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What was so hard about phase coatings?

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Old Thursday 9th November 2017, 16:54   #1
tenex
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What was so hard about phase coatings?

Discussion of the new nostalgic Trinovid made me aware of something I've never really understood. Zeiss and Leica both laid a huge bet on roof prisms in the early 1960s, and were surely aware of the image degradation they suffered from despite their high cost. Yet it took 25 years to develop phase correction coatings that would deliver image quality comparable to much cheaper porro binos... why? What was really so hard about it? That's a rather technical question, but does anyone here know?

(Somehow the gamble worked: enough people bought them for many years anyway... I gather that means image quality really isn't paramount for many people, even those who can afford Leica/Zeiss, despite all the nitpicking of alphas that goes on in some circles.)
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Old Thursday 9th November 2017, 19:58   #2
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It is I think multilayer and an exacting coating, which was probably difficult and expensive to apply.

Once factories were geared up for it, it probably became cheaper and commercially viable.
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Old Thursday 9th November 2017, 20:27   #3
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Originally Posted by tenex View Post
Discussion of the new nostalgic Trinovid made me aware of something I've never really understood. Zeiss and Leica both laid a huge bet on roof prisms in the early 1960s, and were surely aware of the image degradation they suffered from despite their high cost. Yet it took 25 years to develop phase correction coatings that would deliver image quality comparable to much cheaper porro binos... why? What was really so hard about it? That's a rather technical question, but does anyone here know?

(Somehow the gamble worked: enough people bought them for many years anyway... I gather that means image quality really isn't paramount for many people, even those who can afford Leica/Zeiss, despite all the nitpicking of alphas that goes on in some circles.)
You also have to remember that there are many, many people who can't see well enough to tell the difference.
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Old Thursday 9th November 2017, 20:29   #4
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It may be that there was enough improvement in usability with the roofs to justify their selection.
My understanding is that roofs are relatively easy to make waterproof and much less likely to lose collimation than their porro counterparts.
However, I don't really know why that is so or even whether it is actually true.
Perhaps WJC or some other expert could shed light on the issua.
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Old Thursday 9th November 2017, 20:38   #5
Gijs van Ginkel
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In 1943 a type of phase coating was suggested in the Zeiss workshop and it was described in a paper by Prof. Joos. It took until 1988 before the first binoculars with phase coatings were produced. However, many roof prism binoculars were already produced since 1897.
One must not forget that the controlled application of ultrathin layers under low pressure was necessary to apply the phase coatings and it had to be done under very well controlled conditions and, not to forget, it needed the proper choice of coating materials and that also was a completely new game in science and all that together needed some research and that takes time and it costs money.
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Old Saturday 11th November 2017, 02:21   #6
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I just find it hard to imagine how it took 25 years to figure it out, when it must have been a high priority for research, and polarization of light had been well understood for a long time. It would be interesting to understand the technical challenge better.

I do understand the reasons (convenience etc) why people bought roof prisms anyway. I was curious to try a more compact bino myself and around 1988 (horrible timing!) I finally decided to get a Zeiss 8x30, must have been one of the very last made without phase coatings. I used it for some time; never loved the view, but it worked. Eventually I replaced it with a Leica 10x32 BN which I still have and enjoy today. A huge improvement.
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Old Saturday 11th November 2017, 08:01   #7
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It may be that there was enough improvement in usability with the roofs to justify their selection.
My understanding is that roofs are relatively easy to make waterproof and much less likely to lose collimation than their porro counterparts.
However, I don't really know why that is so or even whether it is actually true.
Perhaps WJC or some other expert could shed light on the issua.
Covered fairly well on pages 33-38 in Binoculars: Fallacy & Fact. The Zeiss paper by Weyraush and Dorband (1988) cover it more precisely, with excellent graphics.

Buy the book, anyway. I need the burger money. ........... Boy, I hate it when the tongue goes all the way through the cheek like that.

Bill
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Old Friday 24th November 2017, 15:45   #8
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I'd be interested to know if there have been improvements in phase-coatings over the years, as there have been with anti-reflection coatings? We know that Zeiss' T* anti-reflection coatings are constantly being worked on - I wonder if similar efforts are being made with the P phase coatings to achieve objectives such as (a wild ass guess on my part) superior contrast, etc. P* is supposedly the same as P, but that was well over a decade ago...
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Old Friday 24th November 2017, 16:02   #9
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Book is in-bound, hope I can learn from it - might be a challenge for someone like me.
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Old Friday 24th November 2017, 16:04   #10
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P* is supposedly the same as P, but that was well over a decade ago...
Hey Patudo.
P is more than supposedly the same as P*, it is exactly the same. The asterisk was added to the P solely to give it the same format as T*: a letter, followed by an asterisk.

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Old Friday 24th November 2017, 16:30   #11
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Thanks, Troubadour; now if I may be so bold, were the phase coatings used in FLs and subsequent Victorys improved from those used in the Dialyts?

Best regards
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Old Friday 24th November 2017, 19:09   #12
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Thanks, Troubadour; now if I may be so bold, were the phase coatings used in FLs and subsequent Victorys improved from those used in the Dialyts?

Best regards
patudo
This is a good question and I don't know the answer. However there is one clue. Zeiss has a person whose job it is to improve the T* coatings so it is logical to guess that someone is responsible for keeping P* coatings competitive too.

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Old Friday 24th November 2017, 21:03   #13
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Patudo, post 11,
Probably not, the coatings used worked well, so improvements seem not necessary in my opinion.
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Old Friday 24th November 2017, 21:45   #14
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Buy the book, anyway. I need the burger money. ........... Boy, I hate it when the tongue goes all the way through the cheek like that.

Bill
If you need burger money, ok, but the book is only for dummies

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Old Friday 24th November 2017, 22:03   #15
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If you need burger money, ok, but the book is only for dummies
You called?
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Old Saturday 25th November 2017, 21:17   #16
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You called?
Thanks for the laugh!!
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Old Saturday 25th November 2017, 21:29   #17
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In 1943 a type of phase coating was suggested in the Zeiss workshop and it was described in a paper by Prof. Joos.
Hi Gjis,

do you happen to have a reference for that? I only knew Paul A. Mauer: Phase compensation of total internal reflection from 1966 (https://www.osapublishing.org/josa/a...josa-56-9-1219) and of course the 1988 article in Deutsche Optikerzeitung: A. Weyrauch, B. Dörband: P-Belag: Verbesserte Abbildung bei Ferngläsern durch phasenkorrigierte Dachprismen. In: Deutsche Optikerzeitung. Nr. 4, 1988.

Joachim
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Old Sunday 26th November 2017, 05:07   #18
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If you need burger money, ok, but the book is only for dummies
Yes, I know. That's why there are so many great reviews on the back from optical engineering professors, authors, and industry professionals and so many 5-star ratings from readers on Amazon.com. So, please don't waste your money. cat:

Bill

PS The "burger money" is a long-standing joke.
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Old Sunday 26th November 2017, 08:04   #19
Gijs van Ginkel
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Joachim, post 17,
The article by prof. Joos is published in Zeiss Nachrichten from 1943, I will search for the paper in my archive and let you know when I found it.
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Old Sunday 26th November 2017, 08:30   #20
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Hi Gjis,

do you happen to have a reference for that? I only knew Paul A. Mauer: Phase compensation of total internal reflection from 1966 (https://www.osapublishing.org/josa/a...josa-56-9-1219) and of course the 1988 article in Deutsche Optikerzeitung: A. Weyrauch, B. Dörband: P-Belag: Verbesserte Abbildung bei Ferngläsern durch phasenkorrigierte Dachprismen. In: Deutsche Optikerzeitung. Nr. 4, 1988.

Joachim
Hi Joachim,

The original work seems to be

G. Joos: Die Bildverschlechterung durch Dachprismen und ihre Behebung. In: Zeiss Nachrichten. 4, 1943, S. 9

but I have no copy of that. What I do have is a copy of Mahan's work, just two years later. Therein he mentions Joos' paper which he came to know only after having finished his own studies:

A. I. MAHAN: Focal Plane Anomalies in Roof Prisms. In: Journal of the Optical Society of America. Vol. 35, No. 10, 1. October 1945, p. 623–635

Both, Joos and Mahan were apparently on the same track and worked it out independently, but due to the WWII troubles none of them knew about the other one's efforts. They recognized the existence of that interference, but - as far as I remember - they hadn't come up with a solution yet. It had been noted that the performance of the prism improved once the roof-faces were coated with metal layers. I can only guess that the partial polarization, which otherwise occurs during total internal reflection, did not (or: to lower extent) show up in presence of that thin (and probably rather disordered) metallic layer. Yet: Who wanted to coat such a roof-prism with another two metal layers, thus inducing additional loss of light?

Cheers,
Holger
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Old Sunday 26th November 2017, 10:42   #21
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Joachim, post 17 and Holger, post 20,
The reference given by Holger of the article by Prof. Joos is correct: Zeiss Nachrichten 4, heft 9, 1943, pages 221-227.
On page 221 Joos writes about the correction for the phase shift by roof prisms: "Die von Herrn Werkmeister Wendland gemachte Beobachtung, dass durch Versilberung der Flächen eine wesentliche Besserung zu erziehlen ist, gab den Anlass, der Ursache dieser Bildverschlecterung nachzugehen,....
After discussion about the cause of the phase shift on pages 221-226, Joos writes on page 227:
"Nachdem so die Ursache der merkwürdigen Bildverschlechterung gefunden ist, muss sich auch ein Weg zur Beseitigung angeben lassen. Dieser ist durch den Ersatz der Totalrefexion durch metallische Reflexion vorgezeichnet (DRP. angemeldet). Aus den Werten der optischen Konstanten der Metalle liesse sich, allerdings mit erheblichen Rechenaufwand, das für jedes Prisma güstigste Metall ermitteln. Einfacher ist aber bei der geringen Anzahl der in Betracht kommenden Metall der Weg des Probierens. Auf diese Art wurde gefunden, dass bei den vorkommenden Prismen eine Belegung der Dachflächen mit Aluminium die Störungen so gut wie restlos beseitigt. Allerdings muss wegen der zweimalige Reflexion ein Lichtverlust von etwa 25% in Kauf genommen werden".
So the cause of the phenomenon is clear and so is a solution, but it would take some time before the instrumentation and most suited materials and measuring techiques were available to deposit ultrathin layers with hardly light losses could be applied obviously.
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Old Sunday 26th November 2017, 11:02   #22
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Thanks to Gijs and Holger for casting a light on this historical development.

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Old Sunday 26th November 2017, 11:54   #23
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Joachim, post 17 and Holger, post 20,
The reference given by Holger of the article by Prof. Joos is correct: Zeiss Nachrichten 4, heft 9, 1943, pages 221-227.
..... Auf diese Art wurde gefunden, dass bei den vorkommenden Prismen eine Belegung der Dachflächen mit Aluminium die Störungen so gut wie restlos beseitigt. Allerdings muss wegen der zweimalige Reflexion ein Lichtverlust von etwa 25% in Kauf genommen werden".
So the cause of the phenomenon is clear and so is a solution...
Gijs van Ginkel
Excellent research!
One wonders whether a more careful metal deposition possible with todays techniques could avoid that 25% light loss. It might be considerably simpler to implement.
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Old Sunday 26th November 2017, 15:50   #24
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Thanks to Gijs and Holger for casting a light on this historical development.
Thanks indeed to Gijs and Holger - so the effect was known and understood but the means to properly mitigate it were not yet known.

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Old Sunday 26th November 2017, 15:56   #25
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The reference given by Holger of the article by Prof. Joos is correct: Zeiss Nachrichten 4, heft 9, 1943, pages 221-227.
On page 221 Joos writes about the correction for the phase shift by roof prisms: "Die von Herrn Werkmeister Wendland gemachte Beobachtung, dass durch Versilberung der Flächen eine wesentliche Besserung zu erziehlen ist, gab den Anlass, der Ursache dieser Bildverschlecterung nachzugehen,....
After discussion about the cause of the phase shift on pages 221-226, Joos writes on page 227:
"Nachdem so die Ursache der merkwürdigen Bildverschlechterung gefunden ist, muss sich auch ein Weg zur Beseitigung angeben lassen. Dieser ist durch den Ersatz der Totalrefexion durch metallische Reflexion vorgezeichnet (DRP. angemeldet). Aus den Werten der optischen Konstanten der Metalle liesse sich, allerdings mit erheblichen Rechenaufwand, das für jedes Prisma güstigste Metall ermitteln. Einfacher ist aber bei der geringen Anzahl der in Betracht kommenden Metall der Weg des Probierens. Auf diese Art wurde gefunden, dass bei den vorkommenden Prismen eine Belegung der Dachflächen mit Aluminium die Störungen so gut wie restlos beseitigt. Allerdings muss wegen der zweimalige Reflexion ein Lichtverlust von etwa 25% in Kauf genommen werden".
So the cause of the phenomenon is clear and so is a solution, but it would take some time before the instrumentation and most suited materials and measuring techiques were available to deposit ultrathin layers with hardly light losses could be applied obviously.
Very interesting stuff! Maybe Zeiss actually used the technique described here, albeit in a scope, not in binoculars, in the Zeiss Dialyt 40x60 (http://www.monocular.info/cz_dial40x60.htm).

The Zeiss 40x60 had an AK prism and came onto the market in 1977, well before phase coatings as we know them arrived in 1988. The optical quality was very good indeed, on a par with scopes using porro prisms like the Zeiss Jena Asiola (http://www.monocular.info/cz_asiola.htm) or the Kowa TS 1. It was also clearly better than its Hensoldt predecessor (http://www.monocular.info/dialyt40x60.htm) or its smaller brother, the Hensoldt 25x56 (http://www.monocular.info/dialyt25x56.htm), so much so that the difference couldn't be explained by probably somewhat more modern coatings. In fact, I used the small Hensoldt for travelling for quite a few years and asked Zeiss if they could "upgrade" it which they declined to do, so I sold it.

A friend of mine had a Zeiss 40x60 for quite a few years, and I remember we both wondered how Zeiss acchieved such optical quality with an AK prism before phase coatings arrived. Maybe that Gijs' posting explains what we saw.

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