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?
[QUOTE=ksbird/foxranch;1069954].... so why is zirconium oxide used so rarely?[/QUOTE]
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