Re: Melles Griot filter info

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

On 29 Feb 00 at 21:38, ADavidhazy wrote:

> > > The rep said it is a short pass filter that transmitted in the
> > > 425nm-885nm range with 50% cut-off at 900nm.
> > Not sure what the author of that article (see your original note below)
> > tried to achieve....doesn't sound related to IR at all....it
> > transmits visible all the way up to IR....it blocks more UV than IR.
> 
> Willem-Jan plus ...
> 
> I suspect that the scheme is to limit the number of IR wavelengths that can
> influence the meter's cell only to those that the film will respond to. Since
> the film's low end response is determined by the 87 filter and the high end by
> the film's inherent decay in sensitivity at the higher end ... near 900nm, it
> is only desirable that the meter also only respond to these wavelengths.
> 
> If the meter cell does not respond beyond 900nm all's well and good but if it
> does and there happens to be an excessive amount of wavelengths above 900nm
> present in a scene, then the meter will respond well and indicate a lot more
> energy present than if the wavelengths above 900nm were eliminated ... which
> is presumably the function of the filter quoted. (Although the filter only
> eliminates 50% of the energy at 900nm)        

Aha....mmm....this begs the question why anyone would mount such a 
deep-IR sensitive sensor in a lightmeter that normally is only used 
to read visible light....whatever it misreads for IR film because of 
this, it would misread perhaps even more for visible applications 
(perhaps the relative amount of error increases?....only a tiny 
amount of wanted IR, and still the same amount of unwanted IR).

(in which case the design should include a IR-block filter right from 
the factory)







 

--                 
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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