Re: polarizing filters with b&w infrared

["Willem-Jan Markerink" <w.j.markerink@xxxxx>]

On 30 Oct 96 at 17:51, Tom Benedict wrote:

> So has anyone tried an IR polarizer?  They're there (at least you can
> order them from optics houses like Newport, Melles-Griot, and Oriel).
> Has anyone tried using them?  They'd be linear polarizers, but I doubt
> anyone is using autofocus for HIE anyway (or did I just put a really big
> foot in my mouth? ;) )

First, like Joe said, polarizers indeed loose their effect beyond 
750-800nm. Actually, you can create a rather dark IR filter by cross 
polarizing two pol's. Near-IR is unaffected, yet visible is blocked.


Thusfar I have tested three species, but all have one or more 
caviats. The most effective polarizer is hideously expensive, about 
US$700 for a piece of 10x10cm. I once managed to get a sample of it, 
but it also incorporates a rather dark red filter, darker than #89B. 
So control in front of a lens is at best difficult.
Next best option took me more than a year to get a a sheet, but my 
enthousiasm was dimmed fast....quality was poor, far from optically 
flat. You could see focus shifts with your naked eye....8-((
Then I got a second best option, but after a few tests I have
difficulty justifying a minor polarization increase with the hassle
of using a gel holder for mounting a piece of pol foil. Beyond a 
certain level of experimentation it becomes counter 
productive....8-))


Btw: with normal pola sheets, one can get the circular variety even in 
foil. A bit thicker than linear, but still one piece of foil (0.8mm 
if I am not mistaken).  

--
Bye,

       _/      _/       _/_/_/_/_/       _/_/_/_/_/
     _/  _/  _/               _/       _/  _/  _/
     _/  _/ illem    _/     _/ an    _/  _/  _/ arkerink
                     _/_/_/  



      The desire to understand 
is sometimes far less intelligent than
     the inability to understand


<w.j.markerink@xxxxx>
[note: 'a-one' & 'en-el'!]

------------------------------

Topic No. 13