Weak polarizing effect

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Saul
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Weak polarizing effect

Post by Saul »

I started playing with polarizing filters on my microscope, but I'm getting very weak effect. What I'm doing wrong ?
For the retarder I'm using a cellophane - can it be crinkled or it should be perfectly flat/smooth ? Distances ? Thicknesses? Placement ? Polarizing filters (I think) are OK - at the right angle they are killing the light. Something else ? I'm using Mitutoyo objectives, AO and Nikon condensers.

Pau
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Post by Pau »

With the same setup without compensators, does the field become dark when the pols are crossed?
If so, the issue is with your compensators. In principle they are to be used with the filters fully crossed (although later you can play with other configurations)
For the retarder I'm using a cellophane - can it be crinkled or it should be perfectly flat/smooth ? Distances ? Thicknesses? Placement ?
In principle flat is the best, distance does not matter, they just need to be placed between both polarizers, of course if not of the highest quality is better to put them over the polarizer before the objective to avoid image degradation.

Cellophane can be OK, although maybe not all of them are equally adequate. Cello tape glued on a glass slide also works well, you can play with different thicknesses adding more layers
Pau

Saul
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Post by Saul »

Hi Pau, thank you for your quick reply !
With the same setup without compensators, does the field become dark when the pols are crossed?
Yes, field become dark
Cello tape glued on a glass slide also works well...
On the back side of the slide ? I tried to place between slide & condenser (but it was crinkled), nothing ...
...you can play with different thicknesses adding more layers
Interesting, I thought, that with the thinner cellophane effect will be stronger

Pau
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Post by Pau »

Saul wrote:
Cello tape glued on a glass slide also works well...
On the back side of the slide ? I tried to place between slide & condenser (but it was crinkled), nothing ...
No, I was referring to a blank slide. The ability to rotate the compensator is very important. Usually the stronger effect is produced when the plate optical direction is at 45 deg. of the polarizers.
If you use it like in a typical microscope the usual place will be between the polarizer and the condenser
...you can play with different thicknesses adding more layers
Interesting, I thought, that with the thinner cellophane effect will be stronger
If I recall well one layer of tape is like a 1/4 plate, two layers like a 1/2 plate and so, of course not well calibrated..it depends of its thickness and of the traction strength applied when gluing it
Pau

zzffnn
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Post by zzffnn »

Saul,

I had good results with the clear cellophane from this package:

https://www.amazon.com/Hygloss-78548-Sh ... _in_o0_img

One sheet is 1/4 plate. I can try to find mine and send them your way, if I can find them (I just moved my house and have not been able to find everything from boxes).

I placed cellophane between two microscope slides (to hold them flat) and taped on the glass slides, instead of cellophane. I placed the DIY retarder in between condenser (bottom lens) and bottom polarizer (which is in turn placed on field iris of Optiphot). You have to rotate retarder to get best effects.

Saul
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Post by Saul »

Thanks Fan ! Don't go through you boxes yet :)

Before your message I tried one thing, using old aperture mount O-ring from the DIY AO-Nikon condenser, window sealing heat shrink film, double-stick tape and wife's hairdryer:

Image

Initial quick test shows some signs of life (need to prepare a better slide).
I can rotate ~60 degrees, I hope it will be enough ...

Saul
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Post by Saul »

OK, looks like the condenser was the cause - after removing it, polarizing effect is much more stronger. Is there any way around to get same strong effect with the condenser ? Placing polarizing film filter on the top of the condenser ?

Pau
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Post by Pau »

Do you have the same issue with different condensers?
Possible causes
1. Condenser is dirty (easy to see!)
2. condenser lenses have strain and alter the polarization of light. There are pol condensers meant to work with pol, although I've got good results even with cheap no name condensers.

You can try to put the polarizer over the condenser if you have enough room, optically it must work

What polarizer do you use?
An easy mistake is putting a circular pol with the plate wave facing the condenser, if they are camera filters they must be placed with the front side facing the condenser
Pau

Saul
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Post by Saul »

Pau wrote:Do you have the same issue with different condensers?
Tried with the Nikon & AO only. Will try with Amscope & Quodmaster (this one has a rotatable filter changer, one of them (looks like, have to check) is pol, so should be ok option (only problem - to adapt to the Optiphot)
You can try to put the polarizer over the condenser if you have enough room, optically it must work
Have a small pol, will try. But will be a problem with the retarder ...
What polarizer do you use?
An easy mistake is putting a circular pol with the plate wave facing the condenser, if they are camera filters they must be placed with the front side facing the condenser
Yes. Field becomes dark when the pols are crossed. Bottom - circular pol, behind the objective - linear

JohnyM
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Post by JohnyM »

How does the conoscopic image look like? Is it even? Can you take a picture of it? Preferably with 40x objective.

ChrisR
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Post by ChrisR »

I have played with a type of tape which was found to be very good; the type which is completely transparent even when on the roll. Try
" AIEX Clear Sticky Tape Ultra Transparent Adhesive Tape ". About 300nm per layer.

Using a screen as a pol source and a handy analyser we made national flags by building up the colours with the retarder tape.
Image
Chris R

Saul
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Post by Saul »

JohnyM wrote:How does the conoscopic image look like? Is it even? Can you take a picture of it? Preferably with 40x objective.
Hmm, how to get that in the easiest way ? I have somewhere small piece with kind a diffraction cell ... 5mx5mm approx

Saul
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Post by Saul »

ChrisR wrote:...Try " AIEX Clear Sticky Tape Ultra Transparent Adhesive Tape "...
Hmm, worst case scenario it will be used for the packing :)

Pau
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Post by Pau »

To see the conoscopic image you can
- just remove the eyepiece (but its small and so not very easy to see)
- use a telescope eyepiece (usual for centering phase contrast, often labeled P, Phase or Phako)
- Use a Bertrand lens -usually only present in pol microscopes-
Pau

Saul
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Post by Saul »

The learning curve (for me) - not every cellophane is the same , not every objective is the same for cross-pol ... and not for the every specimen. Lot of variations.

Picture (quick test stack) with the same AO condenser, different retarder and (lost my patience keeping pol filter and falling patches of the cellophane from the illuminator) assembly, to keep and adjust polarizer/retarder on the Optiphot illuminator. Now I can rotate each one separately and everything together .

Image

Assembled:

Image
Last edited by Saul on Mon Jan 07, 2019 10:31 am, edited 1 time in total.

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