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    Re: Sextants with Polarizing filters
    From: Peter Fogg
    Date: 2006 Jan 25, 08:32 +1100

    > 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 ...
    
    
    

       
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