DIC Question

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LouiseScot
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Re: DIC Question

#91 Post by LouiseScot » Thu Jun 10, 2021 9:32 pm

Ah, ok jmp, thanks. Sounds like I'm trying too hard. I did post one pic of the two prisms together - 'Prism A+B'. I think I need to go back a step and try again. Also I think I'd get improved results if I could cut the prisms better. I only have a hacksaw so they always end up cut a little wonky! I notice that (unsurprisingly) the patterns change as the polarizers are rotated. I think the prisms are supposed to be at 45 deg relative to the cross polarizers? Is that how Nomarski prisms sit? I can get a dark band with a single prism but even with two I can't seem to get the symmetric dark central destructive two-prism interference pattern. Presumably the two prisms have to be adjusted such that each has exactly the same bending moment?
I have to do jury service tomorrow but will do some more at the weekend.

Thanks

Louise
A Nikon CF plan 20x; A Swift 380T; A DIY infinity corrected focus rail system with a 40x/0.65 Olympus Plan, a 10x/0.30 Amscope Plan Fluor, and a 20x/0.75 Nikon Plan Apo

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75RR
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Re: DIC Question

#92 Post by 75RR » Thu Jun 10, 2021 9:37 pm

.
I notice some damage on the prisim from the screws (see image below)

wonder if a metal sheath on the edge might protect it and yet give the same effect?

Might stop it twisting as well.
.
Image
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LouiseScot
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Re: DIC Question

#93 Post by LouiseScot » Thu Jun 10, 2021 9:46 pm

75RR wrote:
Thu Jun 10, 2021 9:37 pm
.
I notice some damage on the prisim from the screws (see image below)

wonder if a metal sheath on the edge might protect it and yet give the same effect?

Might stop it twisting as well.
.
Image
I may be guilty of overtightening... I might see if I can get better results with a nut on the end of the screw - maybe a t-nut. It's supposed to be a dowel-shape so there is point contact but I don't know how to reproduce that with set screws. Dome nuts might be a possible.

Louise
A Nikon CF plan 20x; A Swift 380T; A DIY infinity corrected focus rail system with a 40x/0.65 Olympus Plan, a 10x/0.30 Amscope Plan Fluor, and a 20x/0.75 Nikon Plan Apo

jmp
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Re: DIC Question

#94 Post by jmp » Fri Jun 11, 2021 1:33 am

LouiseScot wrote:
Thu Jun 10, 2021 9:32 pm
Ah, ok jmp, thanks. Sounds like I'm trying too hard. I did post one pic of the two prisms together - 'Prism A+B'.
Sorry, I didn't see the Prism A+B picture before. Though now that I've seen it, it appears to be the same photo as Prism B, right?
LouiseScot wrote:
Thu Jun 10, 2021 9:32 pm
I think I need to go back a step and try again. Also I think I'd get improved results if I could cut the prisms better. I only have a hacksaw so they always end up cut a little wonky!
I cut my prisms with a serrated x-acto blade and then I sand the edges with ~400 grit sandpaper in water, mostly to avoid small pieces of the polycarbonate falling into the microscope tube when I insert the sliders. Yet, from what I've seen, I doubt that the fabrication quality of the prisms would have an impact at this stage. Your current prisms should work and let you observe the interference patterns.

I would worry more about the damage done by the screws, as 75RR mentions. Note that my prisms also have some damage from the screws, though less marked because I have not tightened them that much. Still, this damage should not prevent you to see the interference patterns. These polycarbonate prisms appear to be quite forgiving in terms of the abuse they can take.
LouiseScot wrote:
Thu Jun 10, 2021 9:32 pm
I notice that (unsurprisingly) the patterns change as the polarizers are rotated.
If I keep the polarizers crossed and then I rotate the prism I see the same interference pattern but rotated in the same degree as the rotation that I applied to the prism. The interference pattern rotates with the prism and fringes are always parallel to the long edge of the prism.
LouiseScot wrote:
Thu Jun 10, 2021 9:32 pm
I think the prisms are supposed to be at 45 deg relative to the cross polarizers? Is that how Nomarski prisms sit?
I don't know.
LouiseScot wrote:
Thu Jun 10, 2021 9:32 pm
I can get a dark band with a single prism but even with two I can't seem to get the symmetric dark central destructive two-prism interference pattern. Presumably the two prisms have to be adjusted such that each has exactly the same bending moment?
When using two prisms at the same time, how are you orienting them? I only see the dark fringe if the bending moment from both prisms is applied from the same direction. The bending moment does not have to be the same though. I've looked at this and the interference patterns from the individual prisms are different, yet I still see the dark fringe when using both prisms together. But if I rotate one of the prisms 180deg the interference pattern changes, most fringes disappear and the space between fringes and their width increases notably.

Once I get a chance I'll capture a few videos to better illustrate what happens under these different conditions.

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Re: DIC Question

#95 Post by LouiseScot » Fri Jun 11, 2021 10:40 pm

Thanks for the input, jmp! Unfortunately I've not been able to do any more today. Will try and get on tomorrow (Saturday).
Sorry, the second Prism B image I posted was actually a duplicate of the two-prism image (Prism A+B). I was having trouble uploading so the duplication must have been a consequence of that.
I'll have to try remaking the prisms - maybe I can partially cut through with a box knife before sawing. I'm rather lacking in facilities for doing such things in my living room.

Have you tried measuring your delta-y?

More to follow!

Louise
A Nikon CF plan 20x; A Swift 380T; A DIY infinity corrected focus rail system with a 40x/0.65 Olympus Plan, a 10x/0.30 Amscope Plan Fluor, and a 20x/0.75 Nikon Plan Apo

Hobbyst46
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Re: DIC Question

#96 Post by Hobbyst46 » Sat Jun 12, 2021 9:10 am

LouiseScot wrote:
Fri Jun 11, 2021 10:40 pm
Thanks for the input, jmp! Unfortunately I've not been able to do any more today. Will try and get on tomorrow (Saturday).
Sorry, the second Prism B image I posted was actually a duplicate of the two-prism image (Prism A+B). I was having trouble uploading so the duplication must have been a consequence of that.
I'll have to try remaking the prisms - maybe I can partially cut through with a box knife before sawing. I'm rather lacking in facilities for doing such things in my living room.

Have you tried measuring your delta-y?

More to follow!

Louise
To bypass any imperfections on the edge of the prism, due to cutting, perhaps an edge protector could be 3D printed ? I mean, a narrow ח-shaped profile? so the tightening screws will press through this profile on the prism edge. A metal profile would be even better but difficult to find one with an internal space of 2.4 or 3.1 mm. Ancient 3.5" floppies had something like that...

Incidentally, some CD/DVD boxes, the "newer" thinner boxes, just a few mm overall thickness, are made of plates, the ends of which are like L-shaped profiles. Perhaps an improvised edge protector can be made by cutting and gluing parts from these plates.

LouiseScot
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Re: DIC Question

#97 Post by LouiseScot » Sat Jun 12, 2021 9:18 am

Hobbyst46 wrote:
Sat Jun 12, 2021 9:10 am
LouiseScot wrote:
Fri Jun 11, 2021 10:40 pm
Thanks for the input, jmp! Unfortunately I've not been able to do any more today. Will try and get on tomorrow (Saturday).
Sorry, the second Prism B image I posted was actually a duplicate of the two-prism image (Prism A+B). I was having trouble uploading so the duplication must have been a consequence of that.
I'll have to try remaking the prisms - maybe I can partially cut through with a box knife before sawing. I'm rather lacking in facilities for doing such things in my living room.

Have you tried measuring your delta-y?

More to follow!

Louise
To bypass any imperfections on the edge of the prism, due to cutting, perhaps an edge protector could be 3D printed ? I mean, a narrow ח-shaped profile? so the tightening screws will press through this profile on the prism edge. A metal profile would be even better but difficult to find one with an internal space of 2.4 or 3.1 mm. Ancient 3.5" floppies had something like that...
I think the forces of the screws on anything 3D printed would be too great. I'm trying out the t-nuts at the moment.
Louise
A Nikon CF plan 20x; A Swift 380T; A DIY infinity corrected focus rail system with a 40x/0.65 Olympus Plan, a 10x/0.30 Amscope Plan Fluor, and a 20x/0.75 Nikon Plan Apo

LouiseScot
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Re: DIC Question

#98 Post by LouiseScot » Sat Jun 12, 2021 10:12 am

Using the t-nuts is looking positive:
Holder-T-nuts.jpg
Holder-T-nuts.jpg (49.95 KiB) Viewed 435137 times
Also the 2 prisms image is looking better and more like it! I made sure both holders were the same way around - thanks jmp!
A+B_T-nutsCrop_0167.jpg
A+B_T-nutsCrop_0167.jpg (38.82 KiB) Viewed 435137 times

The black fringes are still confined to the lower half of the prisms which is the tensile side. Perhaps the tensile force is influenced by the r-value and the diameter of the lower supports relative to the set screws/t-nuts? Still, I'll try them out on the optical setup next :) Also, the frame itself is still tending to bend even though it's pretty hefty and printed at 100% infill. I really don't know how Rathi's group avoided frame bending (they didn't mention it). It might be possible to add some metal brackets. If it all works, though, all well and good!

Louise
A Nikon CF plan 20x; A Swift 380T; A DIY infinity corrected focus rail system with a 40x/0.65 Olympus Plan, a 10x/0.30 Amscope Plan Fluor, and a 20x/0.75 Nikon Plan Apo

jmp
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Re: DIC Question

#99 Post by jmp » Sat Jun 12, 2021 1:46 pm

Hi Louise,

Is difficult to see in this picture:

Image

Are the short edges of the prism touching the sides of the frame? What about the prism edge and the bottom of the frame? From that photo it looks like there is uneven bending, as if the prism is making contact with the guide on the left. I would assume that there's space and that the prism is not touching the frame at those points, but I'm asking just in case.

In any case, it's looking better, great progress!

I have not yet had time to measure the displacements. I might give it a try tomorrow. I been thinking about how to modify my sliders to be able to apply more bending force to the prism without deforming the frame, and to be able to hold 2 prisms on each slider if possible. I'm toying with the idea of using the metal contact from euro-style terminal blocks as the inner adjustable points of contact with the prism, the profile is rounded on one side, and has two threaded holes on the other so it seems it might be a good fit to improve the way in which I can adjust the force on the prism.
Last edited by jmp on Sat Jun 12, 2021 2:57 pm, edited 1 time in total.

LouiseScot
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Re: DIC Question

#100 Post by LouiseScot » Sat Jun 12, 2021 2:48 pm

jmp wrote:
Sat Jun 12, 2021 1:46 pm
Hi Louise,

Is difficult to see in this picture:

Image

Are the short edges of the prism touching the sides of the frame? What about the prism edge and the bottom of the frame? From that photo it looks like there is uneven bending, as if the prism is making contact with the guide on the left. I would assume that there's space and that the prism is not touching the frame at those points, but I'm asking just in case.

In any case, it's looking better, great progress!

In my case, I have not yet had time to measure the displacements. I might give it a try tomorrow. I been thinking about how to modify my sliders to be able to apply more bending force to the prism without deforming the frame, and to be able to hold 2 prisms on each slider if possible. I'm toying with the idea of using the metal contact from euro-style terminal blocks as the inner adjustable points of contact with the prism, the profile is rounded on one side, and has two threaded holes on the other so it seems it might be a good fit to improve the way in which I can adjust the force on the prism.
Hiya

There is a fair gap between the short edges and side of the frame. The screw tension may have been uneven in the pic I took. but I try to get them even in use :) Unfortunately I lack dexterity and grip... Um, do you know what the gap between the two dark lines should be in mm? And how that relates to the position of the specimen for each of the two prisms? I'm thinking the two prisms should be adjusted differently (as per Rathi). So they need to work together when in situ as opposed to just between the cross polarisers outside the optical system. I'm a bit in the dark, so to speak, re the comparison with Nomarski prisms. Are the fringe patterns as seen with naked eye (via cross-polarisers) i.e. with no magnification. I don't pretend to understand the mathematics and optical theory so I'm just trying to find my way. I'll look up how Nomarski is set up in practice - I have some info about that but no experience at doing it.

Cheers

Louise
A Nikon CF plan 20x; A Swift 380T; A DIY infinity corrected focus rail system with a 40x/0.65 Olympus Plan, a 10x/0.30 Amscope Plan Fluor, and a 20x/0.75 Nikon Plan Apo

LouiseScot
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Re: DIC Question

#101 Post by LouiseScot » Sat Jun 12, 2021 7:24 pm

LouiseScot wrote:
Sat Jun 12, 2021 2:48 pm


Hiya

Um, do you know what the gap between the two dark lines should be in mm? And how that relates to the position of the specimen for each of the two prisms? Ignore that!

I'll try what Viktor said and look at one prism via the objective i.e. the condenser prism, and see if I have the black interference line centred in the fov. If so, Then I'll add the second prism and keep my fingers crossed!


Cheers

Louise
A Nikon CF plan 20x; A Swift 380T; A DIY infinity corrected focus rail system with a 40x/0.65 Olympus Plan, a 10x/0.30 Amscope Plan Fluor, and a 20x/0.75 Nikon Plan Apo

LouiseScot
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Re: DIC Question

#102 Post by LouiseScot » Sat Jun 12, 2021 9:50 pm

I had a quick look with one prism (the condenser prism) and, with a bit of fiddling about, was able to see the fringes but there seemed too many of them, and too close together. So I'll relax the screw tension a bit and try again tomorrow.

Louise
A Nikon CF plan 20x; A Swift 380T; A DIY infinity corrected focus rail system with a 40x/0.65 Olympus Plan, a 10x/0.30 Amscope Plan Fluor, and a 20x/0.75 Nikon Plan Apo

hans
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Re: DIC Question

#103 Post by hans » Sun Jun 13, 2021 2:12 am

Interesting to see these experiments and results. Hope this is not too much of a side-track but was wondering, lacking a microscope with easy access to a conjugate of the back focal plane (like jmp's inverted or Loiuse's custom setup), if anyone has heard of concrete ideas or experiments in the direction mentioned at the end of the Rathi paper:
A challenge with the adoption of Sanderson prisms for DIC imaging is the placement of a prism in the BFP of the objective. For budget and educational settings, it will likely be possible to incorporate the Sanderson prism into low-magnification objectives, since they have relatively long focal lengths. Unusual objectives, with access to the BFP, would be necessary for higher-magnification systems. Alternatively, and potentially with more benefit, it may be possible to create more sophisticated refractive index gradients that replicate Nomarski prism divergence profiles.

LouiseScot
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Re: DIC Question

#104 Post by LouiseScot » Sun Jun 13, 2021 9:23 am

hans wrote:
Sun Jun 13, 2021 2:12 am
Interesting to see these experiments and results. Hope this is not too much of a side-track but was wondering, lacking a microscope with easy access to a conjugate of the back focal plane (like jmp's inverted or Loiuse's custom setup), if anyone has heard of concrete ideas or experiments in the direction mentioned at the end of the Rathi paper:
A challenge with the adoption of Sanderson prisms for DIC imaging is the placement of a prism in the BFP of the objective. For budget and educational settings, it will likely be possible to incorporate the Sanderson prism into low-magnification objectives, since they have relatively long focal lengths. Unusual objectives, with access to the BFP, would be necessary for higher-magnification systems. Alternatively, and potentially with more benefit, it may be possible to create more sophisticated refractive index gradients that replicate Nomarski prism divergence profiles.
Hi Hans

I think it's open to experimentation. As mentioned, low power objectives have BFPs outside of the barrel so there is scope for fitting prisms just above it, by some means. My 'custom' approach really only consists of an additional ~200mm lens to extend the BFP. In this case I've used a 208mm DCR-150 lens as they are good quality. I will look into the possibility of shortening that fl, if I can get everything else up and running! Also, with the incorporation of the 200mm lens, there's no reason why a finite setup couldn't be used, as far as I can see. You'd just need to customise the condenser, I think. Correct me if I'm wrong, anyone.

Louise
A Nikon CF plan 20x; A Swift 380T; A DIY infinity corrected focus rail system with a 40x/0.65 Olympus Plan, a 10x/0.30 Amscope Plan Fluor, and a 20x/0.75 Nikon Plan Apo

LouiseScot
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Re: DIC Question

#105 Post by LouiseScot » Sun Jun 13, 2021 7:19 pm

Hobbyst46 wrote:
Sat Jun 12, 2021 9:10 am
LouiseScot wrote:
Fri Jun 11, 2021 10:40 pm
Thanks for the input, jmp! Unfortunately I've not been able to do any more today. Will try and get on tomorrow (Saturday).
Sorry, the second Prism B image I posted was actually a duplicate of the two-prism image (Prism A+B). I was having trouble uploading so the duplication must have been a consequence of that.
I'll have to try remaking the prisms - maybe I can partially cut through with a box knife before sawing. I'm rather lacking in facilities for doing such things in my living room.

Have you tried measuring your delta-y?

More to follow!

Louise
To bypass any imperfections on the edge of the prism, due to cutting, perhaps an edge protector could be 3D printed ? I mean, a narrow ח-shaped profile? so the tightening screws will press through this profile on the prism edge. A metal profile would be even better but difficult to find one with an internal space of 2.4 or 3.1 mm. Ancient 3.5" floppies had something like that...

Incidentally, some CD/DVD boxes, the "newer" thinner boxes, just a few mm overall thickness, are made of plates, the ends of which are like L-shaped profiles. Perhaps an improvised edge protector can be made by cutting and gluing parts from these plates.
Hiya

Just to let you know - in the end I decided that it doesn't look like I'll have to use as much bending pressure via the screws as I'd thought. So I've 3D printed some clip-like things which will mate with the M5 screw ends. I've done it so that the contact point is a curved surface so that should be better. I've not cut any new prisms - will try and do that tomorrow.
I think I'm making progress now though the fringes incorporating the dark band is still towards the bottom of the prism. I've no idea why. However, initial experiments seem to suggest that the objective prism needs to have higher tension than the condenser prism. Also, it looks like I need to turn the objective prism 'upside down' relative to the condenser one i.e. rotated 180 deg in the plane of the specimen. When I do, then the camera view goes clear as opposed to still having some 'pastel-coloured' fringes. I tried it out on a slide (a Chinese) coloured Volvox. I notice now when I go in and out of focus I get a double image and the focus seems more critical now. That suggests something is going on!

Here's a couple of pics: (40x objective)
VolvoxCrop30pc.jpg
VolvoxCrop30pc.jpg (48.63 KiB) Viewed 435008 times

100% crop:
VolvoxCrop100pc.jpg
VolvoxCrop100pc.jpg (99.55 KiB) Viewed 435008 times
I don't know if I'm seeing DIC yet but making progress!

Louise
A Nikon CF plan 20x; A Swift 380T; A DIY infinity corrected focus rail system with a 40x/0.65 Olympus Plan, a 10x/0.30 Amscope Plan Fluor, and a 20x/0.75 Nikon Plan Apo

Scarodactyl
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Re: DIC Question

#106 Post by Scarodactyl » Sun Jun 13, 2021 8:09 pm

That does look promising. I think a stained slide is going to mask the effect, though I am not sure what the ideal subject is for DIC. Diatoms?

LouiseScot
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Re: DIC Question

#107 Post by LouiseScot » Sun Jun 13, 2021 8:13 pm

Scarodactyl wrote:
Sun Jun 13, 2021 8:09 pm
That does look promising. I think a stained slide is going to mask the effect, though I am not sure what the ideal subject is for DIC. Diatoms?
Thanks. I'll try some diatoms and maybe some other things. There are some faint fringe colours so probably need some tweaks - at least!

Louise
A Nikon CF plan 20x; A Swift 380T; A DIY infinity corrected focus rail system with a 40x/0.65 Olympus Plan, a 10x/0.30 Amscope Plan Fluor, and a 20x/0.75 Nikon Plan Apo

LouiseScot
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Re: DIC Question

#108 Post by LouiseScot » Mon Jun 14, 2021 6:41 pm

I cut a couple of fresh prisms today. The 3D-printed 'screw defenders' seem to work ok :) . I noticed with the new prisms that the strongest fringes occur towards the top this time. That suggests the fringe positioning is a property of the polycarbonate rather than anything to do with the frame or supports.
I'm a bit stuck now because I'm unsure how to set the two prisms up. I'm presuming they aren't both set to be the same tension/bending moment? The condenser prism is the beam splitter whereas the objective one is the combiner. The recombining takes place post objective so the objective prism can't be the same? I get the idea of matching fringes to what you see with a Nomarski prism for a given objective magnification (40x for me at the moment - looks like that is the black fringe plus only the nearest edges of the proximal coloured fringes, although it's hard for me to judge when is just right) but I'm assuming the condenser prism isn't set the same? Rathi did their setup by matching measured beam divergence to their existing Nomarski prisms. If the condenser prism fringe spacing is different then I think I'll have to see if I can measure its beam divergence by some means. I've been trying trial and error but not getting very far.

Louise
A Nikon CF plan 20x; A Swift 380T; A DIY infinity corrected focus rail system with a 40x/0.65 Olympus Plan, a 10x/0.30 Amscope Plan Fluor, and a 20x/0.75 Nikon Plan Apo

tpruuden
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Re: DIC Question

#109 Post by tpruuden » Fri Jun 25, 2021 4:03 am

Might it be, that the Hoya polarizer(s) are not inear but circular? If so, just reversing the polarizer helps. Following is the illustration of the circular/linear polarizer effect with single prism:

https://drive.google.com/file/d/188LW5W ... sp=sharing
https://drive.google.com/file/d/17xea5u ... sp=sharing

LouiseScot
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Re: DIC Question

#110 Post by LouiseScot » Fri Jun 25, 2021 9:30 am

tpruuden wrote:
Fri Jun 25, 2021 4:03 am
Might it be, that the Hoya polarizer(s) are not inear but circular? If so, just reversing the polarizer helps. Following is the illustration of the circular/linear polarizer effect with single prism:

https://drive.google.com/file/d/188LW5W ... sp=sharing
https://drive.google.com/file/d/17xea5u ... sp=sharing
Thanks, but I bought the Hoyas specifically because they are linear. Your prism holder looks neat, though it's hard to tell what the dimensions of it are?
My fringes are narrower (so mostly coloured) and not centred. If I slacken off the tension I can get broader bands but they become diffuse and I don't get the dark destructive interference band. As mentioned above, Whether my fringes are biased towards the top or bottom of the prism seems to depend on how I've cut them from the sheet. I've not done much with it since my previous post. I'm not sure how much difference the prism thickness makes (mine is 3mm so not far off Rathi's) or even whether the particular manufacturer matters (I'm in UK and just bought some small sheets off ebay).

Louise
A Nikon CF plan 20x; A Swift 380T; A DIY infinity corrected focus rail system with a 40x/0.65 Olympus Plan, a 10x/0.30 Amscope Plan Fluor, and a 20x/0.75 Nikon Plan Apo

tpruuden
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Re: DIC Question

#111 Post by tpruuden » Fri Jun 25, 2021 1:45 pm

May still be worth excluding filter type problem - the same circular filter may be called linear, assuming in photography use the orientation stays always the same.

LouiseScot
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Re: DIC Question

#112 Post by LouiseScot » Sat Jun 26, 2021 2:25 pm

tpruuden wrote:
Fri Jun 25, 2021 1:45 pm
May still be worth excluding filter type problem - the same circular filter may be called linear, assuming in photography use the orientation stays always the same.
Hi

They are definitely linear. I have a circular one for a camera and appreciate the difference, and have observed it with the prisms.

This is the Hoya:
https://www.amazon.co.uk/gp/product/B00 ... UTF8&psc=1

My polycarbonate (Palsun) does act as a waveplate though. If you place an unstrained piece between cross polarisers it makes the view go from dark to clear (with a tint). I have a 6mm sheet (Impex brand) which does the same so I presume it's the same with all polycarbonate?

ps I've checked the behaviours of the Hoyas against a linear polarisation sheet and two other polarisers - all the same.

Louise
A Nikon CF plan 20x; A Swift 380T; A DIY infinity corrected focus rail system with a 40x/0.65 Olympus Plan, a 10x/0.30 Amscope Plan Fluor, and a 20x/0.75 Nikon Plan Apo

hans
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Re: DIC Question

#113 Post by hans » Sat Jun 26, 2021 9:21 pm

LouiseScot wrote:
Sat Jun 26, 2021 2:25 pm
My polycarbonate (Palsun) does act as a waveplate though. If you place an unstrained piece between cross polarisers it makes the view go from dark to clear (with a tint). I have a 6mm sheet (Impex brand) which does the same so I presume it's the same with all polycarbonate?
Just checked a few scraps of sheets I had laying around (unknown brand/origin, thicknesses in the 1/8 to 1/4" range) and they all show birefringence like this with no external stress. Certainly seems like that could bias the fringe pattern as you suggest. Did you see that mentioned in any of the papers? I don't remember seeing it mentioned but it has been a while and I didn't read the papers very carefully. Two of the papers specifically mention bar stock purchased from McMaster-Carr, I wonder if bars are manufactured differently than sheets might show less inherent birefringence? I will add a couple bars to my next order. They are cheap and the sheets I have are pretty scratched up anyways.

Scarodactyl
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Re: DIC Question

#114 Post by Scarodactyl » Sat Jun 26, 2021 10:36 pm

Plastic is made of organic polymers which are chiral, so you expect them to be optically active. That's why you can use a plastic bag or sheet as a sprt of ersatz wave plate.

hans
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Re: DIC Question

#115 Post by hans » Sat Jun 26, 2021 11:34 pm

Scarodactyl wrote:
Sat Jun 26, 2021 10:36 pm
Plastic is made of organic polymers which are chiral, so you expect them to be optically active.
Different effects, I think. With the polycarbonate sheets, packing tape, whatever is used in circular polarizing filters, etc. the transmission between crossed polarizing filters depends strongly on angle and goes through four dark/light "cycles" when rotated through 360 degrees. If I understand correctly this indicates linear birefringence with transmission when the optic axis is at a multiple of 45 degrees from the initial linear polarization and extinction when the optic axis is parallel or perpendicular to it. I don't think this necessarily requires chirality and remember reading somewhere that the birefringence in plastic films was caused by linear alignment of molecules as a result of stretching at some point in the manufacturing process.

For optical activity due to chirality the behavior between crossed polarizing filters does not depend on the angle of the initial linear polarization relative to the sample, as far as I understand. (And in the cases like sugar solution there is no large-scale anisotropy at all so how could rotation matter?)

tpruuden
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Re: DIC Question

#116 Post by tpruuden » Sun Jun 27, 2021 12:39 am

LouiseScot wrote:
Sat Jun 26, 2021 2:25 pm

My polycarbonate (Palsun) does act as a waveplate though. If you place an unstrained piece between cross polarisers it makes the view go from dark to clear (with a tint). I have a 6mm sheet (Impex brand) which does the same so I presume it's the same with all polycarbonate?
So, this seems to indicate stressed material sheets. I used 3 mm Fluke scopemeter screen cover material for tests (assumed polycarbonate). This material has slight polarizing effect unstrained, 10..20% maximum brightening between crossed polarizers. How about hanging piece of material from the edge in oven and keeping it around 110..150c few minutes to remove residual stresses?

hans
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Re: DIC Question

#117 Post by hans » Sun Jun 27, 2021 1:45 am

tpruuden wrote:
Sun Jun 27, 2021 12:39 am
So, this seems to indicate stressed material sheets.
In the sheets I have the birefringence is very uniform across the entire sheet right up to the edge, just like packing tape. Not obvious to me how that could be explained by an internal stress distribution...?

tpruuden
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Re: DIC Question

#118 Post by tpruuden » Sun Jun 27, 2021 4:58 am

hans wrote:
Sun Jun 27, 2021 1:45 am

In the sheets I have the birefringence is very uniform across the entire sheet right up to the edge, just like packing tape. Not obvious to me how that could be explained by an internal stress distribution...?
I am not sure either but there are references of compression induced increase of impact strength of polycarbonate plates up to 700%. This might create quite uniform internal stress, that is better visible as gradient from the edge direction, not from side of the sheet. Pure speculation from my side...

https://onlinelibrary.wiley.com/doi/abs ... /app.50125

LouiseScot
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Re: DIC Question

#119 Post by LouiseScot » Sun Jun 27, 2021 3:50 pm

Sorry for late reply but I've not been getting email notifications. I'm in the UK and I'm wondering whether there is some difference with European polycarbonate and US made? Obviously the Rathi lab used US material and also mentioned using a half waveplate for measuring deflections. When viewing my 3mm polycarbonate (UK Palram/Palsun) between cross polarisers I see a fairly even-coloured hue (usually purplish but can vary across/between sheets). e.g.
Test_162000.jpg
Test_162000.jpg (19.01 KiB) Viewed 434571 times
You can see the top sector where its dark, and the polarisers are crossed, but there's no polycarbonate between. It's otherwise still darker than normal but not as dark as the cross polarisers alone.
Does anyone else see this? If not, I'll have to try and get a different make though my 6mm 'Impex' brand behaves the same.

Cheers

Louise
A Nikon CF plan 20x; A Swift 380T; A DIY infinity corrected focus rail system with a 40x/0.65 Olympus Plan, a 10x/0.30 Amscope Plan Fluor, and a 20x/0.75 Nikon Plan Apo

hans
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Re: DIC Question

#120 Post by hans » Sun Jun 27, 2021 7:44 pm

LouiseScot wrote:
Sun Jun 27, 2021 3:50 pm
Does anyone else see this?
How does it behave when you rotate it, relative to the direction of your polarizers? Have you checked the orientation of the Hoya PLs you have? I read conflicting things about whether or not the mark on the ring is supposed to be aligned accurately with the polarizer direction and some reports of them not being aligned even in new ones. I have several I bought used and they were all misaligned. In all mine the filters are held in with a circular spring clip and can rotated relative to the alignment mark without too much difficulty while pinching the filter between thumb and finger. I aligned them approximately using reflection from a glass microscope slide near Brewster's angle as a reference.

As mentioned in #113 and #115 all the pieces I have laying around act like waveplates. Unsure of absolute retardance but judging from the interference color charts probably third order or higher.

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