DIC Question

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hans
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Re: DIC Question

#151 Post by hans » Tue Jun 29, 2021 9:29 pm

viktor j nilsson wrote:
Tue Jun 29, 2021 9:01 pm
I got it from a post by abednego:
https://www.photomacrography.net/forum/ ... 84#p244784
viktor j nilsson wrote: I am not really planning to shell out the dough needed to put together a DIC system, but I have been reading a lot of DIY DIC threads with great interest lately. (is that the first symptoms of DIC stepwise acquisition disorder?).
Uh oh, symptoms are starting to show, and we can see how your case turned out...

viktor j nilsson
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Re: DIC Question

#152 Post by viktor j nilsson » Tue Jun 29, 2021 10:07 pm

hans wrote:
Tue Jun 29, 2021 9:29 pm
viktor j nilsson wrote:
Tue Jun 29, 2021 9:01 pm
I got it from a post by abednego:
https://www.photomacrography.net/forum/ ... 84#p244784
viktor j nilsson wrote: I am not really planning to shell out the dough needed to put together a DIC system, but I have been reading a lot of DIY DIC threads with great interest lately. (is that the first symptoms of DIC stepwise acquisition disorder?).
Uh oh, symptoms are starting to show, and we can see how your case turned out...
Haha, thanks for reminding me! :lol:

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75RR
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Re: DIC Question

#153 Post by 75RR » Sun Jul 11, 2021 5:55 am

.
re prism interference pattern

came across this, thought the prism interference patterns per objective type would be of interest

https://micro.magnet.fsu.edu/primer/tec ... ation.html
.
Attachments
nomarski objective prisms.png
nomarski objective prisms.png (115.84 KiB) Viewed 311826 times
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LouiseScot
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Re: DIC Question

#154 Post by LouiseScot » Sun Jul 11, 2021 8:45 am

75RR wrote:
Sun Jul 11, 2021 5:55 am
.
re prism interference pattern

came across this, thought the prism interference patterns per objective type would be of interest

https://micro.magnet.fsu.edu/primer/tec ... ation.html
.
Thanks, though I think everyone is familiar with that reference :) My project is suspended until next month as I have other things I need to 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

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75RR
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Re: DIC Question

#155 Post by 75RR » Sun Jul 11, 2021 8:51 am

LouiseScot wrote:
Sun Jul 11, 2021 8:45 am
75RR wrote:
Sun Jul 11, 2021 5:55 am
.
re prism interference pattern

came across this, thought the prism interference patterns per objective type would be of interest

https://micro.magnet.fsu.edu/primer/tec ... ation.html
.
Thanks, though I think everyone is familiar with that reference :) My project is suspended until next month as I have other things I need to do.

Louise
It just struck me that you all seem to be trying for an interference pattern more suitable to a 60x objective
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LouiseScot
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Re: DIC Question

#156 Post by LouiseScot » Sun Jul 11, 2021 8:57 am

75RR wrote:
Sun Jul 11, 2021 8:51 am
LouiseScot wrote:
Sun Jul 11, 2021 8:45 am
75RR wrote:
Sun Jul 11, 2021 5:55 am
.
re prism interference pattern

came across this, thought the prism interference patterns per objective type would be of interest

https://micro.magnet.fsu.edu/primer/tec ... ation.html
.
Thanks, though I think everyone is familiar with that reference :) My project is suspended until next month as I have other things I need to do.

Louise
It just struck me that you all seem to be trying for an interference pattern more suitable to a 60x objective
I've been using a 40x which isn't vastly different to a 60x, but the setup is different to a normal dic with the objective (polycarbonate) prism positioned a long way from the objective.

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

#157 Post by tpruuden » Wed Jul 14, 2021 6:03 pm

Palsun 3mm UV protected material seems to behave the same here - the fringes are shifted. Even then, from experiments I believe weak DIC effect can be created even with that material.

LouiseScot
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Re: DIC Question

#158 Post by LouiseScot » Wed Jul 14, 2021 6:29 pm

tpruuden wrote:
Wed Jul 14, 2021 6:03 pm
Palsun 3mm UV protected material seems to behave the same here - the fringes are shifted. Even then, from experiments I believe weak DIC effect can be created even with that material.
Hiya

Just to be clear... Are you saying there's a difference in optical properties (from a DIC point of view) between UV protected and unprotected polycarbonate?

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

#159 Post by tpruuden » Wed Jul 14, 2021 10:35 pm

LouiseScot wrote:
Wed Jul 14, 2021 6:29 pm

Just to be clear... Are you saying there's a difference in optical properties (from a DIC point of view) between UV protected and unprotected polycarbonate?

Louise
I believe the UV protection acts as a waveplate (not sure, how much exactly the polarization is shifted). They say, that it is a co-extruded layer:

"The PALSUN range of products consists of flat polycarbonate sheets, which have a co-extruded UV protective layer on one side (PALSUN
UV2 products offer co-extruded UV protective layer on both sides)."

LouiseScot
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Re: DIC Question

#160 Post by LouiseScot » Wed Jul 14, 2021 10:42 pm

tpruuden wrote:
Wed Jul 14, 2021 10:35 pm
LouiseScot wrote:
Wed Jul 14, 2021 6:29 pm

Just to be clear... Are you saying there's a difference in optical properties (from a DIC point of view) between UV protected and unprotected polycarbonate?

Louise
I believe the UV protection acts as a waveplate (not sure, how much exactly the polarization is shifted). They say, that it is a co-extruded layer:

"The PALSUN range of products consists of flat polycarbonate sheets, which have a co-extruded UV protective layer on one side (PALSUN
UV2 products offer co-extruded UV protective layer on both sides)."
Hi again

So does non-uv protected polycarbonate have no such waveplate properties? I can't seem to find any suppliers of non uv protected polycarbonate sheet here in the UK. Presumably it's all only made for glazing purposes.

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

#161 Post by MichaelG. » Thu Jul 15, 2021 6:28 am

LouiseScot wrote:
Wed Jul 14, 2021 10:42 pm
I can't seem to find any suppliers of non uv protected polycarbonate sheet here in the UK. Presumably it's all only made for glazing purposes.
.
Also used for machine safety guards, riot shields, and the visors on motorcycle helmets
… but it may all be UV protected : I don’t know.

MichaelG.
Too many 'projects'

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75RR
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Re: DIC Question

#162 Post by 75RR » Thu Jul 15, 2021 7:34 am

This is what Wiki says:

"Standard polycarbonate resins are not suitable for long term exposure to UV radiation. To overcome this, the primary resin can have UV stabilisers added. These grades are sold as UV stabilized polycarbonate to injection moulding and extrusion companies. Other applications, including polycarbonate sheets, may have the anti-UV layer added as a special coating or a coextrusion for enhanced weathering resistance."


https://en.wikipedia.org/wiki/Polycarbonate
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LouiseScot
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Re: DIC Question

#163 Post by LouiseScot » Thu Jul 15, 2021 8:14 am

MichaelG. wrote:
Thu Jul 15, 2021 6:28 am
LouiseScot wrote:
Wed Jul 14, 2021 10:42 pm
I can't seem to find any suppliers of non uv protected polycarbonate sheet here in the UK. Presumably it's all only made for glazing purposes.
.
Also used for machine safety guards, riot shields, and the visors on motorcycle helmets
… but it may all be UV protected : I don’t know.

MichaelG.
As far as I can make out, all polycarbonate is UV stabilised. Most polycarbonate also has one or two UV blocking coatings. Palram make a version called 'Paltuf' which doesn't have any UV coatings but I can't find anyone that sells it in the sort of sizes/quantities I'd want e.g. a single A4 sheet... I'll keep looking.

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

#164 Post by LouiseScot » Thu Jul 15, 2021 9:52 am

I think this UV coating business is all a red herring! Neither Rathi, Biss or Sanderson himself specify prisms should be constructed from non-standard polycarbonate sheet. Biss mentioned 'Lexan' from McMaster-Carr though the latter only say their polycarbonate is 'comparable' to Lexan, Makrolon etc.
The UV coating probably only affects wavelengths shorter than the visible violet so shouldn't affect the general visible spectrum. So I'll continue with the standard Palsun for now. I'm busy with other things at the moment but I expect to resume DIC investigations mid-August.

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

#165 Post by hans » Thu Jul 15, 2021 4:29 pm

75RR wrote:
Thu Jul 15, 2021 7:34 am
This is what Wiki says: ...
McMaster-Carr has three different types advertising resistance to: [impact], [impact, UV], [impact, UV, scratch]. These probably correspond to the three cases mentioned on Wikipedia: UV stabilizer added to the bulk material; additional UV-resistant coating (maybe the 15-50 um thickness Hobbyst46 mentioned in #138); coextrusion of some thicker surface layer that additionally gives scratch resistance.
tpruuden wrote:
Wed Jul 14, 2021 10:35 pm
I believe the UV protection acts as a waveplate (not sure, how much exactly the polarization is shifted). They say, that it is a co-extruded layer...
I only bought the basic type but there was already very strong birefringence in the 1/8" material, 30X greater than the birefringence in the 7/32" material (#137), and enough to bias the central dark band way off the edge of the bar with reasonable bending. That gives some evidence that strong birefringence can come from the base material itself (depending on manufacturing process maybe?) without any extra coatings. Also, polycarbonate is specifically chosen for its strong photoelasticity, does not seem surprising that it would also be prone to strong inherent birefringence depending on manufacturing process. (Either due to internal stress, molecular alignment without stress, or both -- still not sure I understand that relationship mentioned in #135.)
LouiseScot wrote:
Thu Jul 15, 2021 9:52 am
I think this UV coating business is all a red herring!
Seems likely, but would be interesting to compare. I already added UV/scratch-resistant sheets to my cart during the earlier discussion but have not needed to order anything else from McMaster yet.

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Re: DIC Question

#166 Post by LouiseScot » Thu Jul 15, 2021 4:45 pm

hans wrote:
Thu Jul 15, 2021 4:29 pm
75RR wrote:
Thu Jul 15, 2021 7:34 am
This is what Wiki says: ...
McMaster-Carr has three different types advertising resistance to: [impact], [impact, UV], [impact, UV, scratch]. These probably correspond to the three cases mentioned on Wikipedia: UV stabilizer added to the bulk material; additional UV-resistant coating (maybe the 15-50 um thickness Hobbyst46 mentioned in #138); coextrusion of some thicker surface layer that additionally gives scratch resistance.
tpruuden wrote:
Wed Jul 14, 2021 10:35 pm
I believe the UV protection acts as a waveplate (not sure, how much exactly the polarization is shifted). They say, that it is a co-extruded layer...
I only bought the basic type but there was already very strong birefringence in the 1/8" material, 30X greater than the birefringence in the 7/32" material (#137), and enough to bias the central dark band way off the edge of the bar with reasonable bending. That gives some evidence that strong birefringence can come from the base material itself (depending on manufacturing process maybe?) without any extra coatings. Also, polycarbonate is specifically chosen for its strong photoelasticity, does not seem surprising that it would also be prone to strong inherent birefringence depending on manufacturing process. (Either due to internal stress, molecular alignment without stress, or both -- still not sure I understand that relationship mentioned in #135.)
LouiseScot wrote:
Thu Jul 15, 2021 9:52 am
I think this UV coating business is all a red herring!
Seems likely, but would be interesting to compare. I already added UV/scratch-resistant sheets to my cart during the earlier discussion but have not needed to order anything else from McMaster yet.
I just feel that one or other of the Sanderson prism related authors would have mentioned something re the different types/coatings if it was relevant to either Schlieren/beamsplitting or DIC. Still, it would be interesting to have a comparison.

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

#167 Post by hans » Thu Jul 15, 2021 6:32 pm

My guess is that UV protection is not the main cause of birefringence with no bending moment applied. But not sure the inherent birefringence, whatever the cause, is a red herring. Many photos posted earlier show quite strong inherent birefringence, maybe not as much as my 1/8" plain TUFFAK bar (which I did not take a photo of) but much stronger than the 7/32" bar: In all those cases it looks like the inherent birefringence is strong enough that if the bending moment is reduced to get a gradient (fringe density) more like real DIC prisms (as 75RR has suggested) then the central dark band will be biased all the way off the edge of the beam.

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Re: DIC Question

#168 Post by LouiseScot » Thu Jul 15, 2021 6:47 pm

hans wrote:
Thu Jul 15, 2021 6:32 pm
My guess is that UV protection is not the main cause of birefringence with no bending moment applied. But not sure the inherent birefringence, whatever the cause, is a red herring. Many photos posted earlier show quite strong inherent birefringence, maybe not as much as my 1/8" plain TUFFAK bar (which I did not take a photo of) but much stronger than the 7/32" bar: In all those cases it looks like the inherent birefringence is strong enough that if the bending moment is reduced to get a gradient (fringe density) more like real DIC prisms (as 75RR has suggested) then the central dark band will be biased all the way off the edge of the beam.
Hiya

Both of the ones of mine you refer to are images taken with the prism under stress i.e. with a bending moment. Same with jmp's, I think. But there is always waveplate behaviour but with little inherent birefringence - just a backround hue which varies across a plate. I don't know what causes the waveplate properties or how that varies between different makes or types. I presume it's just a property of all
polycarbonate

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

#169 Post by hans » Thu Jul 15, 2021 8:15 pm

LouiseScot wrote:
Thu Jul 15, 2021 6:47 pm
Both of the ones of mine you refer to are images taken with the prism under stress i.e. with a bending moment. Same with jmp's...
Yes, quite a large bending moment judging from the fringe density. I was referring to the bias/shift of the fringe pattern away from the center of the beam, which as far as I understand is a result of the inherent birefringence combining with the stress-induced birefringence from the applied bending moment. If the inherent birefringence is so strong that the center of the fringe pattern only becomes visible with extreme (much higher fringe density than normal DIC) bending moment applied, seems like that would be a problem?
LouiseScot wrote:
Thu Jul 15, 2021 9:52 am
Neither Rathi, Biss or Sanderson himself specify prisms should be constructed from non-standard polycarbonate sheet.
LouiseScot wrote:
Thu Jul 15, 2021 6:47 pm
I don't know what causes the waveplate properties or how that varies between different makes or types. I presume it's just a property of all polycarbonate
The difference between 1/8" and 7/32" plain TUFFAK is huge. The original Sanderson paper says 6.35 mm thickness and I don't remember seeing any indication that the later authors tried thinner, so if they all bought ~1/4" from McMaster-Carr maybe they just never encountered material with strong inherent birefringence?

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Re: DIC Question

#170 Post by LouiseScot » Thu Jul 15, 2021 9:09 pm

hans wrote:
Thu Jul 15, 2021 8:15 pm
LouiseScot wrote:
Thu Jul 15, 2021 6:47 pm
Both of the ones of mine you refer to are images taken with the prism under stress i.e. with a bending moment. Same with jmp's...
Yes, quite a large bending moment judging from the fringe density. I was referring to the bias/shift of the fringe pattern away from the center of the beam, which as far as I understand is a result of the inherent birefringence combining with the stress-induced birefringence from the applied bending moment. If the inherent birefringence is so strong that the center of the fringe pattern only becomes visible with extreme (much higher fringe density than normal DIC) bending moment applied, seems like that would be a problem?
LouiseScot wrote:
Thu Jul 15, 2021 9:52 am
Neither Rathi, Biss or Sanderson himself specify prisms should be constructed from non-standard polycarbonate sheet.
LouiseScot wrote:
Thu Jul 15, 2021 6:47 pm
I don't know what causes the waveplate properties or how that varies between different makes or types. I presume it's just a property of all polycarbonate
The difference between 1/8" and 7/32" plain TUFFAK is huge. The original Sanderson paper says 6.35 mm thickness and I don't remember seeing any indication that the later authors tried thinner, so if they all bought ~1/4" from McMaster-Carr maybe they just never encountered material with strong inherent birefringence?
Oh ok - it wasn't clear to me that you were referring to the shift of the fringe pattern. In 'Flow visualization using a Sanderson prism' Schulz et al (2019) refer to this shift that I've observed as well "During initial testing, the polycarbonate material used revealed that when different sets of prisms were placed in position and stressed, the fringes that formed, formed on opposite sides. Some developed concentrated
fringes at the top of the prism, whereas others developed them in the bottom part of the prism as shown in Fig. 4 under same loading conditions"
Schulz2019.JPG
Schulz2019.JPG (28.18 KiB) Viewed 311610 times
Maybe it's this that causes the shift from the center?

Rathi used 3/32" polycarbonate in their prism. The nearest I could get was 3mm. So maybe any intrinsic birefringence not apparent, as you suggest.

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

#171 Post by hans » Fri Jul 16, 2021 5:11 am

Regarding whether the fringe pattern is shifted toward the compressive or tensile side of the beam... should depend on whether the optic axis of the inherent birefringence is perpendicular or parallel to the beam axis, I think. You are cutting beams from sheets like Schulz and also saw both cases, right? Were you keeping track of what orientation you were cutting the beams relative to the original sheet? Perhaps Schulz were not and that explains the variability. Biss and Rathi both used pre-cut bars, seems likely those would have been cut with consistent orientation relative to the original sheet.

I was thinking 1/4" for Rathi, clearly you read the papers more carefully. The bias in Rathi fig 3(b) for the 3/32" bar from McMaster looks like maybe a little less then half my 7/32" bar from McMaster, so plausible it could have similar inherent birefringence with the thickness difference accounting for the greater bias in mine. The odd thing is both the 1/16" and 1/8" inch I have from McMaster show very strong inherent birefringence, impossible to get a fringe pattern looking like Rathi fig 3(b) from those as far as I can see. Guess I might as well order a 3/23" bar as well, only $1.72...

Looking at the papers again I see Biss fig 4 shows relatively large bias while fig 9 does not but I don't see that mentioned anywhere.

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Re: DIC Question

#172 Post by LouiseScot » Fri Jul 16, 2021 8:56 am

hans wrote:
Fri Jul 16, 2021 5:11 am
Regarding whether the fringe pattern is shifted toward the compressive or tensile side of the beam... should depend on whether the optic axis of the inherent birefringence is perpendicular or parallel to the beam axis, I think. You are cutting beams from sheets like Schulz and also saw both cases, right? Were you keeping track of what orientation you were cutting the beams relative to the original sheet? Perhaps Schulz were not and that explains the variability. Biss and Rathi both used pre-cut bars, seems likely those would have been cut with consistent orientation relative to the original sheet.

I was thinking 1/4" for Rathi, clearly you read the papers more carefully. The bias in Rathi fig 3(b) for the 3/32" bar from McMaster looks like maybe a little less then half my 7/32" bar from McMaster, so plausible it could have similar inherent birefringence with the thickness difference accounting for the greater bias in mine. The odd thing is both the 1/16" and 1/8" inch I have from McMaster show very strong inherent birefringence, impossible to get a fringe pattern looking like Rathi fig 3(b) from those as far as I can see. Guess I might as well order a 3/23" bar as well, only $1.72...

Looking at the papers again I see Biss fig 4 shows relatively large bias while fig 9 does not but I don't see that mentioned anywhere.
My A5 sized sheets were bought precut. I cut the prisms from them. I originally did that without regard to orientation but then soon realised it made a difference! The solid precut bars aren't available here. It's a pity Rathi and others weren't more specific about the polycarbonate. It seems that it's possible to get 'food grade' polycarbonate without UV additives but couldn't find any sources here in the UK.

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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Re: DIC Question

#173 Post by MichaelG. » Fri Jul 16, 2021 12:05 pm

LouiseScot wrote:
Fri Jul 16, 2021 8:56 am
It seems that it's possible to get 'food grade' polycarbonate without UV additives but couldn't find any sources here in the UK.
.
These people might be able to advise:

https://www.sndsolutions.co.uk/polycarbonate-vs-petg/
It is possible to buy Food Grade Polycarbonate, however this is quite a rare material. S&D are one of the only companies in the UK to have stocked 3mm Food Grade Polycarbonate for chute fabrications.
MichaelG.

.
Edit: __ see also : http://www.vacuumformers.co.uk/moulding-materials.aspx
Too many 'projects'

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Re: DIC Question

#174 Post by LouiseScot » Fri Jul 16, 2021 12:33 pm

MichaelG. wrote:
Fri Jul 16, 2021 12:05 pm
LouiseScot wrote:
Fri Jul 16, 2021 8:56 am
It seems that it's possible to get 'food grade' polycarbonate without UV additives but couldn't find any sources here in the UK.
.
These people might be able to advise:

https://www.sndsolutions.co.uk/polycarbonate-vs-petg/
It is possible to buy Food Grade Polycarbonate, however this is quite a rare material. S&D are one of the only companies in the UK to have stocked 3mm Food Grade Polycarbonate for chute fabrications.
MichaelG.

.
Edit: __ see also : http://www.vacuumformers.co.uk/moulding-materials.aspx
Thanks Michael, but they are 'fabricators' rather than sellers of sheet material. I'll only bother buying some if it's both cheap and easy to get hold of, and only just to compare with 'standard' sheet. As it's a polymer there could be variation of optical properties between manufacturers so I might try and get another brand just to see if there's any obvious difference.
Louise
ps I ordered a 12"x6" sheet of Lexan so will see how that behaves, when I can
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

#175 Post by hans » Fri Jul 16, 2021 3:27 pm

viktor j nilsson wrote:
Sun Jun 06, 2021 10:02 pm
One thing that I started thinking about is if the Sanderson prisms are stable over time, or if the shearing characteristics will change over time due to fatigue. Have you read anything about this?
Just noticed this is mentioned briefly in section 2 of:
Biss et al. - Differential schlieren-interferometry with a simple adjustable Wollaston-like prism
We further found that, due to the creep dependency of the photoelastic constants, one should avoid leaving the Sanderson prism loaded for extended periods of time, i.e., hours [18].

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Re: DIC Question

#176 Post by LouiseScot » Fri Jul 16, 2021 3:36 pm

hans wrote:
Fri Jul 16, 2021 3:27 pm
viktor j nilsson wrote:
Sun Jun 06, 2021 10:02 pm
One thing that I started thinking about is if the Sanderson prisms are stable over time, or if the shearing characteristics will change over time due to fatigue. Have you read anything about this?
Just noticed this is mentioned briefly in section 2 of:
Biss et al. - Differential schlieren-interferometry with a simple adjustable Wollaston-like prism
We further found that, due to the creep dependency of the photoelastic constants, one should avoid leaving the Sanderson prism loaded for extended periods of time, i.e., hours [18].
Yes, I read that previously but in that case they were talking about much larger beam deflections. With Rathi and DIC it's only 10's of microrads. In any case it's not a problem in practice, I don't think.
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

#177 Post by hans » Sat Jul 17, 2021 12:27 am

Not ideal, but maybe useful as a workaround if low-birefringence material is hard to find -- stacking two beams with optic axes of the inherent birefringence 90 degrees apart should cause inherent (but not stress-induced) birefringence to cancel. Seems to work reasonably well in a crude test, first photo is a single beam showing high bending moment necessary to get the central dark band visible, second photo is the stacked beams with lower bending moment:
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LouiseScot
Posts: 1167
Joined: Tue Jul 07, 2020 1:51 pm
Location: Scotland

Re: DIC Question

#178 Post by LouiseScot » Sat Jul 17, 2021 8:45 am

hans wrote:
Sat Jul 17, 2021 12:27 am
Not ideal, but maybe useful as a workaround if low-birefringence material is hard to find -- stacking two beams with optic axes of the inherent birefringence 90 degrees apart should cause inherent (but not stress-induced) birefringence to cancel. Seems to work reasonably well in a crude test, first photo is a single beam showing high bending moment necessary to get the central dark band visible, second photo is the stacked beams with lower bending moment:
That looks like magic! I can't see how you are doing the bending?

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
Posts: 986
Joined: Thu May 28, 2020 11:10 pm
Location: Southern California

Re: DIC Question

#179 Post by hans » Sat Jul 17, 2021 11:16 pm

I haven't tried to make any sort of fixture yet, still holding the beam between my left and right hands with locking pliers and bending manually as in the previous photo with the larger 1/4" x 1" x 8" bar. The shiny things in the upper left and lower right corners are the jaws grabbing the ends of the beam. Scale may be confusing, these beams are small, roughly 10 x 50 mm cut in perpendicular directions from 1/16" TUFFAK sheet with strong inherent birefringence. The 1/16" sheet is convenient because it can be cut with heavy-duty scissors. Photos are with the camera in fully manual mode on a tripod with 10 second timer so I press the shutter release then quickly position the beam just behind the polarizing filter and fiddle with bending/twisting/shearing to get a nice fringe pattern that looks like pure bending. Took a few attempts to get decent photos.

LouiseScot
Posts: 1167
Joined: Tue Jul 07, 2020 1:51 pm
Location: Scotland

Re: DIC Question

#180 Post by LouiseScot » Sat Jul 17, 2021 11:18 pm

hans wrote:
Sat Jul 17, 2021 11:16 pm
I haven't tried to make any sort of fixture yet, still holding the beam between my left and right hands with locking pliers and bending manually as in the previous photo with the larger 1/4" x 1" x 8" bar. The shiny things in the upper left and lower right corners are the jaws grabbing the ends of the beam. Scale may be confusing, these beams are small, roughly 10 x 50 mm cut in perpendicular directions from 1/16" TUFFAK sheet with strong inherent birefringence. The 1/16" sheet is convenient because it can be cut with heavy-duty scissors. Photos are with the camera in fully manual mode on a tripod with 10 second timer so I press the shutter release then quickly position the beam just behind the polarizing filter and fiddle with bending/twisting/shearing to get a nice fringe pattern that looks like pure bending. Took a few attempts to get decent photos.
Oh ok

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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