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Quarter wave plate and specular reflections

 
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boomblurt



Joined: 18 Aug 2013
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Location: Australia

PostPosted: Sat Feb 20, 2016 1:11 am    Post subject: Quarter wave plate and specular reflections Reply with quote

I have a SMZ-U stereo microscope and am considering getting an epi illuminator. After lots of googling I am still confused as to how a quarter wave plate reduces specular reflections when using coaxial episcopic illumination.

Can anyone help me understand please?
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g4lab



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PostPosted: Sat Feb 20, 2016 1:41 am    Post subject: Reply with quote

http://ipl.physics.harvard.edu/wp-uploads/2013/03/15c_s07_7.pdf

https://en.wikipedia.org/wiki/Polarizer#Circular_polarizers

http://www.u-oplaz.com/new/waveplate.htm

http://serinet.meei.harvard.edu/faculty/peli/papers/JAOA%201986%20Circular.pdf

The above one has a good diagram showing how it works to suppress glare on crt monitors. It works the same way on the incident illuminator. The light is also polarized.

The techniques don't work on metal or other electrically conductive surfaces.

Almost all coaxial illuminators come with a quarter wave plate attachment. The Zeiss version for their SV6/11 stereo has the quarter wave plate mounted in a plane that is non parallel to the front of the objective. There are also techniques included to reduce the glare from optics and internal components. A friend of mine got a CoAx illuminator for his M400 and thought it was worthless until I told him to find some quarter wave plate and apply it to the front of the objective. He could then see into gemstones with the coax illuminator. I have illuminators like that for the Zeiss SV11 , the Wild M400 and M series and Nikon SMZ10 they all came with quarter wave plates. As do zeiss Antiflex objectives. for compound scopes (incident light scopes) The quarter wave plate needs to be in a rotatable mount.


Last edited by g4lab on Sat Feb 20, 2016 2:05 am; edited 5 times in total
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Pau
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PostPosted: Sat Feb 20, 2016 1:48 am    Post subject: Reply with quote

Zeiss, who seems invented the system, calls it Antiflex

It serves to avoid reflections in dark subjects and to eliminate internal reflections of lens elements surfaces
From Microscopy from the very beginning Zeiss booklet, page 32
https://www.google.ca/url?sa=t&rct=j&q=&esrc=s&source=web&cd=21&cad=rja&uact=8&ved=0ahUKEwj2waPU9YXLAhXDVRoKHRbKC4A4ChAWCE0wCg&url=http%3A%2F%2Fwww.usask.ca%2Fbiology%2Fscopes%2FMicroscopyBasics.pdf&usg=AFQjCNGQCidL8oEbVcZ2H1nZMShIGL97dw&sig2=dzltM-LE6A0NuXTK7blVNA&bvm=bv.114733917,d.d24

Quote:
When objectives of a very low magnification are used, a
so-called Antiflex cap (a rotatable ?/4-Platte in front of the
objective) permits otherwise unavoidable reflection to be also
eliminated from “dark” sample surfaces.


See also Handbook of Incident Light Microscopy, page 39:
https://www.google.ca/url?sa=t&rct=j&q=&esrc=s&source=web&cd=23&cad=rja&uact=8&ved=0ahUKEwilo6Hp9YXLAhWOhhoKHZ-IBrI4FBAWCCgwAg&url=http%3A%2F%2Fwww.microscopy-uk.org.uk%2Fzeiss%2FZeiss-Incident-1-Qualitative-a.pdf&usg=AFQjCNEfDvNcxxjREHoehGxqQJThBCBtig&sig2=MlmuysQ7azTiRc69G0Ovdg&bvm=bv.114733917,d.d24

I only have used it (but few times, so little experience) with low power microscope objectives. Some high end stereos and macroscopes like the Nikon AZ use it.
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boomblurt



Joined: 18 Aug 2013
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PostPosted: Sat Feb 20, 2016 2:39 am    Post subject: Reply with quote

So if I understand correctly the emitted light is polarised before it is reflected down and passes through the objective, and then goes through a crossed analyzer as it goes back up, past the illuminator to the eyepiece. The twofold pass through the 1/4 wave plate, located between the objective and specimen, rotates the direction of polarization so it can pass the analyzer, while other (internal reflection) light is blocked?

So ... the purpose is to reduce internal reflections - not glare off the specimen? No wonder I was confused. Razz

EDIT : That seems to be, I think, the idea of the Antiflex system (Pau's link, page 39). But I think I need to chew this over some more, in view of Gene's last link that show reduction of specimen reflections. Thanks for the links.
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g4lab



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PostPosted: Sat Feb 20, 2016 3:12 am    Post subject: Reply with quote

The reflection it supresses is on the specimen. It is only internal reflection if the "specimen" is the surface of an old CRT monitor behind one of these screens that work on the very same principle.
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boomblurt



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PostPosted: Sat Feb 20, 2016 5:35 am    Post subject: Reply with quote

Gene, is the diagram on your last link at page 299 the right idea - normally reflected light reverses its rotation of polarity and therefore is not passed back through the linear polarizer (after the 1/4 wave retarder)?

EDIT: With a crude setup of a torch (flashlight), linear polarizer and piece of cellophane as a workable 1/4 wave retarder I am able to see an effect of reduced glare that varied with rotation of the cellophane. (The cellophane was tested as a wave retarder by looking at polarized light - a laptop LED screen - with the polarizer between it and me. With the polarizer set for maximum extinction the cellophane was rotated and light varied as expected - four min/max transmissions per rotation. I had a tried a normal photography CPL filter but something was going on there as light could only be extinguished with the polarizer closest to the screen).
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rjlittlefield
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PostPosted: Sat Feb 20, 2016 10:49 am    Post subject: Reply with quote

boomblurt wrote:
I had a tried a normal photography CPL filter but something was going on there as light could only be extinguished with the polarizer closest to the screen).

Here is a reliable demonstration.

Take the CPL filter to an ordinary mirror. Reverse the filter so that its normal front is facing your eye and its normal rear (with external threads) is toward the mirror. Using only one eye, look through the filter at its reflection in the mirror. The filter will appear opaque black -- you will not be able to see your own eye although you will still be able to see other parts of your face around the filter. This is because light going out through the filter becomes circularly polarized, the mirror reverses the direction of polarization, and the filter then blocks that light on its return.

It works the same way in a microscope. The combination of linear polarizer and 1/4 wave retarder causes the light to be circularly polarized, anything that acts like a mirror surface reverses the polarization, and the retarder+polarizer blocks that light on return. The bit about "acts like a mirror surface" includes any lens surface, specular reflections from the specimen, and reflections from most types of metal. It does not include subsurface reflections from organic material.

When you're demonstrating with a CPL, the effect depends on orientation of the filter. If you do not reverse the filter, leaving its normal rear with external threads facing your eye, then it will not go opaque black, only a little dim. This is because failing to reverse the filter sets up an optical chain that is retarder/polarizer/mirror/polarizer/retarder. The mirror is then working with linear polarized light, whose direction it does not affect, so the polarizers are effectively parallel and not crossed. To get the darkening, you need to reverse the filter so that the chain is polarizer/retarder/mirror/retarder/polarizer, so that the mirror can work on the circular polarized light to reverse its direction.

--Rik
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Pau
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PostPosted: Sat Feb 20, 2016 11:10 am    Post subject: Reply with quote

The 1/4 wave plate doesn't eliminate subject reflections but allows to control them like rotating one of the polarizers to intermediate positions between parallel and crossed. It really allows to better contrast than plain bright field.
To really suppress specular reflections on subject what you need is cross polarization. I do it in my stereo without Antiflex with a ring polarizer over the ring fiber optic illuminator or with polarizers on the tips of fiber optics bundles (no actual epi).

Take a look at http://www.photomacrography.net/forum/viewtopic.php?p=154251 and follow the links in it.

From http://zeiss-campus.magnet.fsu.edu/articles/basics/reflected.html
Quote:
In cases where objectives of very low magnification are used in reflected polarized light, a rotatable optical plate (termed an Antiflex cap) consisting of a one-quarter wavelength lambda plate is placed on the objective front lens element to block reflections from the objective itself. The Antiflex method is also particularly useful when the specimen has very low reflectivity, such as would be observed in coal samples.

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JH



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PostPosted: Sat Feb 20, 2016 11:48 am    Post subject: Reply with quote

Interesting discussion!

If it is of any interest I used an ordinary cirkular polarizing camera filter between the ant and the objective in the third picture in this post

http://www.photomacrography.net/forum/viewtopic.php?t=22034

Regards Jörgen
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boomblurt



Joined: 18 Aug 2013
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PostPosted: Sat Feb 20, 2016 8:49 pm    Post subject: Reply with quote

Thanks all for your information and patience. Rik's explanation (same idea as the diagram in Gene's last link, p299) makes sense. I haven't used any of this equipment so am trying to understand purely on the basis of (my very limited) theory, but things seem to be a bit clearer now - hopefully.

Most of the links I had seen before - eg the Zeiss handbook was linked on one of the Yahoo groups but that mentions a crossed polarizer/analyzer. This seems to be a different setup to a single polarizer+retarder and although the principles are the same it confused me. I haven't been able to find any specifics of the SMZ-U coaxial illuminator.

Regards my LED monitor 'tests' I had simply(!) assumed that the photographic CPL was composed of a separate polarizer and wave plate that rotated independently of each other (as in my cellophane combo). This would be a very bad way to design a CPL filter. As a side note, the cellophane worked better than I expected.

Rik's mirror experiment was interesting and seemed to work well with a CPL filter. But when I tried it with the analyzer from my compound microscope (~1/2" diameter) which presumably is also circularly polarized (as it will only cross polarize in one direction) the effect was smaller. Could this be due to the amount of reflections between the mirror and the polarizer, or something else?
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rjlittlefield
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PostPosted: Sat Feb 20, 2016 9:47 pm    Post subject: Reply with quote

boomblurt wrote:
Rik's mirror experiment was interesting and seemed to work well with a CPL filter. But when I tried it with the analyzer from my compound microscope (~1/2" diameter) which presumably is also circularly polarized (as it will only cross polarize in one direction) the effect was smaller. Could this be due to the amount of reflections between the mirror and the polarizer, or something else?

Given the behavior of only cross polarizing in one direction, and darkening some but not completely in the mirror test, I would guess that the analyzer is an elliptical polarizer. See https://www.newport.com/images/webDocuments-EN/images/18954.pdf for discussion.

The technique that Pau describes, rotating the 1/4 wave plate to control the reflections, is based on elliptical polarization.

--Rik
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boomblurt



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PostPosted: Sat Feb 20, 2016 9:52 pm    Post subject: Reply with quote

Aaahh .. thanks Rik!
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