Polarizer for epi-illumination?

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pwnell
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Joined: Fri Dec 18, 2009 4:59 pm
Location: Tsawwassen, Canada

Polarizer for epi-illumination?

Post by pwnell »

As you know trying to take photos with a fluorescence excitation source using a 50/50 beamsplitter for normal white light epi illumination does not always work well. I usually suffer from reflections, huge lack of contrast and general washed out appearance.

I was wondering, if I add a polarizer to my fluorescent arm so that I have both a polarizer and an analyzer in the light path - would this help get rid of reflections?

curt0909
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Joined: Wed Oct 26, 2011 10:06 am
Location: Pittsburgh, PA

Post by curt0909 »

Yes, thats a common problem with epi bright field, especially if a coverslip is used. Having crossed polarizers in the light path will greatly reduce reflection. Just make sure to align the polarizers for max extinction(crossed).

discomorphella
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Location: NW USA

Post by discomorphella »

I know that Leitz makes a POL cube that does exactly that, and so does Olympus. I just don't recall what their catalog number is for it. Its basically a linear polarizer and polarizing beamsplitter combination that fits into one of the positions in your epifluorescence illuminator. That will give you cross-polarized illumination/imaging which will suppress the reflections. I find that I don't always like to have completely crossed pol conditions, so instead of the cube I sometimes use a combination of a rotatable linear polarizer in the illumination arm, then a nonpolarizing beamsplitter where your normal epiflourescence cube would go ( in the objective "infinity" space before the tube lens) and then a linear polarizer after that in its usual spot. You can also put a QWP or HWP or other optics into the illumination train as well.

David

edit, just thought of something else too, you need to adjust your partial coherence (the aperture in the condenser input plane, which is conjugate to your objective back focal plane) as well, to obtain decent images. You can also put various masks in the BFP for your epi illuminator and get forms of oblique illumination just like for transmitted light. This often helps with contrast etc as well.

pwnell
Posts: 2029
Joined: Fri Dec 18, 2009 4:59 pm
Location: Tsawwassen, Canada

Post by pwnell »

Thanks for the information. @discomorphella - do you perhaps know the Olympus catalogue numbers for these masks?

discomorphella
Posts: 607
Joined: Sun Oct 01, 2006 7:26 pm
Location: NW USA

Post by discomorphella »

I've never seen an official Olympus oblique mask for epi-illumination. I have seen homebrew ones in lab, but never one sold by Olympus. These were basically pieces of milled black Delrin sheet glued to a glass disc that fit into the slider (that normally held a darkfield disc) in a BH2/BMJ epi metallurgical illuminator. I will look for the Olympus version of a POL cube.

David

curt0909
Posts: 609
Joined: Wed Oct 26, 2011 10:06 am
Location: Pittsburgh, PA

Post by curt0909 »

I had one of these crossed polarizer cubes for the Nikon 'phot' series fluorescence illuminators as well. It was labeled 'IGS' but I'm not sure what that means. Another method(and cheaper) would be to place a high quality glass linear polarizer above the 50/50 mirror. Place a cheap camera polarizer filter or edmund optics polarizer film anywhere between the light source and 50/50 cube.

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