Leica and lens element anti-reflective coating

[wlaidlaw]

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I have been trying to find some information about Leica's history of lens coating. I know they were one of the first companies to offer a coated lens to the general public in late 1945/early 1946, rather than military or government applications, which had previously been the preserve of coated lenses. I believe the method used by Leica, was similar to that developed in the 1930's by Zeiss and used the drip/vacuum evaporative method of applying the magnesium fluorite coating. Whereas this is an effective method of applying an anti-reflective coating, it has long term problems, in that the coating is soft and prone to cleaning deterioration for external elements. The coating is also porous and hygroscopic, which can promote fungus growth in the inside of lenses. 

 

At a later point coating was then done at some lens makers, by Electron Beam Evaporation (EBE) and whereas this improved the consistency of the thickness of the coating, it was still porous and hygroscopic. I believe that most of the development of this technique was lead by the astronomical optics industry, to improve the coating on telescope mirrors. The next development by Fuji, driven by the requirement for better lenses for the 1964 Tokyo Olympics, was multi coating but I can find little description of the methods they used. 

 

Modern coatings, I understand, are made by either Ion Assisted Deposition (IAD) or Magnetron UHF Sputtering (MS).

 

What techniques did Leica use at various times and what was the changeover date from the early porous and soft lens coatings to modern hard and impervious coatings? What technique do Leica use today?

 

As an aside, Alan Starkie of Cameraworks-UK, tells me he has bought a second hand machine from the USA for vacuum coating. He hopes to be able to modify this to a sputter coating machine with a UVF radio frequency driver and will then be able to offer a hard MC re-coating service for early Leica lenses. 

 

Wilson

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[jaapv]

One of the lens coating pioneers was " de Oude Delft, founded by van Leer optical Industries. As it was  Jewish company, the expertise was "exported" to Germany in the early 1940-ies. How it was integrated in the developments by Zeiss and Leitz I do not know.

[pgk]

A quick glance in my reference books suggests that the earlier method was limited in that it could only be used with coating materials of relatively low melting point (magnesium fluorite). Later methods overcame this and enabled more materials of higher melting points to be used and thus multi-coating could be produced using a variety of different materials of differing refractive indexes. No dates given nor more extensive info - but I'll see if I have anything more later. I think that you might be able to find a Journal for the relevant optical scientific sector which may well contain papers on coatings - probably you would need to pay for access if old copies have been digitised though.

Edited February 16, 2018 by pgk

[wlaidlaw]

One of the lens coating pioneers was " de Oude Delft, founded by van Leer optical Industries. As it was  Jewish company, the expertise was "exported" to Germany in the early 1940-ies. How it was integrated in the developments by Zeiss and Leitz I do not know.

 

My father had his Summar coated in the Netherlands in the late 1940's but I do not know by which company. Given that my father had close connections with the jewish community through business (most of his export agents for Scottish textiles around the world, were jewish), it may well have been a successor company to de Oude Delft. 

 

Wilson

[jaapv]

Yes, the fin was resurrected after the war as Oldelft with help of Philips. They did offer a coating service.

[UliWer]

The first industrial method for coating lenses was developed by the Ukranian physicist Alexander Smakula for Carl Zeiss Jena.

Though this method was not applied to „normal“ Zeiss lenses sold on the customer market before the war. Normal Zeiss lenses even with high serial numbers in the 2.000.000 range which were built during wartimes in most cases were uncoated. One may find some with coating - marked with the famous red *T* - though one cannot be sure if this coating was applied during wartimes, or only afterwards - which was very often the case. It seems as if coating was only applied for special purposes. I am not sure which demands during the war led to the coating, perhaps it was most interesting for photography from aircrafts.

 

It is not completely clear whether Leica lenses were coated by Zeiss as well or whether Smakula‘s technology was „given“ by Zeiss to Leitz to use it for the same purposes. The latter is very probable since Leitz was able to use coating almost immediately after the war when the Zeiss production was vanishing due to war destruction and post-war disassembly. Wartime economy forced the two competitors to cooperate - and it looks as if Leitz profited a lot more from it than Zeiss.

 

The Summarex seems to be the only Leitz lens which was generally coated for the whole production. But examples from wartime being very rare you can‘t even be sure about this. With Summitars and other types you also don’t know whether the coating was applied later. Leitz even advised to have older lenses like the Summar coated well into the 50s as their new production was not sufficient to fulfill the demand on the German market.

 

The coating by Oude Delft is interesting, since it raises the question whether they used their own technology or also made use of the Smakula invention. The latter would have been possible as all german patents were declared void after the war.

[Michael Geschlecht]

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Hello Everybody,

 

I think that multi-coating might have actually been a Leitz Invention by a branch of theirs in San Diego, USofA. Something to do with periscope manufacture & the complex optics of periscopes which sometimes had to be used in adverse conditions.

 

Just as the original Magnesium Fluoride coatings in the 1930's were designed for anti-aircraft guns, since at that time attacking airplanes often came "Out of the Sun".

 

I think Leitz originally called their coating "staggerred coating" since instead of giving all surfaces all of the layers of all coating materials they calculated which layer on what surface needed which color or clear spacer or what adhesive & applied what was needed accordingly. Because in some situations multi-coating may not improve image quality & might even be less helpful than less layers, etc.

 

Best Regards,

 

Michael

Edited February 16, 2018 by Michael Geschlecht

[wlaidlaw]

Hello Everybody,

 

I think that multi-coating might have actually been a Leitz Invention by a branch of theirs in San Diego, USofA. Something to do with periscope manufacture & the complex optics of periscopes which sometimes had to be used in adverse conditions.

 

Just as the original Magnesium Fluoride coatings in the 1930's were designed for anti-aircraft guns, since at that time attacking airplanes often came "Out of the Sun".

 

I think Leitz originally called their coating "staggerred coating" since instead of giving all surfaces all of the layers of all coating materials they calculated which layer on what surface needed which color or clear spacer or what adhesive & applied what was needed accordingly. Because in some situations multi-coating may not improve image quality & might even be less helpful than less layers, etc.

 

Best Regards,

 

Michael

 

Michael, 

 

I have a feeling that staggered coating might not be multi-coating on one element but different types of single coating being applied to different elements. As I understand the coating processes, multi-coating can only be done if you are using Ion Assisted Coating or Sputtered Coating and not drip/evaporative coating. From various articles I have read on my researches, Leica were very conservative in adopting more modern coating methods and were using drip/evaporative coating into the 1960's. Now it may be that their San Diego branch (presumably a sub division of the Canadian Branch, which was doing all sorts of military work), were doing things differently to Wetzlar at the time. I did wonder if Leica might have switched in 1959, as the coating on my 5cm/f1.5 LTM Summarit from 1959 is in very good condition but it is more likely it has either had little use or been carefully looked after. Every article I read, gave Fuji the credit for the invention of multi-coating in the early 1960's with Pentax following soon after with their SMC lenses. 

 

Wilson

[Michael Geschlecht]

Michael, 

 

I have a feeling that staggered coating might not be multi-coating on one element but different types of single coating being applied to different elements. As I understand the coating processes, multi-coating can only be done if you are using Ion Assisted Coating or Sputtered Coating and not drip/evaporative coating. From various articles I have read on my researches, Leica were very conservative in adopting more modern coating methods and were using drip/evaporative coating into the 1960's. Now it may be that their San Diego branch (presumably a sub division of the Canadian Branch, which was doing all sorts of military work), were doing things differently to Wetzlar at the time. I did wonder if Leica might have switched in 1959, as the coating on my 5cm/f1.5 LTM Summarit from 1959 is in very good condition but it is more likely it has either had little use or been carefully looked after. Every article I read, gave Fuji the credit for the invention of multi-coating in the early 1960's with Pentax following soon after with their SMC lenses. 

 

Wilson

 

Hello Wilson,

 

I read a piece by Leitz years ago where they said that their coating  process was the same type of multiple layer coating process that includes different colors (materials), clear spacing layers, adhesive layers, etc. that "multi-coating" does. Except that: Which color was in which layer & how many + which spacers, etc: Was determined by them: By factors such as: The refractive index of a specific element & its individual rate of curvature.

 

Leitz said, at the time, that: The lower the refractive index & the steeper the rate of curvature, coupled with a greater number of elements: The more need that there was for more & different layers. They also said: The flatter the curves & the higher the refractive index of the glasses, in lenses with fewer elements: Meant that these lenses: Did not always benefit additionally from additional layers: Which, they also said: They found at times to be counterproductive: That is: With higher refractive index glasses & flatter curves & less lens surfaces: Lenses sometimes provided better transmission, etc with less, but individually chosen, coatings.

 

Best Regards,

 

Michael

Edited February 17, 2018 by Michael Geschlecht

[cameraworks]

When we think of AR coatings, we generally think of materials that have been evaporated or 'sputtered' under high vacuum conditions and this is currently industry standard. Another technique that has been gathering pace for a number of years is 'Solgel' technology, where coatings can be applied by either dip or spin coating, without the need for vacuum conditions. It is possible to attain high quality thin film coatings but the field doesn't lend itself to mass production and is still considered somewhat experimental. I have also been interested to discover exactly which process Leitz used on their early lenses. Clues come from studying the actual coatings. They are relatively open structured and adsorb moisture, making them ideal fungus breeding grounds under the right circumstances. I've also observed that on some slightly later lenses, the outer coatings are somewhat more robust than the coatings on the internal elements. If vacuum deposition was used, I suspect it was reserved only for outer elements. After some research I discovered that a technology was developed early in the 20th century that I suppose is the precursor to modern Solgel techniques. I reverse engineered an early chemistry recipe that results in a colloidal suspension of silica nanoparticles that can be applied by spin or dip coating. Cryolite can also be used. What I found surprised me very much indeed. I tested the coating on a prepared microscope slide with an original visible light transmission of about 96.8%. After coating on one side, the transmission increased to 98.2% which is pretty impressive. The coating aggressively adheres to glass surfaces and can only be removed afterwards by polishing with cerium oxide. The coating exhibits the familiar blue colour of older coated lenses. The only problem I'm having is getting a smooth clean coating without defects on lenses. It's a cosmetic issue but we had to do a lot of experimentation before we could routinely apply accurate beamsplitter thin films to M2/3 prisms by sputter coating. This will no doubt be the same process, at which point we may perhaps offer this as a service but I don't have much free time to devote to this.

Edited July 25, 2019 by cameraworks
grammar

[wlaidlaw]

Alan,

Leica certainly described their early coating method as “Drip Coating” and there is a perception that it is less robust and maybe not quite as effective as the contemporaneous Zeiss T* or Canon coatings. I don’t know about  its efficacy but from my own experience, the Zeiss and Canon coatings seem to have lasted better. I believe Leica used a fluorite salt. My father had his Summar coated in around 1948 in Holland, most likely by Oude Delft. That had a brownish appearance. By 1965, the coating was virtually all gone on the front element, I suspect mainly due to my father’s habit of cleaning his lens with his tie or pocket silk handkerchief. I had the lens re-coated when his IIIa was having a CLA at Wallace Heaton in 1965. The new coating was purple in colour. Jim Lager of the LHSA would probably be the best person to give you chapter and verse on coatings. He was very helpful to me when I was researching the origins of the 39mm x 26tpi lens mount. 

Wilson

[George Furst]

On 2/18/2018 at 1:46 AM, Michael Geschlecht said:

 

Hello Wilson,

 

I read a piece by Leitz years ago where they said that their coating  process was the same type of multiple layer coating process that includes different colors (materials), clear spacing layers, adhesive layers, etc. that "multi-coating" does. Except that: Which color was in which layer & how many + which spacers, etc: Was determined by them: By factors such as: The refractive index of a specific element & its individual rate of curvature.

 

Leitz said, at the time, that: The lower the refractive index & the steeper the rate of curvature, coupled with a greater number of elements: The more need that there was for more & different layers. They also said: The flatter the curves & the higher the refractive index of the glasses, in lenses with fewer elements: Meant that these lenses: Did not always benefit additionally from additional layers: Which, they also said: They found at times to be counterproductive: That is: With higher refractive index glasses & flatter curves & less lens surfaces: Lenses sometimes provided better transmission, etc with less, but individually chosen, coatings.

 

Best Regards,

 

Michael

No wonder the Summarex f1.5, 8.5 cm lens was perhaps coated from early production. It has some extremely curved inner lens surfaces on the 7 elements that make up this lens so that there are a total of 14 surfaces to reflect light. Also the fact that it was a fast lens designed for low light conditions means that the transmittal of light was critical. I would not be surprised if this lens was coated from its first production in 1941. Would be nice to have someone who has an early example provide information on whether it is coated. Of course it could have been coated later as many of the earlier lenses of this nature were.