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wrb75
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AR coatings: An explanation

AR Coatings on Watch Crystals
An Explanation and FAQ’s

AR coatings are designed to reduce light reflection (increase transmission) by optical interference. The principal is the same as in noise canceling headphones. The AR coatings bend the incoming light wavelengths and eventually one wavelength cancels out another. Generally, reflection is reduced to < .5% per side and transmission is therefore increased to over + 99% depending on the substrate and its thickness. Uncoated, clear glass (2mm) reflects 8% and transmits ~91% (the difference is absorption).
The AR layers are nanometer (billionth of a meter) thick layers of a metal oxide material. The materials my include titanium or zirconium oxides individually or in combination. The outside layer is silicon dioxide. The multiple layers, from three up to 20 or more, are on both sides of the substrate. Typically, there are at least six total layers; three on each side of the crystal. The coatings are applied most often in a vacuum deposition system. Targets of the metal oxide are vaporized in a vacuum chamber with high energy and in the vacuum are attracted to the crystal of an opposite electrical charge and bond to the crystal surface in a very thin and uniform thickness. The layers each have a different index of refraction and hence bend the light differently creating the optical interference, canceling of reflected light.
What is the major benefit of AR coated crystals? In my opinion, the major benefit of an AR coating is increased light transmission. With more ambient light striking the dial, the more color and texture you are able to see. The side benefit is reduced reflection that, if not reduced, is annoying.
Why is there a bluish color of reflection with AR coated crystals? It is a color chosen by the optical engineers to be less garish than, let’s say, red. The thickness of the layers determines the color of reflection. Optical engineers can design the optical system to reflect any color, but blue is often chosen as being less intense and its complimentary color of transmission being a light orange /yellow which is less noticeable and intrusive. The coatings on watch crystals transmit a light yellow (kind of like incandescent light) because its complementary blue color is being reflected / taken out of the visible color. You could have a violet / red color of reflection and a slight greenish color of transmission, but that would be a not be a great system for a watch crystal.

Why do I sometimes see a different color transmitted onto the watch dial? At an angle the color and intensity of the reflected light shifts slightly to violet at an angle and the color of transmission changes with it. In addition, the color of the ambient light affects what you see. Daylight has a much different Kelvin temp than incandescent for fluorescent light and that too affects what is reflected from and transmitted to the dial.
Why do I see more reflected light at an angle? AR coatings are angle dependent. The maximum reduction in reflected light is at 90 degree viewing angle. At an acute viewing angle, the % R increases.
Why coat both sides of the crystal? Coating only one side of the substrate is half as effective as coating both sides. If you only coated the backside of the crystal next to the dial, you would get ~ 4% reflection off the front side plus ~ .5% on the backside. The total reflection would be high at +4.5% vs. the goal of < 1% total reflection. Two sided coatings are, therefore, optimal for minimal reflection and to obtain maximum light transmission onto the watch dial. The small negative is that the AR coatings are softer than the crystal itself and can scratch. But with care this can be minimized. It is cheaper to replace the whole crystal than to have an existing one recoated.
I am not an optical physicist or scientist so this is a laymen’s explanation of how AR systems work. I was on the Management, Sales and Marketing side of a AR coating company. I hope this is helpful and if there is an error in my explanation, I trust it will be corrected by those more knowledgeable.