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    Re: Sextants with Polarizing filters
    From: Peter Fogg
    Date: 2006 Jan 27, 11:42 +1100

    George contends:
    > Reduction due to applying a perfect polariser to unpolarised light - 1
    > stop (as it should be
    > theoretically).
    Presumably this comes from the idea that 50% of the diffused light is being
    passed, and presumably what George means by a "perfect polariser" is one
    without inherent density (including surface reflections, etc). Of course
    this perfect thing only exists in the imagination.
    In practice single polarizing filters lose between one and a half and two f/
    stops. An exception may be when they are faced with light that is already
    polarized (which happens in nature, and also when accepting light from the
    first polarizing filter) and then they lose about a stop. While confirmable
    by experiment (as I did yesterday) it is such an established fact that
    polarizing filters often come with this printed information (1.5 to 2 stops)
    provided by the manufacturer, as indeed normally do all photographic filters
    - it may be important to know in advance the density loss of a filter you
    plan to make an exposure through.
    > Additional light loss due to passage of polarised light through a perfect
    > aligned polariser - 0
    > stop.
    > Additional light loss at each passage through a real polarising filter,
    > due to its imperfections -
    > 0.5 stop.
    As outlined above, I think this idea bears further examination before being
    > Not significantly. You could shine light through the first polaroid ...
    As a keen amateur of matters pedantic, George, I'm sure you will be happy
    for me to point out that Polaroid is a registered trademark of the company
    founded by the remarkable Dr Land. On the other hand, you are welcome to
    spell polarizing anyway you like, although I bet you pronounce it with a
    'z', and not polariceing?
    > And it's got nothing at all to do with navigation. Sorry about that.
    Wrong again! (said he cheerfully). Polarization seems to have an application
    in sextant filters - remember? Also Polaroid sunglasses (and those made by
    other manufacturers) have an important navigational role to play when worn
    on-board by navigators, for a variety of reasons including the reduction of
    glare and their ability to reduce reflections from the surface of water.

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