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Beat,
You are correct, and you really do not have to worry about any spectral
shift of the angled filter(s) as you would only need a very small tilt to
successfully "walk" the reflected light out of the light path. One engineer
suggested 1-2 degrees of relative off normal angle would do the trick, and
that would not appreciably shift the interference coatings performance.
Only drawback would be if the scattered light managed to find its way around
the filter and into the optical path.
Dan
Dan Osborn
Fluorescence Microscopy Product Manager
Omega Optical, Inc.
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-----Original Message-----
From: Confocal Microscopy List [mailto:[log in to unmask]]On
Behalf Of Beat Ludin
Sent: Friday, March 14, 2008 5:38 AM
To: [log in to unmask]
Subject: Re: interference filters
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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
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