ANTI-REFLECTIVE COATINGS-AR

Written by Optician Club on . Posted in Freeform Rx Lenses, Optician News

Anti-reflective lenses usually have three or more thin layers of ceramic or metal oxide on each side of the lens. By varying the refractive index of each layer, reflections from one layer can be made to interact with reflections from the next layer. The basic concept for how anti-reflective coatings work is the optical interference model. Each coating layer in the AR stack combines with previous layers to cancel out a broad range of light waves by introducing opposite, or destructive, waves that are out of phase. Thus, reflective properties of that range of light are neutralized. By applying the principle of optical interference, we are able to increase light transmission to nearly 99 percent in most cases. The result is a reduction of reflections over a broad range of wavelengths… and clear, crisp vision for your patients.

STANDARD LENS WITHOUT AR
Standard, uncoated lenses can decrease light transmission by up to 15 percent of available light. This is caused by visible reflections on the front and back surface of the lens, as well as internal reflections. The higher refractive index of a lens material equates to the greater incidence of reflections and glare.

ANTI-REFLECTIVE COATINGS

Surface reflections are also increased as index increases. As a result, it is important that all lenses, especially high index lenses, include anti-reflective treatment. This is why high index lenses, especially for Cracking and Crazing of AR coating1.67 MR-7 lenses, andCracking and Crazing of AR coating high index 1.74 lenses,  should always be delivered with an integrated AR. How is the best AR constructed and how does it work?

High performance AR is composed of a combination of the following:

• Bare front and back lens surfaces
• Special Cracking and Crazing of AR coatingHard Coatings applied directly to the lens surface (1)
• High energy bombardment etches and activates the surfaces (2)
• Application of the anti-static layer
• Application of a specialized AR stack of low and high index materials (3)
• Application of the super hydrophobic, oleophobic treatment (4)
• Sometimes a lab will apply a special surface to allow for edging without slippage
• Systematic QC inspection.

ANTI-REFLECTIVE COATINGS

All of this results in more transmission, highly durable, easy-to-clean and stays cleaner longer lens for your patients/consumers. AR lenses improve visual acuity, and the overall aesthetics of glasses. They allow more light to reach the eye, providing better vision and reduced reflections, so patients see better and others see their eyes more clearly. Good AR treatments eliminate glare from overhead lights, computer screens, TVs and headlights. This means that glare that would normally cause eyestrain and fatigue, or make night driving more difficult, is virtually eliminated. Lesser performing AR does not apply the same scratch resistance to the back of lenses. Since 40 percent of all scratches occur on the back lens surface, double side hard coating of the same quality is important.

The demand for thinner and lighter lenses has led to an increase in mid- and high-index substrate usage. Most hardcoats index of refraction closely matches that of hard resin (1.49). When the index of refraction of the coating matches that of the substrate this eliminates the reflections from the light passing through the interface of coating to lens. If there is a mismatch between hardcoat refractive index and lens refractive index, the result is birefringence.

ANTI-REFLECTIVE COATINGS

Birefringence appears as a rainbow pattern on the lens surface. If the coating and lens have the same index of refraction, the light passing through “sees” the two as one continuous substrate and the reflections at the interface are minimized. As the use of higher index substrates has increased, so has the availability of higher index coatings.

Scratch Resistant Coatings Technology for Lenses

Written by Optician Club on . Posted in Freeform Rx Lenses, Optician Tools

Scratch Resistant Coatings Technology (Hard Coating) provide lenses with durability and increased longevity. A high quality thermally cured dip hardcoat provides the best scratch resistance and provides for the most durable ingredient for an Anti-Reflective coating application. Plastic lens materials scratch easily than glass materials, and therefore benefit greatly with application of a durable hardcoat.

Two of the more common types of hard coats are:

Thermally Cured Coatings — Thermally cured coatings were traditionally used only by lens manufacturers and referred to as factory coatings or the front side coating on a semi-finished lens blank. They have good to excellent abrasion resistance and AR compatibility. They allow the option of using primers to achieve better adhesion, tintability and impact enhancement. Most thermally cured coatings are designed for adhesion to a single substrate. This makes them ideal in the lens manufacturing environment. The use of a primer, however, allows the same coating to be used on multiple substrates or even as an overcoat (more details below). This has resulted in increased usage of thermally cured coatings at labs and retailers. Thermally cured coatings are generally thought to be more compatible with AR and mirror coatings. Thermally cured coatings can be spin or dip coated. Thermally cured coatings
have longer cure times (typically one to four hours) and have limited substrate compatibility.

Scratch Resistant Coatings-Dip

Check the Features and Benefits of Hard Coating Lenses

UV Cured Coatings — UV cured coatings are traditionally used in the laboratory environment and at some lens manufacturers. The advantages of UV cure coatings are quick cure time and multiple substrate compatibility. This makes them ideal for the laboratory or retailer where multiple substrates are used and quick turnaround times are expected. UV cured coatings can be applied by spin coating, dip coating or in-mold coating. Spin coating is the most typical method of application. UV cured coatings are typically lower in abrasion resistance and often less compatible with AR and mirror coatings. UV cure coatings often have good steel wool abrasion resistance, but do not perform as well as thermally cured coatings in the Bayer Abrasion Test (more on test methods below).

Scratch Resistant Coatings-Spin

One of the most important things to understand related to this topic is that lenses have what is scientifically referred to as a coefficient of thermal expansion. This is a fancy way of saying that the lenses expand and contract with temperature. When the lens expands and contracts at a different rate than the AR coating, the AR coating can, and very often will, crack. (Keep in mind the AR coating is very, very thin relative to the lens.) The highest coefficient of thermal expansion in lenses belongs to Hi Index, then CR39 and Mid Index. Poly and Trivex have relatively little thermal expansion. If we are going to see temperature related cracking, we will typically see it first in Hi Index and then follow it down. One hardly ever sees temperature related cracking in Poly.