Re: Hoya R72 InfraRed filter

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

On  5 Sep 99 at 0:33, Marco Pauck wrote:

> > Another question, in Hoya's catalougue, I saw 2 infrared filters, the 
> > R72 & the M90. I'd suppose the M90 lets in less light than the R72. 
> > Only question is, what exatcly is it supposed to let in & what is it 
> > meant for ? 
> 
> The R72 cuts off at about 720nm and the RM90 at 900nm, hence their
> names. The R72 is actually a very versatile filter that may be used
> with Kodak HIE as well as Konica Infrared 750 and Inford SFX200 with
> good results. I use it a lot. The RM90 is very 'strong' and usable for
> Kodak HIE only.

It might look strong on paper, but the one I have resembles an #88A 
in Wratten *visual* terms....it simply leaks significantly below 
780nm (the limit for eye vision at daylight intensity)....
Or: not just to the level of being able to see the filament in a
light bulb, but to the level of seeing the entire bulb as well as
surrounding objects.
I recently fooled around with this RM90 and a Wratten #87A, and even 
my night vision scope shows me that it is different.

I don't believe the B+H/Heliopan/Schott equivalents leak this much
visible light, those should be truely opaque. My #87A Wratten filters
definately are....not even a halogen filament will show up (halogen
lights are hotter than ordinary tungsten).


--                 
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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