Parallel vs. crossed polarizers

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microcosmos
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Parallel vs. crossed polarizers

#1 Post by microcosmos » Fri Oct 22, 2021 5:42 am

In polarizing microscopy, the polarizer and analyzer are almost always used together in the crossed or partially crossed position. However, the McCrone article on interference colours mentions the use of parallel polarizers to generate complementary interference colours and "zero in" on retardation values.

I gave it a try, and it seems that parallel polarizers can do even more than help get a more accurate retardation value.

These images show a grain of quartz (the big patch in the centre, straddling the whole field of view) in a thin section through crossed polarizers, without and with a 530 nm tint plate:

ImageImage

And this shows the same view with parallel polarizers, without a tint plate:

Image

With parallel polarizers I can see additional defects (yellow) and roughly parallel streaks (purple) that are difficult or impossible to see in the cross-polarized images, even when I rotate the stage or insert the tint plate.

Has anyone found other things that parallel polarizers can do that crossed ones can't?

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mrsonchus
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Re: Parallel vs. crossed polarizers

#2 Post by mrsonchus » Fri Oct 22, 2021 6:24 pm

Very interesting, a method I've never even heard of. It clearly in this case and of course many others yet to be seen, works very well indeed with a huge increase in contrast between the colours in the quartz patch. The yellow and rust/purple really do contrast well. The purplish of the quart patch is exceedingly clearly delineated as the surroundings are almost whitish.

I'll certainly be giving this technique a run-out with my plant sections, and have a read of the McCrone article hopefully later.

Excellent and interesting post!
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MichaelG.
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Re: Parallel vs. crossed polarizers

#3 Post by MichaelG. » Fri Oct 22, 2021 9:17 pm

There is a very good copy of the Michel-Lévy chart available here:
http://opticalmineralogy.blogspot.com/p ... gramm.html

There is also a useful ‘interactive’ App available for iOS users :
https://apps.apple.com/gb/app/birefring ... 1354688131

MichaelG.
Too many 'projects'

Greg Howald
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Re: Parallel vs. crossed polarizers

#4 Post by Greg Howald » Fri Oct 22, 2021 10:55 pm

I have had success with parallel polarization in mineralogy in thin section, but not as much with other things. I have had no luck at all trying the technique with circular polarization.

microcosmos
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Re: Parallel vs. crossed polarizers

#5 Post by microcosmos » Sat Oct 23, 2021 2:13 am

Thanks John, I'll be interested to hear if the plant sections give any hidden messages with parallels! I don't have any plant sections myself.

Thanks Michael for the chart. I quite like the uncluttered design of that one with its detailed graduations of the axes. There's this pdf version of the Zeiss chart which has infinite resolution being a vector graphic, but I don't like the black border and it doesn't show so many orders of interference colours. As expected the colours on the two charts look quite different - one is a (probably RGB) scan of a paper print that seems to have faded a bit, while the other is a digitally generated pdf probably in CMYK. Which one is more accurate when printed out? Maybe the Zeiss one? But it depends on the printer as well.

However, I personally don't rely too much on accurately estimating interference colours by eye beyond basic initial assessment of the degree of birefringence of the material. If I really need to pin down the retardation accurately, I use compensators that can measure it. So it doesn't really matter which of the above charts I use since the colour part of it is just a rough guide. In this respect Michael's chart is probably better as it shows colours up to higher orders and the scale ticks on the right side are higher-resolution so I can really use it to intersect the quantitative retardation measurements I get from the sample and get a more accurate value for the very high-birefringence substances than I can with the Zeiss chart.

I think the iOS app is a nice innovation for those who are comfortable with those platforms, especially on an iPad whose screen is big enough, but I just love good old paper!

Greg, how have you made use of parallel polarization in mineralogy? Was it for more accurate estimation of birefringence or other things?

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Re: Parallel vs. crossed polarizers

#6 Post by Greg Howald » Sat Oct 23, 2021 12:10 pm

That might be about a 500 page dissertation, but I have several experiments I'm currently working on. One is that ppl can assist in identifying things you didn't know we're there. You already know that you have different colors like yellow and purple in your own photos. What are those things? Michelle - Levi is all about xpl, and no one has developed a means of identification based upon the things you see with ppl.

I'm not really very good with the m-l chart because my eyes ain't so great no more. Can I develope my own spectrometer based on the chart which will measure both frequency of color and intensity, and if so can I computerize that so it will automatically identify the specimen?

Can the use of three polarizing lenses rather than two produce a positive effect or is it a total waste of time?

I have lots of questions. My greatest joy in all this is when I personally find answers.

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