DIC Question

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LouiseScot
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Re: DIC Question

#121 Post by LouiseScot » Sun Jun 27, 2021 11:49 pm

hans wrote:
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.
Hiya

You mean if I rotate the polycarbonate between cross polarisers? The brightness varies and there is a minimum but it's not so dark as without having the polycarbonate sandwiched. I'm thinking this is maybe not so relevant though I'm a bit unsure. I do get the expected interference fringes but they just seem relatively narrow with the dark destructive band either towards the top or towards the bottom of the prism. However, I've been using two of the relatively large open frames based on Rathi's dimensions. Maybe I should go back to using a 'compact' holder for the condenser prism.
The hoya polarisers are camera types so rotatable, so I can set them at any angle relative to the prisms. I can set the polarisers to be crossed/opaque with no prisms in place and then position the two prisms. There doesn't seem to be an issue there. I can't remember if I previously set them to be both crossed and to have a minimum transmittance with unstressed polycarbonate in between, but it's something I'll check out next time. (Out of the top of my head, I think Rathi mentioned something about polarisers being at 45 deg. relative to the prisms - I presume this equates to the minimum transmittance - but might be a red herring!)
Is there someone in the USA who has had the same behaviour with their polycarbonate? Or anyone who has definitely and successfully achieved DIC? (I think @jmp said he did but not seen a post from him since). Anyway, I'm not sure whether all 'standard' polycarbonate sheet is the same? I think you can get sheets that have been hardened or coated (e.g. extra UV coating) but I presume any non-standard things would be stated. I imagine Rathi et al. just used the ordinary non-treated sheet here https://www.mcmaster.com/polycarbonate-sheet-stock/. Apart from it being measured in fractions of an inch thickness, it doesn't seem any different to my sheets.

Hans - sorry I either missed or didn't fully appreciate your previous 113, 115 posts. Are you in the States or Europe?

I'm likely to have to postpone working on this project until after July as I have a course and other things I have to do.

Thanks for everyone's replies :)

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

#122 Post by hans » Mon Jun 28, 2021 12:57 am

LouiseScot wrote:
Sun Jun 27, 2021 11:49 pm
You mean if I rotate the polycarbonate between cross polarisers? The brightness varies and there is a minimum but it's not so dark as without having the polycarbonate sandwiched.
Yeah, the sheets I have get quite dark with the optic axis parallel or perpendicular to the polarizer directions, hard to judge from the photo but probably darker than yours. Extinction in that orientation is quite sensitive to twisting. The optic axis is pretty close to aligned with the original factory edges which seems right for molecular alignment or residual stress from extrusion.
LouiseScot wrote:
Sun Jun 27, 2021 11:49 pm
The hoya polarisers are camera types so rotatable, so I can set them at any angle relative to the prisms. I can set the polarisers to be crossed/opaque with no prisms in place and then position the two prisms.
I think I have the same ones, was just wondering whether you know angle of the prism relative to the polarization, and pointing out a possible source of confusion: mine have little marks on the rotating rings that one might reasonably assume indicate the direction of polarization but are not reliable unless you check and align them against a known reference yourself.
LouiseScot wrote:
Sun Jun 27, 2021 11:49 pm
(Out of the top of my head, I think Rathi mentioned something about polarisers being at 45 deg. relative to the prisms - I presume this equates to the minimum transmittance - but might be a red herring!)
If your sheets are like mine then 45 deg (measuring from an original factory edge) is close to maximum transmission. That is why I suggest checking and marking your Hoya filters relative to some absolute reference like reflection from glass near Brewster's angle.
LouiseScot wrote:
Sun Jun 27, 2021 11:49 pm
Are you in the States or Europe?
Southern California, not far from McMaster, which is why I order fairly often since shipping is usually ~$10 and comes the next day via a courier service. I will order some sheets and bars for comparison since two of the papers specifically mention bars. No idea if they are actually manufactured differently or are just cut-up sheets.

LouiseScot
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Re: DIC Question

#123 Post by LouiseScot » Mon Jun 28, 2021 9:37 am

hans wrote:
Mon Jun 28, 2021 12:57 am
LouiseScot wrote:
Sun Jun 27, 2021 11:49 pm
You mean if I rotate the polycarbonate between cross polarisers? The brightness varies and there is a minimum but it's not so dark as without having the polycarbonate sandwiched.
Yeah, the sheets I have get quite dark with the optic axis parallel or perpendicular to the polarizer directions, hard to judge from the photo but probably darker than yours. Extinction in that orientation is quite sensitive to twisting. The optic axis is pretty close to aligned with the original factory edges which seems right for molecular alignment or residual stress from extrusion.

Yes, same here. There is a point which is darkest but, as you say, very sensitive. In my photo, I just did it to show the passive effect of being between cross polarisers. I don't think I tried to either minimise or maximise transmission through the polycarbonate.

LouiseScot wrote:
Sun Jun 27, 2021 11:49 pm
The hoya polarisers are camera types so rotatable, so I can set them at any angle relative to the prisms. I can set the polarisers to be crossed/opaque with no prisms in place and then position the two prisms.
I think I have the same ones, was just wondering whether you know angle of the prism relative to the polarization, and pointing out a possible source of confusion: mine have little marks on the rotating rings that one might reasonably assume indicate the direction of polarization but are not reliable unless you check and align them against a known reference yourself.

I've no idea. There are arrow/triangle marks on the edge of the rotatable part but I always assumed they were just a point of reference. Having said that, the Hoyas are cross-polarised when one arrow is 90 deg to the other.
LouiseScot wrote:
Sun Jun 27, 2021 11:49 pm
(Out of the top of my head, I think Rathi mentioned something about polarisers being at 45 deg. relative to the prisms - I presume this equates to the minimum transmittance - but might be a red herring!)
If your sheets are like mine then 45 deg (measuring from an original factory edge) is close to maximum transmission. That is why I suggest checking and marking your Hoya filters relative to some absolute reference like reflection from glass near Brewster's angle.

I could do that. Is it the case that maximum transmission through the polycarbonate would be best? Just checking with a small piece of polycarbonate I get minimum transmission at 90 degrees to cross polarisers using linear polarising sheet. Presumably the polarising sheet can be used as a reference?
LouiseScot wrote:
Sun Jun 27, 2021 11:49 pm
Are you in the States or Europe?
Southern California, not far from McMaster, which is why I order fairly often since shipping is usually ~$10 and comes the next day via a courier service. I will order some sheets and bars for comparison since two of the papers specifically mention bars. No idea if they are actually manufactured differently or are just cut-up sheets.
That's a nice place to be! Unfortunately nobody seems to offer bars in the UK. If I could get 20mm bars that would save some of my dodgy sawing!

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

tpruuden
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Re: DIC Question

#124 Post by tpruuden » Mon Jun 28, 2021 11:31 am

LouiseScot wrote:
Sun Jun 27, 2021 3:50 pm
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.

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.
There is minimal stresses in the Fluke screen material (and this is cracked test piece), compared to Hoya filter casing and 2 mm sheet material (crossed polarizers, 45 degrees material orientation):
https://drive.google.com/file/d/18I4q9c ... sp=sharing
https://drive.google.com/file/d/18D9w3e ... sp=sharing
https://drive.google.com/file/d/18KPJ3m ... sp=sharing

Blue hue is seen from the 2mm sheet material, I would say, that it is stressed uniformly.

LouiseScot
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Re: DIC Question

#125 Post by LouiseScot » Mon Jun 28, 2021 11:39 am

tpruuden wrote:
Mon Jun 28, 2021 11:31 am
LouiseScot wrote:
Sun Jun 27, 2021 3:50 pm
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.

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.
There is minimal stresses in the Fluke screen material (and this is cracked test piece), compared to Hoya filter casing and 2 mm sheet material (crossed polarizers, 45 degrees material orientation):
https://drive.google.com/file/d/18I4q9c ... sp=sharing
https://drive.google.com/file/d/18D9w3e ... sp=sharing
https://drive.google.com/file/d/18KPJ3m ... sp=sharing

Blue hue is seen from the 2mm sheet material, I would say, that it is stressed uniformly.
Thanks. About the same as mine then. I'll have to get back to trying to get the DIC working, when I can. I'll make a new condenser prism holder as well.

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

#126 Post by tpruuden » Mon Jun 28, 2021 12:03 pm

LouiseScot wrote:
Mon Jun 28, 2021 11:39 am

Thanks. About the same as mine then. I'll have to get back to trying to get the DIC working, when I can. I'll make a new condenser prism holder as well.
I am atm testing the 2mm sheet pieces in oven at glass transition temperature to see, if the internal stress can be removed. Smells terrible for sure.

Hobbyst46
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Re: DIC Question

#127 Post by Hobbyst46 » Mon Jun 28, 2021 12:10 pm

In the brochures of some sellers of Palram/Palsun products they state that Palsun includes a one-side UV protection. Some other PC sheets seem to carry double-side UV protection. I do not know what those mean, but I would suspect a UV-blocking coat layer of some sort. Would that affect the observed birefringence :?

tpruuden
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Re: DIC Question

#128 Post by tpruuden » Mon Jun 28, 2021 12:29 pm

Hobbyst46 wrote:
Mon Jun 28, 2021 12:10 pm
In the brochures of some sellers of Palram/Palsun products they state that Palsun includes a one-side UV protection. Some other PC sheets seem to carry double-side UV protection. I do not know what those mean, but I would suspect a UV-blocking coat layer of some sort. Would that affect the observed birefringence :?
Good catch, it may act as 1/x waveplate there.



Although the glass transition temperature seems to be way lower than expected, the stresses significantly differ after heating. Considering, that the material shrunk and thickened, I assume, that common plate materials are rolled to thickness and thus having internal stresses. Materials specially targeted as LCD cover glasses seem to be minimally stressed to avoid polarisation changes.

The same 2mm material that was having blue hue earlier:
https://drive.google.com/file/d/18V09VN ... sp=sharing

LouiseScot
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Re: DIC Question

#129 Post by LouiseScot » Mon Jun 28, 2021 12:59 pm

Hobbyst46 wrote:
Mon Jun 28, 2021 12:10 pm
In the brochures of some sellers of Palram/Palsun products they state that Palsun includes a one-side UV protection. Some other PC sheets seem to carry double-side UV protection. I do not know what those mean, but I would suspect a UV-blocking coat layer of some sort. Would that affect the observed birefringence :?
I've dropped the seller a line to check but I would suppose it would be stated if there was. I await a reply :)

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

MichaelG.
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Re: DIC Question

#130 Post by MichaelG. » Mon Jun 28, 2021 1:24 pm

A useful source of background information on Polycarbonate materials, here:
https://www.bpf.co.uk/plastipedia/polym ... onate.aspx

MichaelG.
Too many 'projects'

LouiseScot
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Re: DIC Question

#131 Post by LouiseScot » Mon Jun 28, 2021 1:28 pm

MichaelG. wrote:
Mon Jun 28, 2021 1:24 pm
A useful source of background information on Polycarbonate materials, here:
https://www.bpf.co.uk/plastipedia/polym ... onate.aspx

MichaelG.
Doesn't really help but thanks anyway.
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

#132 Post by LouiseScot » Mon Jun 28, 2021 1:37 pm

Tut, I just lost a post - grrr. Anyway seller tells me that all polycarbonate sheeting has added UV stabiliser. It's just a kind of sunscreen added to the resin. Whether it might affect visible light properties I've no idea but it may not be possible to get it without.

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

#133 Post by hans » Mon Jun 28, 2021 5:15 pm

LouiseScot wrote:
Mon Jun 28, 2021 9:37 am
Having said that, the Hoyas are cross-polarised when one arrow is 90 deg to the other.
Yeah that's a good sign, maybe yours are already aligned.
LouiseScot wrote:
Mon Jun 28, 2021 9:37 am
Is it the case that maximum transmission through the polycarbonate would be best?
The stress gradient (which should be perpendicular to the beam axis in the ideal case) is supposed to be 45 degree between the polarizers for DIC. If you are stuck using material with some preexisting birefringence then basically you just have a choice of how to orient the existing birefringence relative to the beam axis, depending on what orientation you cut the beam out from the sheet. Not clear to me what the least-bad orientation for existing birefringence would be, but I would guess having the optic axis parallel to the beam axis (as you already do) is probably better than having it perpendicular, or at least not as confusing to think about.
LouiseScot wrote:
Mon Jun 28, 2021 9:37 am
Just checking with a small piece of polycarbonate I get minimum transmission at 90 degrees to cross polarisers using linear polarising sheet. Presumably the polarising sheet can be used as a reference?
Yeah, probably. I suggested reflection from plain, uncoated glass because it doesn't require any assumptions about the material. Extinction of the reflection is maximum when the polarizer direction is parallel to the plane of incidence. Blue sky is another reference, but maybe more convenient in Southern California than Scotland...

LouiseScot
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Re: DIC Question

#134 Post by LouiseScot » Mon Jun 28, 2021 5:30 pm

hans wrote:
Mon Jun 28, 2021 5:15 pm
LouiseScot wrote:
Mon Jun 28, 2021 9:37 am
Having said that, the Hoyas are cross-polarised when one arrow is 90 deg to the other.
Yeah that's a good sign, maybe yours are already aligned.
LouiseScot wrote:
Mon Jun 28, 2021 9:37 am
Is it the case that maximum transmission through the polycarbonate would be best?
The stress gradient (which should be perpendicular to the beam axis in the ideal case) is supposed to be 45 degree between the polarizers for DIC. If you are stuck using material with some preexisting birefringence then basically you just have a choice of how to orient the existing birefringence relative to the beam axis, depending on what orientation you cut the beam out from the sheet. Not clear to me what the least-bad orientation for existing birefringence would be, but I would guess having the optic axis parallel to the beam axis (as you already do) is probably better than having it perpendicular, or at least not as confusing to think about.
LouiseScot wrote:
Mon Jun 28, 2021 9:37 am
Just checking with a small piece of polycarbonate I get minimum transmission at 90 degrees to cross polarisers using linear polarising sheet. Presumably the polarising sheet can be used as a reference?
Yeah, probably. I suggested reflection from plain, uncoated glass because it doesn't require any assumptions about the material. Extinction of the reflection is maximum when the polarizer direction is parallel to the plane of incidence. Blue sky is another reference, but maybe more convenient in Southern California than Scotland...
It's fairly easy to arrange the prisms to be 45 deg to the cross polarisers axis. It was actually hot and sunny here earlier :) but has since clouded over :(. Still, tomorrow is forecast to be sunny and warm :)
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

#135 Post by hans » Mon Jun 28, 2021 5:57 pm

Yes, of course as I was writing that, it is actually cloudy here today.

Slight change of subject, but related: some papers on polymers appear to be making a distinction between "intrinsic orientational birefringence" (and a few other similar phrasings) vs. stress-induced (photoelastic) birefringence. I have not found a clear (to me, at least) explanation of the relationship, particularly in the case where the the material is being cooled and stretched at the same time, like extrusion...?

LouiseScot
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Re: DIC Question

#136 Post by LouiseScot » Mon Jun 28, 2021 6:21 pm

hans wrote:
Mon Jun 28, 2021 5:57 pm
Yes, of course as I was writing that, it is actually cloudy here today.

Slight change of subject, but related: some papers on polymers appear to be making a distinction between "intrinsic orientational birefringence" (and a few other similar phrasings) vs. stress-induced (photoelastic) birefringence. I have not found a clear (to me, at least) explanation of the relationship, particularly in the case where the the material is being cooled and stretched at the same time, like extrusion...?
If someone finds a use for it that interests me then I might delve into it more. In the meantime I'll just stick to the DIC challenge!
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

#137 Post by hans » Tue Jun 29, 2021 8:44 am

Placed the order at 10:30 AM and it was dropped off at 3:30 PM, probably a new record. The 7/32" bars are clearly cut from sheet since they have the same cloudy white protective film with "TUFFAK polycarbonate sheet" printed all over as the 1/4" sheet. The 1/8" sheet is also TUFFAK. The 1/8" bar also appears to be cut from a larger sheet but has clear protective film with no branding printed on it.

Interestingly the birefringence of the 7/32" TUFFAK bar is dramatically lower than either of the 1/8" pieces. A bit less than 100 nm retardance, I believe, since if I increase path length somewhat by tilting it away from being perpendicular with the line of sight (by about 45 degrees) it behaves similar a reversed Hoya CIR-PL with little brightness variation as the analyzer is rotated, just some color shift from bluish to brownish.

The unbranded 1/8" bar on the other hand is around 1500 nm retardance despite being half as thick, so more than 30 times greater birefringence. The 1/8" TUFFAK sheet is similar. So it seems like maybe different manufacturing processes are used depending on thickness?

Here is what the 7/32" x 1" TUFFAK bar looks like with stress gradient at roughly 45 degrees between crossed polarizers. Polarizer is my monitor and analyzer is a Hoya CIR-PL installed normally on the camera lens. There is a lot of ugly color banding. I guess some of the interference colors are fairly pure and going out of gamut somewhere? The colors are prettier in person:
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Hobbyst46
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Re: DIC Question

#138 Post by Hobbyst46 » Tue Jun 29, 2021 8:49 am

hans wrote:...
I have been informed that at least one brand of PC for outdoor use is coated against UV, coating thickness can be 15-50 um, and may cover one side or both sides of the sheet, depending on expected market/site demands. FWIW.

LouiseScot
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Re: DIC Question

#139 Post by LouiseScot » Tue Jun 29, 2021 9:18 am

hans wrote:
Tue Jun 29, 2021 8:44 am
Placed the order at 10:30 AM and it was dropped off at 3:30 PM, probably a new record. The 7/32" bars are clearly cut from sheet since they have the same cloudy white protective film with "TUFFAK polycarbonate sheet" printed all over as the 1/4" sheet. The 1/8" sheet is also TUFFAK. The 1/8" bar also appears to be cut from a larger sheet but has clear protective film with no branding printed on it.

Interestingly the birefringence of the 7/32" TUFFAK bar is dramatically lower than either of the 1/8" pieces. A bit less than 100 nm retardance, I believe, since if I increase path length somewhat by tilting it away from being perpendicular with the line of sight (by about 45 degrees) it behaves similar a reversed Hoya CIR-PL with little brightness variation as the analyzer is rotated, just some color shift from bluish to brownish.

The unbranded 1/8" bar on the other hand is around 1500 nm retardance despite being half as thick, so more than 30 times greater birefringence. The 1/8" TUFFAK sheet is similar. So it seems like maybe different manufacturing processes are used depending on thickness?

Here is what the 7/32" x 1" TUFFAK bar looks like with stress gradient at roughly 45 degrees between crossed polarizers. Polarizer is my monitor and analyzer is a Hoya CIR-PL installed normally on the camera lens. There is a lot of ugly color banding. I guess some of the interference colors are fairly pure and going out of gamut somewhere? The colors are prettier in person:
Wow, that's a good image! How did you stress the bar exactly? You've got a nice, almost central, destructive interference band - something I've struggled to get. The colours look good too.

ps I just tried to do the same with my 6mm x 20mm x 110mm bar. Did the same as you did, Hans, with bending performed by hand (I'm a little old lady with small hands, though - I'd need another hand to take a quick photo). I do indeed get the stress induced birefringence but I still struggle to get the dark fringe away from one edge. As soon as I apply more stress the dark fringe narrows significantly. This is the problem I've had all along :( It seems to be a property of the polycarbonate. It may be that I just have to adapt to this property.

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

#140 Post by hans » Tue Jun 29, 2021 10:00 am

I was just bending it by hand with a pair of locking pliers at each end for better leverage and control. Extinction in the center of the fringe pattern is very sensitive to twisting, similar to the case we were discussing a few posts up with the optic axis of the inherent birefringence aligned parallel or perpendicular to the polarizer, so possibly your fixture is inadvertently twisting the beam? I am not sure the 4-roller designs in the the papers are a good idea without precise, rigid construction. They are overconstrained in the sense that they will twist the beam unless everything (the rollers and the faces of the beam and frame where they contact) is perfectly coplanar/parallel.
Hobbyst46 wrote:
Tue Jun 29, 2021 8:49 am
I have been informed that at least one brand of PC for outdoor use is coated against UV, coating thickness can be 15-50 um, and may cover one side or both sides of the sheet, depending on expected market/site demands. FWIW.
McMaster sells UV-resistant variants so I could try those too. But if the coatings are 50 um or less the birefringence would need to be quite high to explain the retardance, not sure if possible unless it was crystalline?

LouiseScot
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Re: DIC Question

#141 Post by LouiseScot » Tue Jun 29, 2021 10:38 am

hans wrote:
Tue Jun 29, 2021 10:00 am
I was just bending it by hand with a pair of locking pliers at each end for better leverage and control. Extinction in the center of the fringe pattern is very sensitive to twisting, similar to the case we were discussing a few posts up with the optic axis of the inherent birefringence aligned parallel or perpendicular to the polarizer, so possibly your fixture is inadvertently twisting the beam? I am not sure the 4-roller designs in the the papers are a good idea without precise, rigid construction. They are overconstrained in the sense that they will twist the beam unless everything (the rollers and the faces of the beam and frame where they contact) is perfectly coplanar/parallel.
Hobbyst46 wrote:
Tue Jun 29, 2021 8:49 am
I have been informed that at least one brand of PC for outdoor use is coated against UV, coating thickness can be 15-50 um, and may cover one side or both sides of the sheet, depending on expected market/site demands. FWIW.
McMaster sells UV-resistant variants so I could try those too. But if the coatings are 50 um or less the birefringence would need to be quite high to explain the retardance, not sure if possible unless it was crystalline?
Hi Hans

I'm not sure if it's a twisting moment which causes the problem since it's so consistent between different thicknesses of sheet and with different frames - as well as bending by hand in this case. It's also a property of the polycarbonate that the dark fringe is biased to either the top or the bottom depending on how it's cut from the larger sheet. I suppose the big question is, does it affect the ability of the stressed prism to perform DIC? A quick measurement of the spacing between the coloured fringes either side of the dark band gives a distance of about 10mm (~ 4/10") with the 6mm prism which is maybe a smaller distance compared to Rathi's Fig 3. Theirs is is more like 16-18mm. It's almost ironic that I get broader bands on my plastic ruler - with no external stress!

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

#142 Post by hans » Tue Jun 29, 2021 5:36 pm

LouiseScot wrote:
Tue Jun 29, 2021 10:38 am
I'm not sure if it's a twisting moment which causes the problem...
Which problem, poor extinction in the center or offset? I saw twisting mainly affecting extinction in the center, not offset.

As far as I can see the offset is exactly what would be expected from the inherent uniform birefringence of the unstressed beam combining with the stress-gradient-induced birefringence. (Assuming the optic axis of the inherent birefringence is parallel to the beam, which seems to be the default case unless the beam is intentionally cut at an angle relative to the original material.) As bending moment is applied the fringe pattern narrows but the color along the central neutral axis (where there is no stress-induced birefringence) stays the same. (This is how I decided ~1500 nm absolute retardance for the 1/8" material without a calibrated compensator.)

If this understanding is correct then it seems like the offset should be equivalent to bias retardation normally introduced intentionally by translating the prism or with de Sénarmont compensator. So as long as the offset is not large enough to put the required part of the fringe pattern "out of view" with the correct bending applied, I don't see any fundamental problem.

LouiseScot
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Re: DIC Question

#143 Post by LouiseScot » Tue Jun 29, 2021 6:26 pm

hans wrote:
Tue Jun 29, 2021 5:36 pm
LouiseScot wrote:
Tue Jun 29, 2021 10:38 am
I'm not sure if it's a twisting moment which causes the problem...
Which problem, poor extinction in the center or offset? I saw twisting mainly affecting extinction in the center, not offset.

As far as I can see the offset is exactly what would be expected from the inherent uniform birefringence of the unstressed beam combining with the stress-gradient-induced birefringence. (Assuming the optic axis of the inherent birefringence is parallel to the beam, which seems to be the default case unless the beam is intentionally cut at an angle relative to the original material.) As bending moment is applied the fringe pattern narrows but the color along the central neutral axis (where there is no stress-induced birefringence) stays the same. (This is how I decided ~1500 nm absolute retardance for the 1/8" material without a calibrated compensator.)

If this understanding is correct then it seems like the offset should be equivalent to bias retardation normally introduced intentionally by translating the prism or with de Sénarmont compensator. So as long as the offset is not large enough to put the required part of the fringe pattern "out of view" with the correct bending applied, I don't see any fundamental problem.
The offset, really. I don't pretend to understand all the technicalities. I just printed out a second lower profile open frame. That should let me have both prisms the same, and with the extinction band in a good place. I'm still unsure how the condenser prism should be adjusted, in terms of tension, relative to the objective one. Rathi says there should be a difference in deflection angle between the two, but how to measure or estimate it?

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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Location: Southern California

Re: DIC Question

#144 Post by hans » Tue Jun 29, 2021 7:07 pm

LouiseScot wrote:
Tue Jun 29, 2021 6:26 pm
I'm still unsure how the condenser prism should be adjusted, in terms of tension, relative to the objective one. Rathi says there should be a difference in deflection angle between the two, but how to measure or estimate it?
The size of the fringe pattern is supposed to match when taking into account the magnification between the respective conjugate planes. Viktor posted an equation representing that in #80:
viktor j nilsson wrote:
Fri Jun 04, 2021 10:15 pm
In regular DIC, the following criteria needs to be met to achieve proper DIC...
I do wonder what the source of that is, and why variable f_ob is primed while f_c is not?

Your case with the extra image relay is a little more complicated because more focal lengths need to be taken into account to determine magnification going between your prisms, but same basic idea, I think.

LouiseScot
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Re: DIC Question

#145 Post by LouiseScot » Tue Jun 29, 2021 7:53 pm

hans wrote:
Tue Jun 29, 2021 7:07 pm
LouiseScot wrote:
Tue Jun 29, 2021 6:26 pm
I'm still unsure how the condenser prism should be adjusted, in terms of tension, relative to the objective one. Rathi says there should be a difference in deflection angle between the two, but how to measure or estimate it?
The size of the fringe pattern is supposed to match when taking into account the magnification between the respective conjugate planes. Viktor posted an equation representing that in #80:
viktor j nilsson wrote:
Fri Jun 04, 2021 10:15 pm
In regular DIC, the following criteria needs to be met to achieve proper DIC...
I do wonder what the source of that is, and why variable f_ob is primed while f_c is not?

Your case with the extra image relay is a little more complicated because more focal lengths need to be taken into account to determine magnification going between your prisms, but same basic idea, I think.
Hiya

So that just means when I look at the bfp then the apparent view of the fringes should match between the two prisms? As simple as that? (But maybe difficult to adjust in practice...) My condenser is set to 15mm but I'm not sure how the infinity objective (Olympus PLN40x/0.65, wd = 0.6mm, ref focal length = tube lens fl = 180mm) is represented in viktor;s equation? I could measure the overall mag with a micrometer slide but not sure if that would help.

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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Location: Southern California

Re: DIC Question

#146 Post by hans » Tue Jun 29, 2021 8:28 pm

LouiseScot wrote:
Tue Jun 29, 2021 7:53 pm
So that just means when I look at the bfp then the apparent view of the fringes should match between the two prisms?
That is my understanding, but I have never actually used a DIC microscope...
LouiseScot wrote:
Tue Jun 29, 2021 7:53 pm
...but I'm not sure how the infinity objective (Olympus PLN40x/0.65, wd = 0.6mm, ref focal length = tube lens fl = 180mm) is represented in viktor;s equation?
The magnification is reference focal length divided by the EFL of the objective, so that objective should have EFL 180 mm / 40 = 4.5 mm.
LouiseScot wrote:
Tue Jun 29, 2021 7:53 pm
I could measure the overall mag with a micrometer slide but not sure if that would help.
If you can take two images, one with the stage micrometer in the condenser FFP and the other with the micrometer in your relay of the objective BFP, then scale them on computer to match, that might be a good way to determine the relevant magnification empirically.

LouiseScot
Posts: 1167
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Location: Scotland

Re: DIC Question

#147 Post by LouiseScot » Tue Jun 29, 2021 8:34 pm

hans wrote:
Tue Jun 29, 2021 8:28 pm
LouiseScot wrote:
Tue Jun 29, 2021 7:53 pm
So that just means when I look at the bfp then the apparent view of the fringes should match between the two prisms?
That is my understanding, but I have never actually used a DIC microscope...
LouiseScot wrote:
Tue Jun 29, 2021 7:53 pm
...but I'm not sure how the infinity objective (Olympus PLN40x/0.65, wd = 0.6mm, ref focal length = tube lens fl = 180mm) is represented in viktor;s equation?
The magnification is reference focal length divided by the EFL of the objective, so that objective should have EFL 180 mm / 40 = 4.5 mm.
LouiseScot wrote:
Tue Jun 29, 2021 7:53 pm
I could measure the overall mag with a micrometer slide but not sure if that would help.
If you can take two images, one with the stage micrometer in the condenser FFP and the other with the micrometer in your relay of the objective BFP, then scale them on computer to match, that might be a good way to determine the relevant magnification empirically.
Ok, thanks. I'll see what I can do :)

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

viktor j nilsson
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Location: Lund, Sweden

Re: DIC Question

#148 Post by viktor j nilsson » Tue Jun 29, 2021 8:40 pm

hans wrote:
Tue Jun 29, 2021 8:28 pm
LouiseScot wrote:
Tue Jun 29, 2021 7:53 pm
So that just means when I look at the bfp then the apparent view of the fringes should match between the two prisms?
That is my understanding, but I have never actually used a DIC microscope...
Exactly. It's as simple as that, the fringes of both prisms should look the same when observed at the BFP. The easiest way to achieve that is in a perfectly symmetrical setup. Two identical prisms, and two identical objectives. One used as objective, and one used as condenser.

LouiseScot
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Joined: Tue Jul 07, 2020 1:51 pm
Location: Scotland

Re: DIC Question

#149 Post by LouiseScot » Tue Jun 29, 2021 8:44 pm

viktor j nilsson wrote:
Tue Jun 29, 2021 8:40 pm
hans wrote:
Tue Jun 29, 2021 8:28 pm
LouiseScot wrote:
Tue Jun 29, 2021 7:53 pm
So that just means when I look at the bfp then the apparent view of the fringes should match between the two prisms?
That is my understanding, but I have never actually used a DIC microscope...
Exactly. It's as simple as that, the fringes of both prisms should look the same when observed at the BFP. The easiest way to achieve that is in a perfectly symmetrical setup. Two identical prisms, and two identical objectives. One used as objective, and one used as condenser.
Ok, thanks, Viktor :)
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

viktor j nilsson
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Location: Lund, Sweden

Re: DIC Question

#150 Post by viktor j nilsson » Tue Jun 29, 2021 9:01 pm

hans wrote: The size of the fringe pattern is supposed to match when taking into account the magnification between the respective conjugate planes. Viktor posted an equation representing that in #80:
viktor j nilsson wrote:
Fri Jun 04, 2021 10:15 pm
In regular DIC, the following criteria needs to be met to achieve proper DIC...
I do wonder what the source of that is, and why variable f_ob is primed while f_c is not?

Your case with the extra image relay is a little more complicated because more focal lengths need to be taken into account to determine magnification going between your prisms, but same basic idea, I think.
I got it from a post by abednego:
https://www.photomacrography.net/forum/ ... 84#p244784
But I'm not sure where it comes from originally.

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