you are getting three times the intensity of the radiation on the cornea
    (less absorption in the 'scope).  To be conservative, get rid of the UV
    before it hits the objective lens.
    
    If you are using a small 'scope on normal land objects (birding, for
    example), the problem is minimal, since there isn't a lot of reflected
    UV.  Some surfaces reflect UV better - snow and water - but I don't know
    how much more this represents.  Sextant users are unique in routinely
    observing the sun and the filters provide comfort for both direct and
    reflected (horizon) views - use them.  (Note that there is always one
    filter less on the horizon than the index mirror - indicating less
    reflected light intensity).
    
    > The article was discussing damage that
    > resulted from the focussing of sunlight through the lens of the eye.
    
    Yes - I'm just trying to make sure someone doesn't read that as applying
    to the outside of the eye too.
    
    > advice given in this article is that anything that attenuates the sun's
    > light enough to comfortably look directly at the sun is sufficient to
    > avoid retinal damage. Can you tell us what your former colleague's advice
    > was?
    
    Well, the article suggests that safe is a lower level than comfortable
    (2 vs 5).  The conventional advice that Ralph gave was comfortable was a
    minimum - and that the levels outside of visible must be confirmed
    (since there is more to it than just retina).  The amount of UV
    attenuation in the eye is dependent on age - the older you are, the more
    yellowed your cornea.  Yellow filters blue, violet and UV - people I
    know who have had corneal transplants with lens replacement (two people,
    both over 60) have expressed amazement at the colours they see now.
    Children's eye's are more transparent and require more protection from
    UV to protect the retina, though not a huge amount more.
    
    The article suggests that smoked filters are ok, whereas Ralph was
    adamantly opposed to them.  I've played with them once long ago and the
    smoke comes off way too easily to be safe - especially in the hands of
    someone who doesn't necessarily understand the concept.
    
    As a person who is slowly losing sight, I feel that it's better to be
    safe than sorry.  Pardon my scepticism and conservatism on these matters.
    
    Mike
    
    
    
    
    
    
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