|The cross section of Satisloh’s Ioncote AR treatment illustrates the many layers of a modern AR stack.|
A modern multilayer AR treatment consists of various layers and processes to ensure its effectiveness.
Premium multilayer anti-reflective (AR) treatments are designed so that each layer of the treatment (or stack) has a specific purpose. The combination of chemistry and physics used in the creation of these treatments produces a product eyecare professionals can sell with confidence. The following is an overview of what each aspect of a modern multilayer AR treatment does.
THE MODERN AR STACK
Early AR lenses used a single layer of magnesium fluoride to create the AR treatment on the back surface of a lens. Since visible light consists of over 400 wavelengths, eliminating only one of them was not particularly effective. To eliminate wavelengths across the visible spectrum, a multilayer treatment was needed. Modern, high-tech AR treatments employ multiple interrelated layers that have different functions. The entire pile is known as the “AR stack.”
The illustration from Satisloh depicts a typical premium AR treatment in cross section. While all AR lens treatments do not follow this exact configuration, this graphic represents all of the major premium AR treatment components.
The first thing that is applied to plastic lenses is a hardcoating. Mechanical differences between the coating and a plastic lens substrate (including differences in elasticity, hardness, and in the rates of expansion and contraction under pressure or as the result of changes in temperature) can easily lead to delaminating, crazing, and scratching. Adding
a hardcoating can solve this problem. Applying the AR treatment to a poorly engineered hardcoating, however, can lead to similar compatibility issues and coating failure.
|Carl Zeiss Vision’s PureCoat AR features a thin, transparent layer of a conductive material that dissipates any buildup of static electricity.|
Current generation nanocomposite hard coatings, like those used in Essilor
of America, Inc.’s Crizal Avancé™ with Scotchgard™ Protector, serve as a mechanical transition between the substrate and the glass-like AR lens treatment. They provide firm support and adhesion properties for the AR treatment while providing a degree of necessary and required flexibility. The result is a treatment that can withstand the rigors most patients will put their glasses through as well as the expansion and contraction of the different materials based on normal environmental temperature changes.
An adhesion layer or some other type of surface preparation is usually employed at this point in the AR treatment process to prepare the surface for the AR layers. In Satisloh’s Ioncote Kappa process depicted in the illustration, a durability base layer is also applied.
Most premium multilayer AR treatment stacks are designed using proprietary chemicals and process combinations. The portion of the AR stack that specifically works to knock out surface reflections usually consists of five layers.
|The top layer in Hoya’s Super HiVision EX3 is formulated to create a smooth surface that repels water and oil.|
These layers are composed of alternating high-index and low-index materials. The first layer is an adhesion layer composed of silicone dioxide, a low-index material with an index of 1.47. This is the layer that is placed onto the hard coating. The second layer consists of a high-index metal oxide, usually zirconium dioxide (ZrO2, index = 2.06) or titanium dioxide (TiO2, index = 2.45). The third layer features low-index silicone dioxide while the fourth layer is a high-index metal oxide. The fifth and final layer is another low-index silicone dioxide layer. Each layer has a different thickness, one-fourth wavelength of the color in the spectrum it is attempting to reflect. For AR treatment purposes, the visible spectrum is considered to range from 400nm and 700nm.
While a quality multilayer AR treatment, like SuperClean AR from Seiko Optical Products of America, Inc., comes close to totally eliminating the ghost images and surface reflections of lenses, some minute degree of light is still reflected. The color and final intensity of the remaining small amount of light reflected from the coating is called its reflex color. This color is the product of the interaction of the incident light with the various films that have been selected for the AR stack and will vary depending on the AR treatment manufacturer. Most AR treatments have a reflex color that reaches peak intensity around 550nm in the yellow-green region of the visible spectrum, which happens to be at the peak sensitivity of the human eye.
When a lens surface is rubbed with a cleaning cloth or tissue, static electricity with a negative charge can build up. This is an interesting problem because the more people clean and rub their lenses, the more static charge it builds up, and the more dust, dirt, lint, etc., it attracts.
|MULTILAYER AR THICKNESS The thickness of a modern anti-reflective (AR) lens treatment falls in the 0.2 to 0.3 micron range. To give you some perspective of just how thick that is in relation to the lens it might sit upon, think of an AR lens treatment’s thickness on a 2.0mm thick lens as equal to 0.25 in. of snow on the top of a 150-story building. The thinness of an AR treatment makes it vulnerable to flexing, cracking, and breaking. To overcome this problem, specially formulated hardcoatings have been developed that flex, thereby helping the coating to “give” instead of crack.|
Carl Zeiss Vision Inc.’s PureCoat™ AR uses a patented anti-static layer: a thin, transparent layer of a conductive material that dissipates any buildup of static electricity on the coating surface. The charge can also be added to the lens material itself. However it is done, the end result helps repel surface particles and defeat static charge buildup. This keeps lenses cleaner longer. It also helps fight scratching because it reduces the frequency of cleaning, which is when many scratches occur.
OLEOPHOBIC AND HYDROPHOBIC
Although their reflecting ability has reached 99% or better, AR lenses still have other issues to face. For example, fingerprints can be highly obvious and oil can accumulate on the lens surface, which is hard to remove. AR treatments, like Super HiVision™ EX3 from HOYA VISION CARE, North America, overcome these problems by creating a top layer that is formulated to create a smooth surface that repels water and oil. This top layer has hydrophobic (water-fighting) and oleophobic (oil-fighting) properties. Hydrophobic properties cause water to bead and run off the surface. Oleophobic properties fight the buildup of facial oils and fingerprints.
Today’s premium multilayer AR treatments are designed so each layer performs a specific purpose. The end result is a durable, long-lasting treatment that improves patients’ vision.
Randall L. Smith is the Opticianry Program Director at Baker College in Jackson, MI.
WHERE TO FIND IT
Carl Zeiss Vision Inc.
800-358-8258 • zeiss.com/lenses
Essilor of America, Inc.
800-542-5668 • essilorusa.com
HOYA VISION CARE, North America
877-528-1939 • hoyavision.com
800-866-5640 • satisloh.com
Seiko Optical Products of America, Inc.
800-235-5367 • seikoeyewear.com