Polarizing filter for microscope objective

[Pau]

Rogelio, here you have, whith its original (if I remember right) plastic bag, the small plastic tip is custom made to facilitate mounting it in my scope

[PauloM]

Pau, are you sure this is a Circular Polarizing Filter (CPL) and not a Linear Polarizing Filter which happens to be circular in shape?

The reason i'm asking is because this is the first time I'm hearing about a CPL being used in a 'scope.

Cheers,
Paulo

[Charles Krebs]

I believe the analyzer in my BH2-UMA in also circular. Circular polarizers can be used for crossed work, you just need to be sure that the "front faces" are facing each other. Circular pols use linear polarizing foils at the front face (facing subject) with a 1/4 wave plate behind it (facing camera body). The 1/4 wave plate alters the linear polarization to avoid the metering and AF difficulties with polarized light. As long as the linear sections face each other you are OK for crossed work.

But there is a very large difference in how camera polarizers seem to work when used for this purpose. The quality of the camera filter (in my experience... such as B+W versus Tiffen, for example), does not necessarily predict how well they will extinguish when crossed (degree of extinction and/or color casts). The ones the microscope manufacturers provide really do seem to work extremely well in these respects.

I've never seen a microscope arrangement that used a polarizer between objective and subject. One will be between the light source and subject, and then one between objective and eyepieces/camera.

While I have never tried it, this Edmund product look as if it might be useful (the spec for transmission when crossed looks very good):
http://www.edmundoptics.com/onlinecatal ... uctid=1912

It would be fine for the filter between light and subject. But not sure how it would be optically in the image path behind the objective.

[PauloM]

Well, live and learn...

I assumed that since in materials science it is important to quantify the birefringency of the specimen, a permanently mounted 1/4 wave plate would make the use of something like the Michel-Levy chart more difficult.

I guess that for other techniques, such as DIC, the presence of said plate it doesn't matter, or is even beneficial.

Regarding the photography-grade optical filters, I too was never able to achieve adequate extinction with them, I always got a uniform deep blue hue for a background. On the other hand, a cheap Zeiss off of ebay works wonderfully.

Truth be told, even with the photo pol filters I was able to make some wonderful observations of the birefringent properties of some substances, so don't let the lack of a 'scope-grade pol filter hold you out.

[Pau]

Paulo,
I think Charles aswered you perfectly. It's very easy to test: My analyzer, like any other circular pol only works with one side facing the subject (or the polarizer for X pol work) but not whith the other side. In linear pols both sides are equivalent.
In fact I also was very surprised when I bought it.

[PauloM]

Pau, if I may ask, which microscope do you have, and for what application do you use the pol filters? DIC? EPI DIC?

[rjlittlefield]

I assumed that since in materials science it is important to quantify the birefringency of the specimen, a permanently mounted 1/4 wave plate would make the use of something like the Michel-Levy chart more difficult.

As I understand the process, the key point is that the 1/4 wave plate is mounted on the side of the polarizer away from the specimen.

In that position, it will not affect the colors produced by the sandwich of linear polarizer + specimen + linear polarizer, so the chart can be used without change.

--Rik

[g4lab]

The petrological pol scope has its analyzer after the objective.

But in gemology sometimes an analyzer is positioned above the specimen but under the objective of a stereoscope and it works well enough.

DIC starts off with polarized light and then uses a birefringent prism to shift the phase of the ordinary and extraordinary rays which then pass through the specimen and may be shifted even more. Then after passing through the objective another birefringent wedge recombines the two rays leaving only the phase shift from the specimen to develope the contrast. So DIC objectives need to be as strain free as pol objectives , probably much more so.

I have looked at numerous microscope objective between crossed polars over the years and never was able to see any anomalous colors. You can easily see such colors in glass or plastic that has strain.

[Chris S.]

Wow--I'm not surprised to be deluged by good advice at this forum, but I sure appreciate it. Thanks much, all!

From what I've learned, I'll definitely place the analyzer (didn't know this term--thanks, Pau) between lens and camera. And it appears I should buy a cheap but good used analyzer on eBay--for a higher extinction factor and lack of potential color casts, compared with standard photographic polarizers.

I'll probably make my own drop-in filter housing--unless someone can point me to something ready-made. I haven't found one. And though Craig's suggestion seems elegant and quite easy to do, Andrew's point that pol filters are not really intended to hold much seems well taken.

Is there any reason that the analyzer has to be close behind the objective? If not, I'm thinking of putting it much closer to the camera. Like many users of Nikon bellows, I need to put an extension tube between it and the camera in order to get sufficient clearance to mount the camera. It occurs to me that I could replace this extension tube with a home-made filter housing--a piece of PVC or other pipe, with Nikon adapters on the ends, and some kind of slot to rotatably hold an analyzer. Does anybody see a glaring error with that approach?

Pau--Yes, I do my exposures by hand. This isn't a big deal. I take a shot and then look at the histogram on my computer screen (I'm shooting tethered, so I quickly see a big image on my screen of what I just shot). The histogram tells me if I need more or less light. Right now, I shoot with speedlights in a dark room--so adjusting light is a matter of moving the flash closer to or farther from the subject, something the camera couldn't do anyway. Or I'll add or subtract diffusion, or adjust the output of the speedlights. I'm mostly using older speedlights that don't communicate with modern digital SLRs--so in any event, I need to adjust light by hand anyway. Again, I don't find this very hard to do at all.

Charles, thanks much for your input. As I said above, I'll go with a surplus analyzer from a quality microscope brand, based on your and other's input. Regarding the Edmund polarizing film, I happen to have a sheet of it on my shelf--purchased it for Xpol closeup photography (pre Bratcam days) but have not gotten around to using it. Those specs do indeed look pretty good for use on the lights. I notice that Edmund also has unmounted glass polarizers, and would perhaps have tried one as the analyzer if not for the advice to get one from a microscope.

G4lab, thanks mucho for letting me know that in some applications, putting the analyzer in front of the objective works OK--I really wondered about that. But since it appears to also be OK to put it behind the objective, I think I'll go ahead and do that, to avoid using up precious working distance.

Again, thanks all. You guys are a huge help.

Cheers,

--Chris

[Pau]

Pau, if I may ask, which microscope do you have, and for what application do you use the pol filters? DIC? EPI DIC?

I use a trinocular Zeiss Standard, mainly whith Leitz objectives and eyepieces.

My main uses are cross polarization for petrographic study (rock thin sections) and to avoid reflections whith incident ligth through fiber optic guides, in the last case also whith Nikon infinite corrected objectives on tele lens and macro lenses on bellows.
Unfortunately I don't have yet DIC stuff

[RogelioMoreno]

I remember had seen a circular polarizer (some text that I read) on a microscope, in that case the circular polarizer was used to convert circular polarized light to linear polarized then an analyzer (linear polarizer) used to check the amount of shift caused by the specimen.

Linear Polarizer => specimen => circular polarized light => circular polarizer => linear polarized light => analyzer (linear polarizer).

I think that it was used to check is the circular polarized light that is output from specimen was left or right handed.

Rogelio

[Chris S.]

Since my remaining questions were buried in my excess of words, let me give them a bump. Any thoughts?

"Is there any reason that the analyzer has to be close behind the objective? If not, I'm thinking of putting it much closer to the camera. Like many users of Nikon bellows, I need to put an extension tube between it and the camera in order to get sufficient clearance to mount the camera. It occurs to me that I could replace this extension tube with a home-made filter housing--a piece of PVC or other pipe, with Nikon adapters on the ends, and some kind of slot to rotatably hold an analyzer. Does anybody see a glaring error with that approach?"

Thanks,

--Chris

[Craig Gerard]

Chris,

In your question you have used "glaring error"; that may be one downside to the 'close to camera' approach - glare - I presume. As an alternative approach, you could add an extension at the lense end of the bellows.

I expect we need to determine what is required for your purposes. If you have the pieces on-hand that I suggested earlier in the thread, then, such an arrangement may help to initially determine which way you need to go regarding a solution.


Craig

[Pau]

"Is there any reason that the analyzer has to be close behind the objective? If not, I'm thinking of putting it much closer to the camera.

I'm not aware of any optical principle for do so, in fact, in some old monocular petrographic scopes it's placed in the middle of the tube. But in modern scopes, with a lot of prisms and lenses in the head it is very logical both by desing constrains and to avoid any strain effect of these optics. Also, microscope analyzers are usually of small diameter (15-30mm) and this will avoid to place it near the camera in a bellows setup.

Like many users of Nikon bellows, I need to put an extension tube between it and the camera in order to get sufficient clearance to mount the camera. It occurs to me that I could replace this extension tube with a home-made filter housing--a piece of PVC or other pipe, with Nikon adapters on the ends, and some kind of slot to rotatably hold an analyzer. Does anybody see a glaring error with that approach?"

I don't know the details of Nikon bellows, but your idea seems complicated (of course I know you are capable of make nice complicated setups ) . Usually you don't need to rotate the analyzer, its easier to rotate the polarizers in the light soures, and almost indispensable if you use more than one light source.
Because I use a vertical setup (both with the bellows and of course with microscope) the gravity nicely holds it over the objective.

[PauloM]

As I understand the process, the key point is that the 1/4 wave plate is mounted on the side of the polarizer away from the specimen.

In that position, it will not affect the colors produced by the sandwich of linear polarizer + specimen + linear polarizer, so the chart can be used without change.

--Rik

Quoting Homer Simpson, "Duh!
/me smacks his forehead!

Thanks Rik!