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    Re: Sextants with Polarizing filters
    From: George Huxtable
    Date: 2006 Jan 25, 00:57 -0000

    Peter Fogg wrote-
    |> From: Frank Reed
    | > ... the idea is to use crossed polarizing filters
    | > which can be rotated to produce any desired  amount of darkening
    | > (within a certain range)...
    | Sounds like a good idea. On the certain range: each polarising filter
    | reduces the brightness of light passing through by about one and a half f/
    | stops, or 150% (this varies between manufacturers; 150/200%). So two used
    | together would already have quite a significant minimal darkening effect,
    | which could be halved by using only one. The maximum effect is complete
    | opacity; no light gets through - try it with a couple of pairs of polarising
    | sunglasses.
    | A great advantage is that polarising filters significantly reduce glare,
    | just what you want reduced when trying to pick out the horizon against
    | sunlight glittering off the sea. I think polarising sunglasses are the best
    | sunglasses for on-board use. Another reason is that by reducing reflections
    | from the surface of the water they allow you to see more clearly below the
    | surface. Just one head of coral unseen can ruin your whole day ...
    Some things I don't understand about Peter Fogg's recent message are
    1. How can you reduce the brightness of light by more than 100%? If you have reduced it by 100%,
    then you have already achieved complete darkness.
    2. Yes, if you introduce one polarising filter, you will approximately halve the light intensity,
    for initially unpolarised light, because you are excluding the 50% of the light that is polarised at
    right angles to the direction of the filter that lets light through. Plus a slight further
    reduction, to allow for losses in the plastic material and in surface reflections.
    But then, if you add a second filter, that's polarised in the same direction as the first, that
    won't halve the light intensity again, because now all the light entering it is polarised in the
    preferred direction. This second filter will therefore cause a slight reduction in intensity, as
    before, because of further losses in that extra layer of plastic and further reflections, but
    nothing like the reduction that the first filter caused. That's a deduction based on theoretical
    reasoning, not on any practical tests by me, so it may possibly be contradicted by experiment. I
    would be interested to learn if it is.
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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