Anti Reflection and other coatings that improve quality of ophthalmic Stock finished lenses
Whenever a light moves from one medium to another (for example, when light enters a sheet of glass after travelling through air), some portion of the light is reflected from the surface (known as the interface) between the two media. The strength of the reflection depends on the refractive indexes of the two media as well as the angle of the surface to the beam of light
Thus the higher is refractive index of the lens the more reflection occurs
The amount of light reflected is known as the reflection loss.
Glass and CR 39 lenses reflection loss is approximately 4%, (In high index lenses reflection loss could be as high as 12%) in the more complicated scenario of multiple reflections, say with light traveling through a window, light is reflected both when going from air to glass and at the other side of the window when going from glass back to air then back to glass ( lenses)and back to air . The size of the loss is the same in both cases. Light also may bounce from one surface to another multiple times, being partially reflected and partially transmitted each time it does so. In all, the combined reflection occurs for example reflection loss when light travels trough the glass is approximately 4% when we double glass layer reflection doubles as well up to approximately 8%
What are the situations the AR coating really needed.
To improve both the vision through the lenses and the appearance of the glasses, an anti-reflective coating (also called AR coating) is applied. Originally developed by Carl Zeiss laboratories in 1935 for optical equipment, anti reflective coatings were widely adopted by other manufacturers. Antireflective coating consist of several microscopic layers of metal oxides applied to the front and back lens surfaces. Designing the proper coating formula is one part of the high art of antireflection coating the second part is equally tricky art of applying the coating under very high vacuum conditions in the proper temperature when optical index and even thickness on the lenses are taken into account. Because of the layering effect, AR coatings sometimes have a hint of green or purple color, depending on the individual manufacturer’s formula.
Anti-reflective coating reduces the glare that you see, as well as the glare that others can see on your lenses. An anti-scratch coating can lengthen the life of your lenses, while hydrophobic coatings keep rain, snow and fog at bay.
left: Lenses without anti-reflective coating. Right: With anti-reflective coating applied to the lenses.
Each layer is scientifically calculated to block reflected light. The result is that you’ll see a reduction in glare, annoying reflections and halos around lights. This is a great safety benefit when you’re driving at night.
Also, anti-reflective coating reduces both internal and external reflections on the lenses themselves, creating a nicer cosmetic appearance. Internal reflections appear as rings that make lenses look thick. External reflections mask your eyes from a clear, complete view when someone is looking at you. So with an anti-reflective coating, eyeglass lenses appear thin or non-existent, and your eyes look more natural.
Anyone on TV or whose photo is taken often benefits tremendously from the coating, but really, all eyeglass wearers would benefit from an anti-reflective coating from a cosmetic point of view. If you have a strong prescription, you can use the AR coating in conjunction with high-index lenses to make your glasses look and feel as thin as possible.
With sunglass lenses, an AR coating is better applied only to the back surface of the lens (the surface nearest the eye). Because sunglass lenses are so dark, the AR coating can wind up looking smeary on the front surface. Coating the back side helps reduce the reflections of light that enter from behind you and bounce off the surface into your eyes. A back-side coated sun lens is much more comfortable than an uncoated sunglass.
From the point of the availability for the retailers AR coatings as well as ophthalmic lenses could be divided to generic prefabricated – applied on the factory or custom made in the laboratory for specific lens. Like in case of the lenses, prefabricated ARcoating, even though basic technology of application is similar, done in the big batches with industrial equipment in standardized procedures. The same quality and appearance kept from batch to batch. Durability of prefabricated coating is usually higher that this of the custom made. We at the K-Mars optical have long experience with both groups. Based on test groups of tens of thousands of lenses we experienced much less pilling or other defects with prefabricated AR coating than with AR custom made in the laboratory. Of coarse when lens in question is custom made (multi focal, progressive or single vision) and we have to apply AR coating for the definite customer benefit we have no choice but to do it in the laboratory, but in case of stock finished lenses we definitely prefer prefabricated AR coating witch is more durable, consistent, and much less (6-5 times) expensive.
Hydrophobic and EZ clean coating
KM300 – Superhydrophobic Coating
By treating a lens with a Hydrophobic (based on Silica polymers) coating we are filling the spaces between the crystals of Metal oxides of which the coating layer consists. There are hundreds of thousands of these crystals that are deposited on the lens surface during the coating process. There are spaces between these crystals or rods into which dirt and grease penetrates during normal use of the lens. The Hydrophobic coating will fill these gaps and create a smooth surface without changing the properties of the A-R coating. Dirt stays on top of the surface and can not go in between the above mentioned gaps.
Next step is a super hydrophobic treatment (KM300 is the K-Mars Brand) which ads antistatic and hydrophilic properties to the lens thus reduces attraction of the dust (charged particles) and makes lens more cleanable with water based solutions. Therefore the lens becomes easily cleanable and less smudgy. The downside is that the premium AR coatings 2 times more expensive than regular custom AR coatings and 10-12 times more expensive than prefabricated AR coating. At K-Mars actually made a breaktrough by applying KM300 super hydrophobic coating on top of our inexpensive yet high-quality prefabricated (stock finished lenses) AR coating. The result is similar to what one would achieve with brand name premium coating at quarter (1/4) of the price!!!.