Good polarizer to put in front of a macroscope / microscope lens?

[Scarodactyl]

Hello everyone, I was hoping to pick some brains on this.
I have been wanting to equip my Leica Z6 Apo macroscope with a linear polarizer for better viewing inclusions inside anisotropic gems and minerals. My ultimate goal is to make a rotating polarizer to fit in the infinity space between the zoom body and head (though it will need to be slim, since it does not like having a lot of extra space there), but that's going to require some custom work and will be further out.
In the mean time I thought I might try Leica/Wild's own in-house solution, just slapping a rotatable polarizer on the end of the objective. There's plenty of working distance for it, and I already have a clamp that grips the outside of the objective which I can attach it to for quick installation/removal. In the past I've tried using a camera polarizer under a stereo microscope, but found that it introduced nasty abberations at higher mag/RI, so I thought I'd ask if anyone here had any experience with this kind of setup and what type of polarizer I should be looking out for.

[Ichthyophthirius]

Hi,

What polariser did you use? The tolerance for glass thickness in front of the lens depends of the NA. From the Olympus front mounted filters I would say that the polariser should be quite thin and idealy be mounted at a slight angle (to avoid reflexions).

Unfortunately, it's difficult to get glass mounted polarisers of that size that are very thin. Camera filters are the cheapest option. Acrylic-mounted polarisers can be thin but they don't have AR coating.

A circular polariser should be an advantage as you don't need a separate depolariser - is a linear polariser essential for your application?

Regards, Ichty

[Scarodactyl]

Thanjs! I don't remember the branding, but I still have it somewhere. It was part of a gemological setup for cross polarization but didn't work well with my stereozoom 7. That was at higher mag and resolution than it was probably tested at.

For my z6 the max NA is 0.234 at 9x.

It would probably be fine if it were circular as long as I orient it to act linear. It is necessary for removing the image doubling produced in birefringent minerals.

I have been pondering getting a thorlabs rotatable mount and trying one of their polarizers--I guess I will see what the thinnest I can find is.

[Ichthyophthirius]

Hi,

So you're using the Planapo 2.0x. I noticed that in the systems diagram, there is no provision for an analyser (10 315 306) mounted in front of the 2.0x. It's only for objectives up to 1.0x. https://downloads.leica-microsystems.co ... ata_EN.pdf

That's because at NA 0.234, the cover glass tolerance is only about 1 mm https://janrik.net/MiscSubj/2016/Tolera ... ctives.png and you also have a highly diffracting gem in the light path.

Maybe a suitable place would be the behind the 2.0x, were you can apparently place a "10 447 460" quarter-wave plate (likely an infinity space suitable for dropping in a polariser).

Regards, Ichty

[Scarodactyl]

Ah yes, that is interesting. I hadn't noticed the little lines didn't connect the 2x to the analyzer, though they did go out of their way to make sure the 2x had the same front diameter as the rest of them (which I'll admit is annoying).
The ultimate goal is to put it into infinity space, though right behind the objective is probably not viable for my usage--a little extra space there makes it start to vignette direct projecting onto APS-C.
I guess I'll try a couple things in front just in case it works, but I'll refocus on getting a good intermediate polarizer instead.
Edit: ah, here's why I was confused:

They changed their recommended configuration somewhere between the older and newer version of the parts diagram. Presumably they designed the intermediate quarter wave plate for the newer 5x objective, and brought the 2x along for the ride.

[Ichthyophthirius]

For a provisional setup you could just use polariser film (usually quite thin) in an old rotatable camera filter mount. Use a circle cutter to get a nice clean cut of the film

[Scarodactyl]

I will try that and report back!

[Macro_Cosmos]

Generally, only rotation on one side is required, right? For my XPL work, only the lightsource polariser actually rotates. I use a small B+W XS-Pro that I got from BH for cheap, I think it was a niche size or something. It was 46mm. Coincidentally, it lies perfectly on the light source output of my microscope too.

I think I have the thorlabs rotation mount you're talking about. It's the CLR-1 right? It works pretty well, rotates nicely.
Simply buy a 25mm or 25.4mm sized linear polariser from Edmind Optics and put it in. Thorlabs' SM1 works well with Mit's M26 threads. I recommend the EO polarisers over Thorlabs' offerings.

I have these in various sizes: https://www.edmundoptics.com/p/254mm-di ... ter/11674/
They work incredibly well.

Do you want high extinction ratio? EO offers high contrast ones for high pricetags: https://www.edmundoptics.com/f/high-con ... ers/12725/
These have 10,000:1 extinction ratio. You only need one however, the polariser covering the light source can be a basic one (ie low extinction ratio). You get the same high extinction ratio. I'd like to say it's theoretically better to have both matched, but I honestly can't see a difference.

Sometimes, EO glass linear polarisers do show up on eBay. I got a couple high contrast 50mm ones recently. It's worth looking out for.

Thorlabs' "glass polarisers" are rather thick. They are just polariser film sandwiched between 2 optical flats. You can do it yourself but you might not be able to mate the optics as well as they can. I still recommend EO ones for being thinner and cheaper.

Both companies offer polariser film that you can cut to the size you want. Thorlabs' film is brittle, you need to cut it with the potection layer still on, using a pair of small surgical scissors. They do not last long at all. I had a couple installed on a setup at work, after about 2 months, the film on the light source's side started to deform and the polarisation membrane started to peel off, needed to install a new one. Therefore I recommend using the higher quality expensive polariser on the objective side and use something cheaper for the light source. Even the glass ones will degrade when being used on light sources.

A cheap method is to use standard 52mm CPLs, mount the objective via an adaptor, and mount the polariser to whatever tube lens system you're using. This allows rotation straight away.

After examining the Z6 issues and maybe finishing the ITL200/CMH200 comparison, I might do one on polarisers. I'm not sure how I would approach it though. I intend to compare CPLs, linear ones, combinations of both, and high contrast ones to see if there really is a difference that justifies their respective pricetags. It would be interesting to me but I doubt people will find it useful.

[Macro_Cosmos]

Additionally, placing the polariser in front of the objective does have a visible impact on image quality. This of course depends on the NA, as I recently learned too. For say the Mitty 10x, it shouldn't matter too much since the NA is 0.28. Higher NA ones that aren't designed to shoot through glass will not like a thick piece of glass sitting in front of the specimen.

[Pau]

Additionally, placing the polariser in front of the objective does have a visible impact on image quality. This of course depends on the NA, as I recently learned too. For say the Mitty 10x, it shouldn't matter too much since the NA is 0.28. Higher NA ones that aren't designed to shoot through glass will not like a thick piece of glass sitting in front of the specimen.

0.28 is safe to see with or without cover glass but standard covers are 0.17mm thick while camera polarizers are several mm. I think it will be clearly affected. Did you try it?

[Macro_Cosmos]

Additionally, placing the polariser in front of the objective does have a visible impact on image quality. This of course depends on the NA, as I recently learned too. For say the Mitty 10x, it shouldn't matter too much since the NA is 0.28. Higher NA ones that aren't designed to shoot through glass will not like a thick piece of glass sitting in front of the specimen.

0.28 is safe to see with or without cover glass but standard covers are 0.17mm thick while camera polarizers are several mm. I think it will be clearly affected. Did you try it?

Great point. I've only tried with polarisation film which is very thin, maybe not 0.17mm but far thinner than glass ones. It's worth a try.

[abednego1995]

Just a musing... but if the WD of the objective is sufficient, shooting off a reflection on a plane piece of glass or someother dielectric reflective surface at its Brewster angle would give pol. Would become an awkward setup but as a possibility.

Cheers,
John

[Scarodactyl]

That is a really cool idea, though I think it would prove a bit impractical.
I am putting a self-made solution on hold because I found a nice cheap schott fostec ring light polarizer/analyzer combo, which should be easy to mount, might work and would also be nice to use as intended.

[R_REISENER]

If your Z6 is mounted on a M400 or M420, you can utilize the polarizer that is built into the vertical illuminator (assuming you have one). I sometimes utilize the VI unit as an analyzer for transmitted light, keeping the VI illumination turned off of course.

[Scarodactyl]

It is, and I do have one and that does work for a fixed angle polarizer (though iirc it starts to darken the corners of the image on my apsc camera at min mag). It isn't too easy to rotate though.