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Christophe, it's a good question. I'm not sure why there are so few "notch dichroic" filters, but I imagine it is because the filter companies don't get a lot of requests for optics like that. Think about it, the dichroics are usually intended to pass broadband light (usually fluorescence) to a detector, and the idea is that you would have also used an exciter and emission filter (positioned normal to the direction of light) to pass and block the laser light respectively. 

Dichroic mirrors are used in laser engines to combine laser beams into one light path, but long pass and short pass mirrors can be used in succession if your arrange your lasers in increasing or decreasing wavelengths, so you don't need a notch filter here either.

Having said that, you can use a notch filter intended for normal incidence of light at some non-zero incident angle, but the position of the notch band with shift up or down the spectrum depending on how you turn it, and the transmission efficiency will lower. There are some newer filters that actually are intended for variable incident angles of light, though. For example, you could use a filter like this from Semrock (no commercial interest):

http://www.semrock.com/FilterDetails.aspx?id=TBP02-440/16-25x36

I think Chroma and Omega Optical (no commercial interest) might also make something similar. Turn this filter about 45 or 50 degrees, and arrange your light sources such that the arc lamp reflects off the mirror (instead of transmitting) and position your laser so that it transmits straight through. I think this would do what you want it to do.

John Oreopoulos
Research Assistant
Spectral Applied Research
Richmond Hill, Ontario
Canada
www.spectral.ca


On 2011-11-30, at 8:32 AM, Christophe Leterrier wrote:

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> To join, leave or search the confocal microscopy listserv, go to:
> http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
> *****
> 
> Hi,  
> 
> On a widefield/FRAP setup, I want to combine a laser at 491 nm (fiber coupling) with an HBO xenon lamp (direct mount) on the back of my scope. The way things are arranged, I need to put a 45° dichroic mirror so that the lamp light will go through the mirror into the scope and the laser light will be reflected into the scope. So I'm looking for a dichroic mirror that would transmit everything (from 400 to 750) but a small window arround 491 nm. The window has to be narrow so that I don't loose too much GFP excitation light from the lamp. Is that kind of dichroïc available? I could'nt find one browsing Chroma or Semrock offers. Of course there is the option of having a custom one made, but I was wondering if someone had a link to an existing offer.
> 
> On a related note, I found lots of notch filters but not a lot of notch dichroics. What is the difference between a simple filter and a dichroic? What happens to the blocked light in the case of a filter, isn't it also reflected like for a dichroic?
> 
> Thanks for your help,
> 
> Christophe
> 
> --
> Christophe Leterrier
> Postdoc
> INSERM UMR641 // Ionic channels Lab
> IFR Jean Roche, Mediterranée University
> Marseille, France
> http://www.cleterrier.net