The AR Coating Process - An Interview with Norm Kester
Back in 2014, before I worked for Laramy-K, I visited a mid-sized, wholesale optical lab in Virginia that did in-house AR coating. I was privileged to get a behind-the-scenes look at the process. I must admit, while I thought I knew quite a bit about AR application, it turns out I was wrong.
The cleanroom where the lenses are prepped was bright, shiny, clinical and high-tech. It was full of digital readout panels, fancy dials and switches, bright colors, all with a whiff of science about it. One of the most startling revelations was this sort of, Wizard of Oz moment, when we opened the door (pulled back the curtain) that led to the area behind the cleanroom. There, in a room the size of a small garage, it looked like a cross between a power station and an oil refinery, complete with huge pipes, large motors, bulky valves, hefty fittings and thick wiring.
“OK,” I thought, “there must be more to this AR application process than I imagined.”
I believe that many ECPs think of AR as being pretty much sprayed onto the lens, not unlike grabbing a rattle-can of Rust-Oleum. I think they see it as just another routine lab function. The reality, of course, is something completely different.
Recently, I decided to learn more. So, instead of talking with people who sell AR, I decided to find the people who actually make AR and the associated processing equipment—the engineers and science people, like Norm Kester, owner of Quantum Innovations. Quantum is the company that provides the equipment and materials Laramy-K Optical uses to produce ICE (colorless AR), TKO, and UVARity premium coatings.
In my talk with Norm, he does a wonderful job explaining some of the extraordinary, complex and fascinating parts of the AR application process. A better understanding of the intricacies involved may give you a greater appreciation and better understanding. You might also find greater confidence in selling and recommending AR coatings to your customers.
John: For my own curiosity, what is the right term or terms, to be using when we talk about AR. Is this process happening at the molecular, nano, microscopic or some other level?
Norm: I guess the right answer would be “all of the above.” Most of what we are doing is at the atomic or molecular level. We are evaporating inorganic materials (ZrO2 zirconium dioxide, SiO2 silicone dioxide, Chrome or chromium, ITO indium tin oxide, etc.) by heating them to their melting or sublimation point to put them into their vapor or gas phase. We are then growing the materials back onto the lens. During the process, we are counting the molecules as they are being deposited onto the lens. The result of this action is measured in nanometers. You can’t see any of this with the naked eye or even standard microscopes. To see a layer of an AR coating, you must use an electron microscope or similar.
Sidebar: A nanometer is one billionth of a meter. To put it another way, that’s one millionth of a millimeter.
John: I saw that the lenses are mounted in razor edged holders and then placed in a vacuum chamber. Is that the right term?
Norm: Yes, it is a vacuum chamber. The lenses are mounted in lens rings. The edges of these rings are made very thin and hold the lens closely so that the vaporized material is not obstructed. This has to be done very carefully or the outcome will be compromised.
John: On my visit to the lab that day, they mentioned extremes of temperatures used in the process. If I understood it correctly, part of what I was seeing in the back room was the equipment that could produce the extremes necessary. Can you tell me what the range is and why you need such extremes?
Norm: This is not an often talked about area of the AR process. But, to heat many of the coating materials to their melting or subliming point, it takes a great deal of energy. The heat required is sometimes thousands of degrees (that is 1400+ in Celsius and 3000+ in Fahrenheit). Of course, cooling becomes extremely important too. Thankfully, all of this is done in a vacuum so heat doesn't transfer easily. The bulk of that equipment you saw in the back room is actually there to pull and maintain the necessary vacuum.
John: Are there other extremes involved in the process? I think I recall some very intense lighting being used inside the chamber?
Norm: There are many crazy things going on in the process. We are bombarding the lens with positive ions with an ion gun and we heat the lens with light/lamps. The entire process is done in a vacuum or negative pressure region and we are using electron beam guns to heat up the coating materials. There are many extremes to pulling off this art of thin film coating.
John: Wait, wait, did you just say ion gun? This is starting to sound more like Star Trek than the Wizard of Oz.
Norm: Yes. The ion gun helps prepare the lens surface for the best possible application results.
John: Speaking of surface preparation, if I have it right the lens is “etched” or roughed up at the surface level to increase coating adhesion. Am I right that it is done in an acid bath?
Norm: Actually it is a high PH, alkali bath that does the etching prior to AR coating. Keep in mind that we are putting molecules of material onto the lens to “grow” the material from the gas phase back into a solid on the surface. So, any material or aberration on the lens surface can destroy the coating. The lens has to be pristine and be prepared to receive the material in the proper way to promote adhesion. We also have the ION source in most AR systems to further promote adhesion by atomic level bombardment of the surface of the lens.
John: Does every material get the same bath?
Norm: We wish that it was that easy. Unfortunately, there are many characteristics to the variety of lens types that are used. Some of them are not really suitable for high PH situations, high heat situations and in some cases for AR coating. But, because of light loss due to the refractive index of the lenses, we must put AR coatings on them. To do this, we must carefully navigate the many pitfalls associated with all of the myriad different lens material types. This is somewhat of an art form and we have pages and pages of data and processing techniques to ensure success. Every time a new lens or lens type comes out, we must learn that one as well and create processing method for it.
John: I was amazed that the coatings are made up of, am I using the right word, “elements?”
Norm: Inorganic compounds such as silicon dioxide and zirconium dioxide are typically used. We generally refer to them as coating materials. There are literally hundreds that could be used as well as some organic compounds. Keep in mind that AR coating is a tiny niche within a much broader industry of thin film, vapor applied sciences. Many, many things get thin film coatings: the ink on our money, telescopes, windows, windshields, watches, phones, TV’s, semiconductors, microprocessors, hard drives and everything in between.
John: Can you tell me anything about the different elements that make up the different layers of the stack?
Norm: I think that most people do not recognize the compounds (as listed above). These compounds are in a lot of things you are surrounded with on a daily basis. But, as I was before coming into this industry, most are unaware of their existence.
John: So the elements are combined like a recipe for for baking. Mix the right elements together and you get a certain type of AR or a certain type of layer?
Norm: Yes. “Mixed” isn’t probably the right term but we do call them recipes. “Layered” is probably a better term. Keep in mind that we are trying to bend light by creating a refractive index that is correct to create a reflected wave of light that is out of phase with the arriving wave of light. To do this, we have to create the correct layering of materials. Each of these materials has a refractive index to bend light subtly until the desired effect is achieved.
John: Then the elements are vaporized to become a gas?
Norm: Yes. The materials are heated to their evaporation point and the vapor then is “airborne.” I put that in quotes because there is no air in a vacuum. But, you get the point. Because we have removed most of the molecules from the inside of the vacuum chamber, the molecules are free to move unimpeded. This allows them to go from the evaporation source to the lens. The vapor is then grown back into the original material until we achieve the proper thickness of that material. The process is repeated until all of the layers have been deposited onto the lens.
John: That gas then settles on the lens surfaces and becomes the coating. What makes it “stick?”
Norm: This is a complicated question. We are not bonding the material to the lens, but there is adhesive strength between the arrive particles/molecules and the lens surface considering the process has been done properly. Depending on many variables, the adhesive strength is at best variable. This is closely controlled by lens heat, kinetic energy of the arriving particle, ION etching, ION assisting, etc.
John: Does the order in which the elements are vaporized determine the layer or AR stack?
Norm: Yes. This is written in the recipe. The recipe is dictated by the refractive index of the lens and the outcome that is trying to be achieved.
John: We always see images of the stack as being individual layers. Is that the right way to think about it?
Norm: It is individual layers that are working in concert to create the desired effect. But, if we could, we would do it with one layer. That technology just doesn’t exist at the current time.
John: I guess this is the same question. We read about “substrate matched AR coatings.” Does that mean each layer in relation to the one above and below it, or just the lens material?
Norm: To do this optimally, the refractive index of the AR coating and the AR coating design would be such that it is created for a specific refractive index of lens. This would create the best overall outcome and give the best light transmission through the lens to the eye. Unfortunately, most AR coating machines are “batch” coaters. This means many lens types (refractive indexes) are AR coated at the same time. So, a “general use” AR coating is used that gives an average effect across many refractive indexes. This sounds easy but is actually quite complicated. Anyone who designs these coatings or is applying AR coating understands this precarious dance.
John: Can you explain how you manipulate a coating to work best with a particular lens material?
Norm: Sure, but there is a lot of science that goes into this. Everything on earth has an optical density. That optical density bends photons in very specific ways. If we think of the wave nature of light, it becomes easier for us to consider how to manipulate light. As the optical density of the material changes, the wave is bent more or less. The number we use to designate this “angle of bending” of the wave is called refractive index. We know precisely how each material will bend light relative to its refractive index. This numerical value and its resulting geometrical angle is used in our calculations to bend light specifically for that refractive index. I wrote a more detailed video blog post on this that may be helpful- AR coating are we looking at this wrong?
John: In our initial email you mentioned, “Most opticians and optometrists don't realize that AR is for increased transmission to the eye. They also don't realize that as refractive index goes up, reflection goes up. Therefore, AR coating becomes critical due to light loss to the eye.”
I know you do a lot of video-blogging and even some classroom presentations on AR to all kinds of groups. Now, I’m a little worried. I’m really curious what ECPs think AR is for, if it isn’t for increased light transmission. I mean, I have always taught—not sold—AR from the simple premise, “More light equals more sight.”
If you are hearing ECPs reducing AR to, “a sprayed on coating that reduces the ugly glare off the front of the lens,” who is to blame here and how do we get the right message out to ECPs?
Norm: I do a lot of ABO and a variety of other training around the country as you have said. My opinion is that we have a lot of very caring practitioners that are hungry for knowledge and training. I speak to 30 - year opticians that haven’t been given the proper training and knowledge. They get what I refer to as “tribal knowledge.” The optician or optometrist gave the person that taught them some information. The telephone game ensues and it eventually becomes a sprayed on coating that reduces the ugly glare off the front of the lens. A cosmetic effect.
In every case where we do AR training, AR sales go up dramatically. The knowledge of the how’s, why’s and the importance of AR coating lenses that are reducing light transmission is given. Then these people, who show up every day wanting to do a great job for their patients, do the rest. Knowledge is the key. I think robust training programs in all aspects of the optical business are missing.
John: I know it isn’t part of the process, but it is a result of the process. You cover an interesting fact during one of your video-blogs, Why Is AR Colored?. I can proudly say I knew the answer to that question! For those of you who don’t know, it is because we want to see a color as proof of the coating.
AR Myth: The color of the coating tells you about the quality of it.
AR Myth: An AR coating has to have a color.
AR Fact: The perfect AR would actually be clear.
John: Norm, thank you. This has been great and I learned a lot.
Dorothy: How do you talk if you don't have a brain?
Scarecrow: Well, some people without brains do an awful lot of talking don't they?