This is the fourth and final part in a series on AR coatings that is going to cover the basics of how AR coatings work. Part three is called Do I Need AR Coating On My Sunglasses?
If you have been following along in this series you know that AR coatings allow direct light to pass through a lens without reflecting off of the surface. To understand how this can be done is it important to know that light travels in waves. There is a principle of wave theory known as "wave interference." There are two types of wave interference. One type is called constructive interference. The other type is called destructive interference, and it is the type that is used to make an AR coating work. The principle of destructive interference says that two light waves traveling along the same medium that have a displacement in the opposite direction will destroy each other when they overlap. So the way an AR coating works is by reversing one light wave against the next one. The net result is that the two waves cancel each other out. Imagine two rocks landing on a calm surface of a pond. Do you see the waves traveling out away from where the rocks hit the water? When the waves collide they will cancel each other out. Light behaves in a similar way. The AR coating effectively reverses one light wave against itself. This causes it to cancel out the next wave and voila, no more glare or reflections. What I have just described can be achieved with a type of AR coating called a single layer coating. It is possible to add multiple layers to cancel out additional wavelengths of light, which will reduce glare more effectively. This effect can be measured with an instrument called a spectrophotometer. Each manufacturer uses its own formula for determining how many layers and how to apply each layer for maximum effectiveness at glare reduction. AR coatings are sometimes referred to as stack coatings. This is because of the multiple layers incorporated onto the lens surface that make up the coating.
In addition to the light canceling layers there are also scratch resistant layers, water resistant layers, and oil resistant layers. High quality AR coatings have these additional layers so that your lenses will be easier to keep clean and will not scratch as easily. The water resistant hydrophobic and the oil resistant oleophobic layers are applied as the top coatings. If you look at the lens surface under a microscope before these coatings are applied it would appear very porous. There would be crevices and pits for oils and dust particles to become trapped, which would make the lenses hard to keep clean. The process of applying the hydrophobic and oleophobic layers involves filling in all those little crevices and pits so that the top layer of the lens is very smooth. It is hard to believe it but there are still some glasses wearers who have yet to experience the vision boosting benefits of a good AR coating. In my view there is no reason whatsoever in this day and age to go without it. So at the end of a long day after you have been looking at your computer screen, checking your smart phone, and just living around all that artificial lighting, your eyes will thank you for wearing lenses that reduce fatigue and increase sharpness.