Modular filter/polarizer rotating holder

[giusciccone96]

I have recently assembled an OFM in order to measure the birefringence of polymer samples.

This requires to have two polarisers (either circular or linear), one placed on the illumination side (before the sample) and the other placed in front of the camera (after the sample). Polarisation microscopy usually requires to freely rotate one of the two polarisers by a given angle.

To achieve this, I and Dr Bruno Tiribilli at the National Research Council in Italy came up with a solution which involves rotating the polariser on the illumination side by hand. We designed a filter holder that incorporates 4 small magnets to which a washer attaches magnetically. The idea is to glue the filter/polariser to the washer and attach the filter magnetically to the holder through the washer. The washer/filter complex can be freely rotated by hand. This solution is modular as one can create a library of washer/filter complexes that can be easily magnetically attached and detached from the microscope based on one’s needs. A few pictures will facilitate my description:

1. top view of 3d printed holder:
   

   

2. bottom view of 3d printed holder with magnets:
   

   

3. washer with attached filter:
   

   

4. full set-up:
   

   

The magnets dimensions are: 3mm diameter and 2mm thickness. They can be purchased online (e.g., from amazon https://www.amazon.co.uk/Magnet-Expert®-3mm-thick-Neodymium/dp/B00TACKU0O/ref=sr_1_8?keywords=3mm+magnets&qid=1636627252&sr=8-8).

Here is the .stl file of the holder:
filter holder openflexure v5.stl (261.1 KB)
It has been designed to have an outer diameter of 1" (25.40mm) so that standard filters from Thorlabs or Edmund optics can be fit easily.

Hope this helps!

Giuseppe.

[WilliamW]

That is very nice, and a neat way of placing it onto the condenser without it all being very thick. Does the holder rotate on the condenser to make the manual adjustment, or do you rotate the washer on the holder?

[giusciccone96]

Hi @WilliamW, thank you. The idea of the magnets is that they allow the washer to be rotated on the holder, while the holder stays in place with a tight fit

[LeedsOFM]

Very helpful, thanks. This would also be a good solution for geological thin sections. I managed to view thin sections in plane polarised and between crossed polars but only by sandwiching the thin section between two sheets of polarising film.

[r.w.bowman]

That’s really nice indeed, the washer is a very elegant solution! Is the washer a standard size (my guess is you’d need M12 or M16 to fit around the condenser)?

[giusciccone96]

The washer is standard size (25.40 mm in diameter approximately) and the central hole has been slightly increased to have a larger area of polariser