why are zirconium oxide coatings so rare? (1 Viewer)

In looking at a variety of lens and coating literature with some Russian and Japanese engineering friends we kept coming across how the best coatings for lenses were zirconium oxide. Glass contracts as it cools and so the surface "splits" and has lots of very tiny "notches" between any smooth areas on the surface. Even the best and finest grit of polishing compound is just dust that scratches off the biggest surface imperfections but leaves behind tiny little scratches. Eventually, even if the surface looks mirror-smooth to our eyes there is a light surface scratching that is the primary offender in causing diffusion and slight grating-diffraction. Even glass (SiO, as opposed to borosilicate or other materials that don't contract when they cool) that seems perfectly smooth never is when measured surface imperfections are measured in angstroms.

This is why light can easily be rejected from uncoated glass surfaces even if it "hits" the glass exactly perpendicularly. That's because all the scratches allow light to strike then at some non-direct angle, the light then bounces around, and some of it reflects back. But coatings "fill in" and smooth the surface really well and the coating to glass surface isn't as "visible" to the light waves, and so when they are direct, the light is more likely to pass right through the coating(s) and the glass it is on.

The only problem to most coatings is that they modify the color of light that is transmitted through the lens or prism. The color of the coatings we see is due to the color of light being reflected or rejected back from the lens or prism. The best multi-coatings seem to use a variety of color coatings to try to better balance the light that passes through.

But zirconium oxide is water white. I've seen some lenses that have been coated with ZiO and the color fidelity as well as light throughput is remarkable. The trouble is that I don't see many products made with zirconium oxide coatings. Maybe once every 5 years some company comes out with a product that uses the zirconium oxide coatings (like Fujinon's EBC coatings on their FMT series bins) and zirconium oxide is harder than most other coating materials so the surfaces stay smoother longer.

Can zirconium oxide really be that much more expensive than magnesium fluoride, zinc sulphide, cryolite or dichroic surface coatings? I've seen lenses coated with tungsten and even gold to reflect IR and that must be incredibly expensive, so why is zirconium oxide used so rarely?

ksbird/foxranch said:

.... so why is zirconium oxide used so rarely?

Click to expand...

How many companies do tell the outside world what their coatings are made of?

I never see any information on this topic.
My impression is that the formulations of the modern multi-layer coatings are well-kept secrets of the trade.

Scientifically it would be interesting but it is understandable that manufacturers do not want to give away details of their technical expertise.

Tom

The tiny defects left on a properly polished surface have no noticeable effect on the images formed by the lens, nor does the constant contraction or expansion of glass produce defects that will eventually degrade the image. There are some glasses that are damaged by moisture, but these are unlikely to be used in modern optics.

Clear skies, Alan

I guess I was not complete in my statement about "glass cooling and contraction", so please excuse my lack of clarity. What I meant to say that when melted or liquid glass was poured for lens blanks, it will contract during cooling and the center potion of the glass and surface area will cool at different rates, so the surface ends up less even and coatings help with the transmission/reflection and dispersion due to surface defects caused when melted lass cools. I've seen uncoated lenses made by 2 different lens makers from the same blanks and depending on the polishing skill and grit size used for the final polishing, there is greater sharpness and reflection-dispersion due to surface irregularities for one vs the other. This is why some manufacturers cut their lens blanks from rods of glass for higher quality end-output and others cast blanks close to the size eventually needed and are willing to accept that all the cooling surface irregularities will not be able to be polished out (so hoefully some of the coatings will "fill in" some of these imperfections).

Actually in this respect the amount of hand polishing labor involved in producing a truly spectacular lenses or prisms now makes super high end porro prism binoculars incredibly expensive anywhere but China. The new Zeiss Classic BGAT* 7x50 Marine which is my 7x50 standard, retails at about US$1500 now. There is allot of hand labor in this product. I expect to see porro prism binoculars of similar quality coming out of China soon and priced under US$600 pr. The Russian Yukon 7x50 WA is nearly as sharp image-wise as the Zeiss 7x50 (except for contrast which is due to the T* Zeiss coatings), but the cost of Russion hand labor to cut and polish lens blanks is so much lower than the German costs that the binoculars prices are ludicrously disparate.

The Baigish 10x40 porros I have are as good or maybe better than the last production Swarovski 10x40 Habicht porros I compare them to when a friend who has them visits. The same is true for the KOMZ 10x40s another friend often brings. They are as good or better than the Swarovskis. The extra hand labor time (and costs) removing surface "lines" and melting-cooling surface splits can visibly improve sharpness. Business associates working for both Arri and Angenieux tell me the same thing about removing surface irregularities to the nth degree for their lenses.

I grind mirrors for telescopes. Getting a 1/20 or more-perfect mirror is very time consuming to start with and then taking all the grinding marks out with finer and finer grit is even more time consuming. Surface lines from any grit produce diffraction from the "grating effect". Gratings are just like prisms in some respects and gratings are nothing more than super-flat pieces of glass that are "scratched" with so many thousands of lines of "scratches" per inch (or cm), (sometimes with dielectric surface coatings). They are designed to break up light beams like prisms and they are useful in spectrometry for this purpose. But random surface scratches on glass, no matter how fine, will also cause dispersion, reflections and diffraction effects that will reduce the quality of an image of one lens or prism vs another even when they are made from exactly the same glass blanks. Coatings are often emplyed to reduce the problems caused by surface imperfection.

Zirconium oxide seems to be one of the best possible coatings in this respect as it can leave behind a much smoother and flatter surface than the glass it is coating and it is water white, which makes for less narrow band color rejection, and color imbalances. Zirconium oxide coatings are as expensive as the cubit zirconium used in simulated diamonds. But the final product when it is used by super high end manufacturers like Fujinon is quite spectacular.

I guess I'm still confused. When the ingredients for a particular glass type are melted and mixed they need to be both uniformly mixed, and slowly and uniformly cooled (annealed). If it is not mixed properly and cooled properly, the indices of refraction will not be uniform. There will always be a slight variation (nothing is perfect), but the tolerances for glass to be used in high power astronomical optics are more severe than for glass used in birding binoculars.

Glass that is not uniformly cooled can also have internal stress, and the completed surfaces will change shape non-uniformly with changing temperatures. Many telescope makers who have done a number of optics eventually run into a piece of glass that "won't hold a figure," and such a poorly annealed blank with internal stress has to be replaced.

Pressed blanks are made by pushing melted glass into a mold. The pressing itself can lead to a variation of indices across the blank. If the variation is high enough and the glass makes it into a completed telescope, high power views of stars will show what looks like pinched optics or astigmatism. Some of the early Meade "apochromats" had this problem. Pressed blanks would, I suspect, be just fine for binoculars.

The folks making high quality optics, like Astro-Physics, don't used pressed blanks, but cut blanks out of larger pieces of glass. This is more expensive, but the resulting blank will almost always be sufficiently uniform to make a fine lens.

If the same finishing methods are used on a pressed blank and a cut blank the surface should be the same. The difference is what happens when the lens is used to form an image.

I'll do another post about the difference between a grating and a polished optical surface later.

Clear skies, Alan

A diffraction grating is made up of very fine, closely spaced groves or scratches. A polished glass surface has some residual random lumpiness or roughness. If it is large scale roughness, it can be detrimental to the image, at least at high powers. Small scale roughness or microripple is not an issue.

See Chapter 13, Roughness, in Harold Richard Suiter's Star Testing Astronomical Telescopes.

Clear skies, Alan

I thought I'd ask for explanations from former workmates at Mamiya and Nikon before I went further with this but while their answers concerning "scratches" put on the glass surfaces by grinding/polishing and the way the skin of a glass surface always has some form of cracking/fissuring due to any type of production methodology heating, there is a general middle ground to their explanations.

There are really only 2 types of glass blanks used in production lenses (and I stress the word production, and not a theoretical lens where a single unit is ground and polished over a period of a year always using optimum conditions and methods, because no production binocular in the world is EVER made that way). One type of blank is cast and since glass is somewhat rope-y this type of glass is a bit more distorted internally, after grinding and polishing. The second type of lens blank is made for use in more expensive lens systems and these blanks are cut from long billets of glass (like round rods). These are less internally distorted but require more grinding to achieve the correct figure. It is even hard to generalize about any lens system because the rear lenses where "comes out of the system" will have a much greater impact on the distortion, diffraction and dispersion of the light entering the "front" lens in the system.

The primary causes of distortions are a lens with a bad figure or internal distortions caused by the rope-y nature of glass itself. But coatings generally do not address distortions. While dispersion can be caused by surface scratches created during grinding and polishing a lens surface, a large amount of dispersion can also be caused by impurities in the glass itself (like dust equivalents, or chemical reactions (like oxidation) that create powder-like substances in the glass, incompletely dissolved doping agents, bubbles, etc., some dispersion can be created by the tiny fissuring that occurs on the surface of glass when it is either cooling from its melted state or when heat is trapped inside glass and stresses the surface during grinding. Even though dispersion is a problem, there is probably not much that can be done about internal dispersion-created problems, but coatings can "fill in" some of the surface fissures on a glass surface that may still be there after grinding/polishing. Unfortunately not all surface fissures can be eliminated in production lens grinding/polishing.

Grinding/polishing can cause glass to be stressed and so any surface weaknesses that are already present in the glass cause the surfaces to make more and new fissures to go with the fissures that occurred when glass was originally "poured". Lenses get hot during grinding no matter what is done to keep a lens cool. But if that was the only problem, then lenses would be allot better than they are today even with multiple coatings. The grinding/polishing process itself is nothing more than using abrasives harder than glass, to scratch away the part of the glass you don't want, to create the lens figure you Do want.

Green light is about 500 nanometers in size/length. Grit that makes a surface scratch of approx. 500 nanometers is in the range of 12,000-9,000 grit size (the bigger the grit size number the smaller the grit). There seems to be no consumer optics product companies that use grits this small in production today to create a surface that even then, would be only 80-90% perfectly reflective (since not all of the reflected light would retain its coherence at that point). But since I haven't talked talked to any of the production managers in German or Austrian homeland factories, making the highest of their high end binoculars, maybe there are German factories using 20,000 grit polishing compounds that only make 250 nanometer width scratches in the surface of glass while they do a final polish. This still means that the glass surface would have non-perpendicular surfaces presented to a light beam over 50% of their surface.

Since there are micro-grit sizes that are even smaller than 40 nanometers diameter (made in very small batches), why aren't these types of grit being used on production of high end binoculars and not just on laser windows and telescopes like Hubble etc. The reason is simple; you can't get any production done with them. Glass may not be the world's hardest substance, but it isn't the softest either. It would make a binocular unbelievably expensive if the grinding/polishing process took 10 years from start to finish to produce each pair of bins. Polishing abrasives are still cutting glass off a lens, even though they are cutting the glass off in extremely fine proportions and very, Very, ... VERY slowly. All polishing compounds for production lenses leave micro-scratches in the glass surface, although if you took a year per surface and kept the lenses optimally cooled during grinding/polishing and kept ALL micro-contaminants out of the grinding/polishing clean rooms, you might get the "scratch" sizes down to about 40-100 nanometers in width but no one could afford that lens set.

So the solution preferred by production facilities making binoculars is to fill in the scratches and fissures with multiple layers of coating materials. Even a single coating may leave some surface irregularities somewhat like one coat of paint on wood. The wood will be somewhat smoother and have less of a matte surface (due to sanding scratches), but a second coat of paint will be a bit smoother and so on until the pigment particle size and the binder material determine final glossiness (finer and finer sanding between paint layers can help this along). Thus it is that lens makers find it easier to fill in grinding/polishing and production fissuring surface scratches and cracks using coatings.

Of all the coatings in the current market, some being hard, others filling in cracks better, others used to balance the colors being rejected by the coatings etc., Zirconium Oxide is optically the best. The problem I found was that it wasn't the hardest type of coating and other coatings didn't always deposit to it the best. And since none of us consumers want our coatings to develop fine cleaning scratches on the lens surface, and we certainly don't want our coatings to flake off unevenly, zirconium oxide is one of the "bete noir" coatings. It is very expensive and doesn't always "play nice with others". But it is one of the rarest coating components because it is "water-white", which means it rejects no visible band colors or to put that another way, it rejects all visible band colors equally. About the only coating that is better is electro-deposited diamond. So while the hardness factor favors diamond coatings, there is really no optical difference between zirconium oxide and diamond coatings.

What I tried to explain previously was why coatings are a more common and actually cheaper solution to surface diffraction problems in lenses, than trying to polish a lens to perfection, because there is absolutely no way to achieve polishing perfection. All polishing compounds are really just scratching the surface of glass lenses. Making any real production precludes going past using 12,000-6,000 size grit for anything but the most experimental or cost-is-no-object-government lens-making. That is why coatings are the real way to cover-up surface scratches now-a-days. Many German lenses made before the 1940s were polished to a greater degree than lenses today. That was the pre-Coatings era and so the surface of a glass lens showed more of the polishing "scratches' made by the polishing grit. To get a truly great lens you needed months of slow polishing with the finest, relatively soft grits.

One more factor needs to be considered here. The surface "scratches" caused by polishing compounds become less of a factor as the lenses get larger. So a lens 1 meter in diameter will exhibit less apparent diffraction and distortion (caused by surface scratches) than a .5 meter lens will appear to exhibit and so on and so on. But of course binocular lenses are tiny by comparison with the Yerkes Telescope 1 meter lens and so the same polishing scratches on a 35mm or 50mm diameter lens may seem truly objectionable, on small binoculars compared to being nearly invisible when using a 1000mm lens. Add to that the fact that binoculars have multiple prisms and it is amazing we see anything through those tiny optical devices at all. But I have always wondered why the coating material that seems to produce the best optical results, is used so rarely. I guess cost and difficulty in use, are the two reasons why.