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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?

Bill

On 6 December 2016 at 09:58, Geoffrey Kolbe
 wrote:
>
>
> 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
>
> 



--
Professor of Applied Mathematics
http://www.maths.manchester.ac.uk/bl

   

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