The Myth About Mid-Index
Anti-Reflection Inc. has been providing customers with anti-reflective coatings for ophthalmic lenses since 2003 and one of the most recognized misconceptions during this time is the notion that mid-index materials (1.55, 1.56) do not require a backside hard coat for anti-reflective coating application.
Due to the hardness of these materials, a backside hard coat is always recommended to aid in the adhesion of the anti-reflective coating stack to the substrate. Imagine trying to paint a piece of metal furniture for your lawn.
The surface would have to be sanded or etched to allow the paint to adhere properly. The harder a substrate is, the more aggressive the etching process would have to be to achieve proper adhesion.
In turn, most hard coatings are very easily etched to create a surface to successfully bond to. With the exception of trivex and polycarbonate, as the index of a mid/hi-index material rises – 1.55, 1.56, 1.60, 1.67, 1.70, the substrate itself becomes increasingly harder. 1.55 and 1.56 sit just above the line for being able to achieve proper adhesion without a hard coat being applied but it is just enough to create problems for your customers down the road.
Following this guideline should increase the lifespan of the anti-reflective coating applied to a mid-index lens significantly decreasing warranty submissions and therefore producing happier customers.