Dielectric and Phase Coatings - do they interrelate? (1 Viewer)

John Russell · Feb 24, 2013

There was a recent discussion here:- http://www.birdforum.net/showthread.php?t=251175 on phase and mirror coatings, which prompted me to go back and investigate some of my bins.

One can check for phase coating by viewing a white TFT screen through the objective (i.e. backwards) and rotating a polarizing filter in front of the objective. In the absence of phase coating the two halves of the exit (now entry) pupil will alternately black out completely either side of the roof edge.

Phase-coated roof prisms will still usually allow one to detect the roof edge but the diiferences between the two sides are less pronounced. However, on two bins with dielectric coatings, a Swarovski SLC and a Kowa Genesis, I noted significant spurious colour differences either side of the roof edge. On a Meopta Meostar, which allegedly has silver-coated prisms, the roof edge was still faintly visible but when rotating the polarizing filter there was a continuous and uniform change from yellow to violet.
With my Swarovski ATM 65HD, which with its 45° Schmidt prism does not require any mirrored surfaces, the effect was similar except that due to the 30x magnification the yellows and violets were so intense that I was unable to see the roof edge.

It would appear that the multiple layers (often over thirty) of dielectric coatings cause arbitrary polarizations of different colours, which interact with the phase coatings on the roof surfaces. I have also often wondered about the lateral shift of different colours at a dielectric mirror surface although this is probably so small as to be invisible.

Dielectric coatings are today a "must have" for any bins with Schmidt-Pechan prisms but perhaps they are not without their disadvantages.

John

ceasar · Feb 24, 2013

Holger Merlitz noted in thread #32 in the above link herein that manufacturers could purchase prisms independently at all kinds of quality levels and that sometimes because of prism dimensions the binocular they were making would require "in house" prisms. So it would follow that there could be many differences among them.

http://www.birdforum.net/showpost.php?p=2668836&postcount=32

I've wondered if this could be the case with the Swift 8.5 x 44 828 Audubon? It would be much improved if fitted with brighter new dielectric prisms. (Let's disregard for a moment that the reason it doesn't have them is because Swift doesn't want to spend the money on them.) Instead, is it possible that the 44mm objectives will not work well with the prisms used in 8.5 x 42 roof prisms and dielectric ones can't be found for a 44mm version and Swift does not have "in house" facilities to make them. 8.5 x 44 roof prism binoculars are an odd configuration. AFAIK there are only 2 of them.

Bob

Gijs van Ginkel · Feb 24, 2013

John,
In beween snowball fights with youngsters:
I am afraid that you are not using the correct method to check for phase correction coatings. You have to proceed as follows: "Two polarizers, the axes of which (transmitting and non-transmitting direction) are clearly marked, should be set up at a sufficient distance from each other for a binocular to be placed between them. Either filtered green lamp light or daylight can be used for testing. In crossed positions, the polarizers do not transmit any test light; in parallel positions they transmit the major part of the test light. If a binocular is placed between the two polarizers with its roof edge parallel to one of the polarizing axes, the following can be distinguished:
-1- binocular without P-coating: the pupil of the binocular appears brighter between crossed polarizers than between parallel polarizers.
-2- binocular with P-coating: the pupil of the binocular appears darker between crossed polarizers than between parallel polarizers.
It should be emphasized again that it is important that the roof edge is parallel to one polarization axis, otherwise it is not possible to obtain meaningful results." (Quotation from Weyrauch and Dörband, the "inventors" of P-coating.
The observations you describe are colorful, but I fear that they do not help us a bit.
Gijs

John Russell · Feb 24, 2013

Gijs,

You are jumping to conclusions again without bothering to read or inwardly digest what has previously been written, and from your descriptions it is obvious that you have not tried "your" method because then you would have observed something completely different.

TFT screens emit polarized light and with a polarizing filter it is possible to block their images completely. There is hence no need for a second filter.

Conducting the test as I described it (or with two filters if you like) a non-phase-coated roof prism will, at a certain filter orientation completely block off one half of the circle so that you see a white semicircle and a black semicircle.

As far as the interaction of dielectric mirror coatings with phase coatings is concerned, that was speculation on my part and I am hoping the experts will chime in on this.

At school I was very weak in Latin and the only quote I remember is: "Those whom the gods wish to destroy they first make mad."
In this sense I think you can be regarded as an instrument of the gods ;-).

John

looksharp65 · Feb 24, 2013

Let's see.

I hold the binocular with the ocular lens facing the TFT. The polarizing filter is between the objectice lens and my eye. Then I turn the PL filter.

The Meopta shows the two prism halves, and the illumination difference is sometimes slightly more, and sometimes absent. The colour I observe varies from true-to-life, over green/yellow to blue/magenta and back to normal. Same hue seen through both prism halves.

My non-P Dialyt reaches a point where one prism half displays a distinctly emerald-green colour while the other is natural. A similar effect can also be observed with a magenta and a brown/yellow hue.

Edit: There is no complete blackout with the Dialyt, the darkest colour I observe is a deep magenta. It just struck me that the Dialyt 10x42 has Abbe-König prisms, hasn't it?

mooreorless · Feb 24, 2013

http://www.birdforum.net/attachment.php?attachmentid=363149&d=1325633817
this is my Swarovski 8x30 SLCneu

http://www.birdforum.net/attachment.php?attachmentid=363148&d=1325633786
and this is the Nikon 8x30 Trailblazer and this is from this thread:

http://www.birdforum.net/showthread.php?t=219014

This is using what John describes with his first post. Sorry I couldn't post like Lars did, you have to click on mine. If you click the bottom link you can see it the way Lars reply is posted.

Troubador · Feb 24, 2013

looksharp65 said:
Let's see.

It just struck me that the Dialyt 10x42 has Abbe-König prisms, hasn't it?

Hi

Actually I am sure the 10x40 Dialyt had SPs. Only the 7x42 and 8x56, the 'long ones' had AKs.

Lee

Gijs van Ginkel · Feb 24, 2013

Hi John,
Happy to be an instrument of the gods, but we still do not have a clear view how you check your measurements according to the required conditions, allthough I also know that a TFT screen emits polarized light. Do you now put your polarizing filter with its polarizing direction parallel to the roof edge or do you do that with the TFT screen using it as a polarizing filter with known polarizing direction? I ask it, since the following posts (not from you) already show that things are not clear.
I will give it another try with the risk that you promote me immediately to a level higher, so no instrument of the gods but directly its supervisor.
All roof prisms whether Abbe-Kónig, Uppendahl or Schmidt-Pechan prisms suffer form the phase problem. Abbe-König roof prisms do not need a perfect surface mirror to prevent light from escaping from one of the prism surfaces, since light incident on the A-K prism surfaces hits at the perfect reflection angle, while this is not the case with Uppendahl and Schmidt-Pechan prisms. A perfect or almost perfect surface mirror on that "defective" prism plane can be applied with layers of silver, aluminium, gold or dielectric material. The polarizers for control of pase correction coatings do not do anyting with this reflected light, so you will not be able to see a sign of this surface while checking phase corrrection coatings.
Gijs
N.B. The snow ball fight is still open for participation

looksharp65 · Feb 24, 2013

Things get even stranger now.

I tried the same procedure with the Nikon HG and the Vortex Fury. Both display similar characteristics as the Dialyt: Different colour hues through the two prism halves. But:

With the HG I observe a so deep magenta that it nearly blacks out the image, while the other half is normal.
I can also see a distinctive colour shift within the prism half: Magenta >> yellow.

The Fury shows the same hue within the prism half but another (none) in the other half.

Steve C · Feb 24, 2013

I was always under the impression all you needed for a check of phase correction was a set of polarized sun glasses. Just open a blank page in the word processor, hold the oculars toward the screen, put the sun glasses in front of the objective, and if you have a phase corrected binocular, you get views like looksharp's pictures. It's a good way to see whether or not the sunglasses are polarized too.

Seems there was a recent thread on this too, but a quick search came up blank.

looksharp65 · Feb 24, 2013

Steve C said:
I was always under the impression all you needed for a check of phase correction was a set of polarized sun glasses. Just open a blank page in the word processor, hold the oculars toward the screen, put the sun glasses in front of the objective, and if you have a phase corrected binocular, you get views like looksharp's pictures. It's a good way to see whether or not the sunglasses are polarized too.

Seems there was a recent thread on this too, but a quick search came up blank.

Obviously, there's something strange going on. The Meostar has the least difference between the two halves, and when there's colour, it's the same in both halves.
The non-P* Dialyt (unless it has been secretly upgraded to P* without having the plate exchanged) is most like the Fury, while the Nikon HG has the most illumination difference between the halves, and even colour difference within the same prism halve.

Frankly, I don't have a clue how to interpret the results :brains:

//L

John Russell · Feb 25, 2013

looksharp65 said:
Obviously, there's something strange going on. The Meostar has the least difference between the two halves, and when there's colour, it's the same in both halves.
The non-P* Dialyt (unless it has been secretly upgraded to P* without having the plate exchanged) is most like the Fury, while the Nikon HG has the most illumination difference between the halves, and even colour difference within the same prism halve.

Frankly, I don't have a clue how to interpret the results :brains:

//L

Lars,

My observations with the 7x42 Meostar concur with yours with the 8x32 Meostar.
The only non-phase-coated instrument available to me is a Vortex doubler, which shows similar blackouts to those Steve (mooreorless) photographed so well on the Eagle Optics extender and the Nikon Trailblazer.

I suspect that your Dialyt is phase-coated. Zeiss introduced the P* coating around 1989 and, AFAIK did not initially identify the original models as such. An 8x20 purchased then was not phase coated and exhibited poor contrast when compared to a 6x18 monocular of 2002 vintage. Zeiss generously updated the 8x20 free of charge some years ago but the phase coatings on both are inferior to current offerings.

John

ceasar · Feb 25, 2013

Troubador said:
Hi

Actually I am sure the 10x40 Dialyt had SPs. Only the 7x42 and 8x56, the 'long ones' had AKs.

Lee

I just got through looking through my 7 x 42 Victory FL LT backwards. Even with their A/K prisms I can see a very thin diagonal line bisecting the circular view. 2 O'Clock to 8 O'Clock and 10 O'Clock to 4 O'Clock. I assume it is a roof edge. I could not see any colors of course.

Bob

brocknroller · Feb 25, 2013

ceasar said:
Holger Merlitz noted in thread #32 in the above link herein that manufacturers could purchase prisms independently at all kinds of quality levels and that sometimes because of prism dimensions the binocular they were making would require "in house" prisms. So it would follow that there could be many differences among them.

http://www.birdforum.net/showpost.php?p=2668836&postcount=32

I've wondered if this could be the case with the Swift 8.5 x 44 828 Audubon? It would be much improved if fitted with brighter new dielectric prisms. (Let's disregard for a moment that the reason it doesn't have them is because Swift doesn't want to spend the money on them.) Instead, is it possible that the 44mm objectives will not work well with the prisms used in 8.5 x 42 roof prisms and dielectric ones can't be found for a 44mm version and Swift does not have "in house" facilities to make them. 8.5 x 44 roof prism binoculars are an odd configuration. AFAIK there are only 2 of them.

Bob

According to Arek, the 828 only has 75% light transmission, which is quite low for a modern roof. Presumably, the 828s have silver coatings on the prism, given Swift's other roofs all have them. Yet, oddly enough, Swift's Website does not state the type of coatings on the 828 prisms. Could it have aluminum coatings???? Their flagship name roof?

http://www.swift-sportoptics.com/binoculars_premier.html

<B>

Holger Merlitz · Feb 25, 2013

Interesting pictures!

Theory tells us that along with each total reflection, polarization effects show up - this means they happen inside every prism, including Porros. The same happens with dielectric mirrors and on metal surfaces, though the precise directions and amplitudes of those polarization vectors differ.

Now how about reflection at the roof? The two half-beams hitting the roof are (total-)reflected and each of them is partially polarized, and when they rejoin later at the image plane, those partial polarization vectors point into different directions, there are interference effects between those different components and the resolution is reduced. The P-coating is supposed to prevent this effect: It is a coating applied to the roof faces and it essentially avoids the partial polarization. The way how they do it is complicated and it is achieved through a multi-layer treatment, but for us it is sufficient to know that these roof faces essentially reflect without any additional polarization.

As cited by Gijs in post #3:

-1- binocular without P-coating: the pupil of the binocular appears brighter between crossed polarizers than between parallel polarizers.
-2- binocular with P-coating: the pupil of the binocular appears darker between crossed polarizers than between parallel polarizers.

It is now understandable: With P-coating, the roof contributes no extra polarization. So with crossed polarization filters, the pupil looks dark. Without P-coating, there are considerable polarization effects that tilt the plane of vibration of the light so that the pupil looks bright even with crossed polarizers.

But: The roof-edge has to be oriented parallel to the polarization direction of one of the two crossed polarizers, otherwise you obtain partial polarization that differs with each of the two roof faces, and in this way you get these funny two-colored pictures. Remind that some amount of polarization always remains due to total reflection at other surfaces of the prism, even though these effects won't reduce the resolution of the image. I guess that is why the pupil is never completely dark, whatever the position of the polarizers.

Cheers,
Holger

looksharp65 · Feb 25, 2013

Finally, the 10x32 FL image. Although it can't be seen in this picture, the FL had the greatest variation of the colours seen while doing the experiment.
A very psychedeliric experience! This is my only binocular with DE coating.

In all images, I tried to turn the filter to show the greatest contrast between the dark and the light prism sides.

//L

John Russell · Feb 25, 2013

Holger Merlitz said:
Interesting pictures!

Theory tells us that along with each total reflection, polarization effects show up - this means they happen inside every prism, including Porros. The same happens with dielectric mirrors and on metal surfaces, though the precise directions and amplitudes of those polarization vectors differ.

Now how about reflection at the roof? The two half-beams hitting the roof are (total-)reflected and each of them is partially polarized, and when they rejoin later at the image plane, those partial polarization vectors point into different directions, there are interference effects between those different components and the resolution is reduced. The P-coating is supposed to prevent this effect: It is a coating applied to the roof faces and it essentially avoids the partial polarization. The way how they do it is complicated and it is achieved through a multi-layer treatment, but for us it is sufficient to know that these roof faces essentially reflect without any additional polarization.

As cited by Gijs in post #3:

-1- binocular without P-coating: the pupil of the binocular appears brighter between crossed polarizers than between parallel polarizers.
-2- binocular with P-coating: the pupil of the binocular appears darker between crossed polarizers than between parallel polarizers.

It is now understandable: With P-coating, the roof contributes no extra polarization. So with crossed polarization filters, the pupil looks dark. Without P-coating, there are considerable polarization effects that tilt the plane of vibration of the light so that the pupil looks bright even with crossed polarizers.

But: The roof-edge has to be oriented parallel to the polarization direction of one of the two crossed polarizers, otherwise you obtain partial polarization that differs with each of the two roof faces, and in this way you get these funny two-colored pictures. Remind that some amount of polarization always remains due to total reflection at other surfaces of the prism, even though these effects won't reduce the resolution of the image. I guess that is why the pupil is never completely dark, whatever the position of the polarizers.

Cheers,
Holger

Holger,

Without questioning the validity of the method quoted by Gijs and repeated above, it is effectively a journey from London to Berlin via New York!

Few of us on BF have access to an optical bench.

We can only determine the orientation of the roof edge using polarizing filters.

We do not know the precise polarizing axes of TFT screens, polarizing filters or sunglasses.

Non P coated roof prisms will exhibit the effect shown in the left picture of post #11 here: http://www.birdforum.net/showthread.php?t=219014. It is possible to obtain this effect with the objective side filter regardless of the orientation of the roof edge to the polarized light source.

If P coatings are effective in reducing polarization at the roof surfaces then both sides of the roof edge should approach symmetry when viewed through the objective side filter regardless of the orientation of the polarized light source to the roof edge.

I repeated the tests today in daylight and used two polarizing filters front and rear to view a white wall with the bins mounted on a tripod, and was thus better able to view on the optical axis and keep the filters perpendicular to the oprtical axis. Good symmetry across the roof edge was achieved by both the Meopta Meostar and a Swarovski SLC with differing orientations of the polarized light source to the roof edge.

Regards,
John

Holger Merlitz · Feb 26, 2013

John Russell said:
Holger,

Without questioning the validity of the method quoted by Gijs and repeated above, it is effectively a journey from London to Berlin via New York!

Few of us on BF have access to an optical bench.

We can only determine the orientation of the roof edge using polarizing filters.

We do not know the precise polarizing axes of TFT screens, polarizing filters or sunglasses.

Non P coated roof prisms will exhibit the effect shown in the left picture of post #11 here: http://www.birdforum.net/showthread.php?t=219014. It is possible to obtain this effect with the objective side filter regardless of the orientation of the roof edge to the polarized light source.

If P coatings are effective in reducing polarization at the roof surfaces then both sides of the roof edge should approach symmetry when viewed through the objective side filter regardless of the orientation of the polarized light source to the roof edge.

I repeated the tests today in daylight and used two polarizing filters front and rear to view a white wall with the bins mounted on a tripod, and was thus better able to view on the optical axis and keep the filters perpendicular to the oprtical axis. Good symmetry across the roof edge was achieved by both the Meopta Meostar and a Swarovski SLC with differing orientations of the polarized light source to the roof edge.

Regards,
John

Hi John,

Using a single filter, it is easy to figure out the precise polarizing axes of the TFT screen, and the roof edge is visible when shining a flashlight into the tube, so it should be no problem to find the proper orientation of the roof edge. Anyway, I didn't try it out, so let me buy a polarizing filter next time I enter a camera store, then I shall do the practical work ;-)

Cheers,
Holger

ronh · Feb 26, 2013

If I might speculate on the origin of Gij's requirement of a known direction of the input polarization, it could be that there is the possibility that an otherwise effective phase coating could still alter the polarization direction of the light.

The only case I can think of where this would matter is actually pretty common: wearing polarized sunglasses to view birds on brightly lit water. The sunglasses wouldn't work so well unless the incoming polarization direction was preserved.
Ron

ceasar · Feb 26, 2013

brocknroller said:
According to Arek, the 828 only has 75% light transmission, which is quite low for a modern roof. Presumably, the 828s have silver coatings on the prism, given Swift's other roofs all have them. Yet, oddly enough, Swift's Website does not state the type of coatings on the 828 prisms. Could it have aluminum coatings???? Their flagship name roof?

http://www.swift-sportoptics.com/binoculars_premier.html

<B>

The Swift website is dated 2011. Eagle Optics lists the 828 as out of stock although that happens with them often. They only list 9 Swift binoculars and the 7 x 36 is not on it.

http://www.eagleoptics.com/binoculars/swift

When I registered mine I called their Lakewood, CO. HQ and spoke to the manager. He told me that they sold as fast as they got them in and they were very rarely returned for problems.

They are made in Japan and they are very similar in construction to my Leupold 7 x 42 BX2 Cascade which is also Japanese and very likely made by the same company. They both have the same integrated diopter and focus wheel, rainguard and very similar eye cups. I'm going to have to compare them as best I can considering their exit pupils. The Leupold has since been upgraded to FMC and renamed Hawthorne.

If Leupold could get an upgrade I don't know why Swift could not unless it is because of it's odd configuration. 8 x 42s are a dime a dozen and there must be lots of different grades of optics available for them as Holger pointed out above. 8.5 x 44 might not have that many options and might require a separate run of upgraded optics which would be reflected in higher prices.

Bob

Dielectric and Phase Coatings - do they interrelate? (1 Viewer)

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