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    Re: Hello and a question about polarizer sun filters
    From: Fred Hebard
    Date: 2016 Dec 6, 10:46 -0500
    The crossed polarizers used in US Navy sextants in WWII and subsequently were, of course, safe to use at the time of manufacture.  They were constructed with sheets of polarizing plastic sandwiched between glass.  It is easy to imagine microscopic cracks developing in the plastic over time, or some other degradation.  The extinction rate of crossed polarizing sheets can vary.

    There should be some mechanism in the polarizer that allows one filter to turn but not the other.  The grease may have congealed, locking them together.

    Fred Hebard

    On Dec 6, 2016, at 4:58 AM, Geoffrey Kolbe <NoReply_GeoffreyKolbe@fer3.com> wrote:

    On 5 December 2016 at 23:13, Wayne Hilliard <NoReply_Hilliard@fer3.com> wrote:

    I guess I have 2 questions. Is this type of filter safe to use and assuming that these are the original filters could they have degraded with age.

    ​I would doubt that....​

    I have contacted the Robert White Company in Boston and have gotten pricing on overhauling this sextant and replacing the filters with new polarizers.  But would like to have input from the folks on here about this subject.

    ​Simple plastic sheet polarisers will ​(when crossed) give an extinction ratio of about 1:500. At the other end of the expense scale, birefringent polarisers (such as Glan-Taylor) will give about 1:
    ​. In my experience is messing about with filters, you want about
    ​ ​
    ​eduction​ in intensity for the sun to be comfortable to look at. The lesson is that cheap sheet crossed polarisers on their own are not safe to look at the sun.

    Then there is the problem of whether the polariser is truly 'neutral' across the whole spectrum and is only effective in the visible, say, but lets through the infra red.

    Geoffrey Kolbe

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