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Thread: Super deep-dive anti-reflective coating questions

  1. #1
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    Super deep-dive anti-reflective coating questions

    Should be fun.

    1. Simple ARCs work because of the 1/4 wavelength rule. But that is a single wavelength at a specific angle. How are multiple wavelengths dealt with? More importantly, how are coatings that deal with blue light created (where a narrow band of wavelengths are more sharply interfered with).

    2. How do oleophobic and hydrophobic coatings work? Is it another layer of some kind of material?

    3. What the heck is destructive interference really? How does one light ray know that another is moving opposite of it? Does it not work if they are a certain distance from each other?

    4. Do the oleophobic and hydrophobic coatings break down over time?

    5. How does ARC really allow higher light transmittance? I get that there's less reflection, and that's aesthetically better. But how does adding a layers on top of something allow MORE light to move through?

    6. Is ARC on the back and front of lenses the same?

    7. How did ARC begin? Did it start off as an opthalmic application?

    8. Is something special done to lenses to insure that ARC adheres when the laser hits the puck? Does the inside of the chamber take on AR qualities as will?

    9. Has AR always been applied the way it is today?


    Phew! that's intense!

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    Master OptiBoarder OptiBoard Silver Supporter Barry Santini's Avatar
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    7. First discovered by accident when hi index flint glass’ surface “oxidized”

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    Master OptiBoarder AngeHamm's Avatar
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    And it was used for scientific and camera lens optics for decades before it was applied to eyewear.
    I'm Andrew Hamm and I approve this message.

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    OptiBoard Professional Mauro.Airoldi's Avatar
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    There are a few too many questions that would require a physics course at the university. A simple suggestion to deepen and to "play" with ARs is to see this site https://www.filmetrics.com/reflectance-calculator
    Use air as a medium, as layer 1 SiO2 (120 nm), as layer 2 TiO2 (5
    nm) and try to play (it is one of the first AR on CR39)
    Regarding question 5, remember that we are talking about the finer thicknesses of the wavelength that we are interested in modifying in reflection. it is as if constructed an "optical trap" that has the same effect as lowering the difference of the refractive index between air and lens.
    6 Usually yes, only some products have deliberately different reflexes
    7 the first Ar were performed hot MgF2 deponent on glass (in vacuum metallizers around the 1930s)
    9 there have been technical implementations mainly related to the introduction of plastics instead of glass.
    8 NO, if not by fury of AR the camera would become transparent however I do not understand what you mean by laser (do not use this technology in the AR.)
    2-4 oleophobic and hydrophobic materials They have the characteristic of having a low wettability, usually PFA or PTFA (as last layer) are evaporated, these products not only improve oleophobic and hydrophobic characteristics but also provide excellent protection against chemical corrosion.

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    Manuf. Lens Surface Treatments
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    Redhot Jumper invented by Alexander Smakula working for Zeiss in 1935..............................

    7.

    Lenses with glass elements artificially "single-coated" by vacuum deposition of a very thin layer (approximately 130-140 nanometers
    ) of magnesium or calcium fluoride to suppress surface reflections were invented by Alexander Smakula working for Zeiss in 1935 and first sold in 1939. Antireflection coating could cut reflection by two-thirds.

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    Manuf. Lens Surface Treatments
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    Redhot Jumper 4. Do the oleophobic and hydrophobic coatings break down over time?


    4. Do the oleophobic and hydrophobic coatings break down over time?
    No they do not. However they are so thin, that they will eventually be rubbed off by the friction of cleaning the lenses.

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    Master OptiBoarder AngeHamm's Avatar
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    Quote Originally Posted by Chris Ryser View Post
    No they do not. However they are so thin, that they will eventually be rubbed off by the friction of cleaning the lenses.
    .....so yes, they do.
    I'm Andrew Hamm and I approve this message.

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    sub specie aeternitatis Pete Hanlin's Avatar
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    (Disclaimer- Essilor Employee)

    I'm primarily a progressive tech guy, but I hang around with people who know a lot about ARC, and some of it has sunk in- so I'll take a shot at some of your questions.
    1. Simple ARCs work because of the 1/4 wavelength rule. But that is a single wavelength at a specific angle. How are multiple wavelengths dealt with? More importantly, how are coatings that deal with blue light created (where a narrow band of wavelengths are more sharply interfered with).
    As you note, a single layer of AR will only allow you to destructively interfere with a single wavelength. That's why pretty much all ARCs are multi-layer. By alternating between low and high index metallic oxides of various thicknesses, you can interfere with a broad band of colors. In fact, the combination of various layers form sub layers that make the design of an ARC stack an interesting exercise in mathematics (one that is far beyond me). As already mentioned, Silicon Dioxide (SiO2) is used in pretty much every ARC, with oxides of elements like Titanium being used for their higher index. By adjusting the thickness of the various layers, you can determine the hue and % (strength) of the reflection. To make a blue filtering stack, you pick the wavelength of blue you want to reflect away and design the stack such that it interferes with the other colors. The resulting residual reflection will be the color you are filtering (and it will likely be stronger and more noticeable than a normal ARC, because you want to reflect that color away from the eye). This is why these coatings are often purplish in appearance.
    2. How do oleophobic and hydrophobic coatings work? Is it another layer of some kind of material?
    Yes, and for the most part everyone uses the same basic concept. The molecule can be described as having two parts. One part might be SiO2 (which loves to adhere to things). If the outer layer of an ARC is SiO2 (as many early coatings were), the lens will be exceptionally hard to clean. However, if you can combine the SiO2 with another molecule (often fluoride, which hates to stick to things), then this dual-molecule will orient itself such that the SiO2 adheres to the lens, leaving the fluoride molecules facing out. Think of the molecules as seaweed on the ocean bed- the fluoride is like the leaves keeping anything from adhering to the ocean floor. As Chris mentioned, these molecules will break off (usually due to abrasion from cleaning), so the trick is to pack as much of the stuff on the surface of the lens as you can. The more you pack on, the higher the "contact angle" of the lens (contact angle is measured by placing a bead of water on the surface and seeing how steep the angle is between the droplet and the lens surface- steeper is better, because it means the surface is not attracting the water. Wax on a car works the same way- increasing the ability of the surface to shed water (that's why water beads on a freshly waxed car).
    3. What the heck is destructive interference really? How does one light ray know that another is moving opposite of it? Does it not work if they are a certain distance from each other?
    As you know, light moves in a wavelike motion- a lot like sound. Bose noise-canceling headphones are a nice representation of destructive interference. The headphones actually have a mic to listen to ambient noises, and then the headphone plays an identical sound inside the headphone that is out of phase with the original sound. When you have two waves in exact opposition, they cancel each other out (imagine a pond with a ripple going across it- if you could figure out a way to create a ripple that was exactly out of phase with the first one, when the ripples intersected they would destroy each other). Another way to think about it is swinging on a hammock. If you move your leg in such a way that it counteracts the swing, the swing will be decreased. Interestingly, this is why many coatings that decrease visible reflections actually intensify UV reflections (because the UV waves are shorter, the extra reflections created by the AR stack are in phase with the UV reflections- which strengthens them).
    4. Do the oleophobic and hydrophobic coatings break down over time?
    Discussed above.
    5. How does ARC really allow higher light transmittance? I get that there's less reflection, and that's aesthetically better. But how does adding a layers on top of something allow MORE light to move through?
    That's an interesting question that is hard to explain (i.e., I've never heard an explanation that I could grasp :) ). My guess would be it has something to do with the other property of light (i.e., it is comprised of photons). When you interfere with the reflection of the waves, my guess would be more photons go through the lens rather than bouncing away- again, sheer speculation. I'll ask the PhD I work with (who used to work in our R&D department) to explain it again to me tomorrow, and will report back if I can make sense of it this time.
    6. Is ARC on the back and front of lenses the same?
    It depends. If you are using AR to filter blue, you definitely do NOT want the front and back stacks to be the same (otherwise, you'll be reflecting blue light back into the eye from the back of the lens). This is why on some blue filtering AR lenses if you look close you will notice TWO residual reflections- one purple (from the front) and one green (from the back). Likewise, while it doesn't matter if the front surface of the lens reflects UV, you don't want the back surface to reflect UV (for the same reason- you don't want to reflect UV back into the eye). Also, since the front surface of a lens is usually subject to more abuse, there's often a thicker layer of a scratch resistance substance (e.g., quartz) built into the stack.
    7. How did ARC begin? Did it start off as an opthalmic application?
    As mentioned earlier, Alexander Smakula is credited with inventing AR in 1935. However, Katherine Blodgett was awarded a patent for AR in 1938.
    8. Is something special done to lenses to insure that ARC adheres when the laser hits the puck? Does the inside of the chamber take on AR qualities as will?
    It depends- some manufacturers have a process step within the chamber that pummels the lenses electronically to create a surface that is more adherent (sort of like lightly sanding a smooth surface before applying paint.
    9. Has AR always been applied the way it is today?
    Actually, AR is applied in at least a couple different ways. The method with which I'm most familiar uses evaporative deposition. Basically, the metallic oxides in the stack are heated into a plasma that rises in the chamber and deposits itself on the lens. It's very similar to holding a lens over a pot of boiling water. The water is heated into steam and rises until it condenses on the lens- creating a very thin film of water on the lens surface. In this process, it is important to have a near perfect vacuum in the chamber- both to avoid melting the lenses (because turning the metallic oxides into plasma requires temperatures above 2,000 degrees) and to avoid having the plasma condensing when it encounters molecules of oxygen, nitrogen, and the other elements that comprise "air."
    Another method is called "sputtering," and I'll admit to having very little knowledge of this process (although I believe it is something akin to spraying the oxides on the lens).


    There are definitely people who are more knowledgeable on the topic who may rightly correct some of the above- but I hope this provides at least some answers to your questions.

    -Pete
    Pete Hanlin, ABOM
    Vice President Professional Services
    Essilor of America

    http://linkedin.com/in/pete-hanlin-72a3a74

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