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Dan -

Good point! Thanks for the explanation. I guess, another solution 
might be to put the filters at a certain distance and at a slight 
angle to each other, if you can afford the slight spectral change.

Beat

At 20:19 12-03-2008, you wrote:
>Beat/ Julio,
>     If you are talking strictly about interference filters (made 
> with dielectric coatings) the Absorbance of a particular filter is 
> mostly only a function of the glass substrate the coating is 
> deposited on, as the filter is working to attenuate incoming light 
> through reflection at the filter's front surface.  If you place two 
> of these in series, you will not see an additive blocking effect 
> because of reflections bouncing back from the second filter to the 
> first, to second, and so on, and there is always some transmittance 
> through the second filter that inhibits the level of blocking you 
> might otherwise expect to achieve.
>If you were to place a low absorbing  piece of glass (such as an NG 
>glass with ~95%T) in between the two filters, then the slight 
>absorption properties of that glass would inhibit the reflections 
>and you would achieve (or nearly so) the additive blocking.
>If you were using absorption glasses only as a blocking component, 
>then you would get the full benefit of their OD blocking at a given 
>wavelength.
>cheers,
>Dan