Re: denomination of Schott/Heliopan filters

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

On  2 Apr 00 at 11:57, Eckhard Stephan wrote:

> Hello everyone,
> 
> during a discussion between Gavin Wrigley and me about his new IR pages
> we came to talk about the denominations used by Schott / Heliopan for
> their glass filters. I always thought, that in names like RG-XXX XXX
> stood for the filter's cut-off frequency. Meanwhile I'm not so sure
> about that anymore. Maybe someone could clarify this? 

It's the 50% transmission mark actually....not 100%....there might not 
even be a point of 100%, neither transmission nor blocking....which 
in fact creates the next problem: the shape of the curve indicates 
whether filters can be compared....a very steep filter has a 
different characteristic than one with a very gradual curve.

I think this applies in particular to the RG9, and Hoya R90 (which in 
fact is not visible opaque, even though it's 50% mark rates it as a 
Wratten #87A).

--                 
Bye,

Willem-Jan Markerink


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


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