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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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