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    Re: Hello and a question about polarizer sun filters
    From: Bill Lionheart
    Date: 2016 Dec 6, 11:03 +0000

    Should we have the sun filters on our older sextants tested across the
    spectrum as IR damage as I understand (from my laser safety course)
    causes damage without initial discomfort?  What would be the easiest
    way to test it?
    On 6 December 2016 at 09:58, Geoffrey Kolbe
    > On 5 December 2016 at 23:13, Wayne Hilliard 
    > 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
    > View and reply to this message
    Professor of Applied Mathematics

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