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.
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 1.67 MR-7 lenses, and 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 Hard 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.
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.
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.