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    Re: Hello and a question about polarizer sun filters
    From: Geoffrey Kolbe
    Date: 2016 Dec 6, 09:51 +0000


    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:
    106 
    ​. In my experience is messing about with filters, you want about
    105
    ​ ​
    ​r
    ​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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