Re: Melles Griot filter info

[ADavidhazy <ANDPPH@xxxxxxxxxxxxxxxxxx>]

> > 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)        

I think I explained this correctly but it is possible I simply obfuscated the
issue and if so please forgive me.

regards,

Andrew  o o  0 0 o . o  Davidhazy, Imaging and Photo Tech
         \/\/\/\/\/\/          http://www.rit.edu/~andpph
__________|        |_____________________________________


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