To complete the stack other materials are used to assist with bonding, to create the abrasion resistant properties and for the water repellent top coat.

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Anti-reflection coatings are made from extremely thin layers of different dielectric materials that are applied in a high vacuum onto both surfaces of the lens. The quality of the AR depends upon the number of layers applied to the lens. The early coatings had only a single layer of magnesium fluoride or perhaps two but nowadays most coatings have at least six layers and are known as broadband coatings.

One of the characteristics of an AR coating is the color of the residual 1% reflection on the lens – sometimes called the bloom. On modern broadband Reflection Free lenses it can be tuned to either a soft green or blue without compromising the quality of the anti-reflection properties. The AR bloom should not be confused with a permanent lens tint. An AR bloom is imperceptible and can only be seen when holding the lens up to sunlight or artificial light.

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The anti-reflection stack is the most important part of the Reflection Free lens. It is made up of quarter wavelength interference layers of alternating high and low index materials. The usual materials are silicon dioxide with a low refractive index of 1.45 and titanium dioxide with the higher refractive index of 2.25.

Anti-reflective coating is also applied on the back surface of sunglass lenses. Coating the back side helps in reducing the lights which comes from behind you and bounces off the surface into your eyes. Back-side coated sunglasses are more comfortable than standard uncoated ones. 11/12/2007