Glass lenses do not require anti scratch coatings. All organic lenses ( polycarbonate, plastic, hi index, Trivex) require that anti scratch properties be applied during the casting manufacturing process or applied later and cured with heat or U.V. Anti scratch single vision, bifocal, and progressive lenses will be molded with anti scratch in the mold. A stock bifocal or single vision lens will then be anti scratch treated on both the front and rear surface. If the lens needs to be surfaced, the surfacing will always be surfaced on the back surface. The surfacing lab must then reapply the anti scratch coating on the back surface and cure it with heat or U.V.
The coatings are made from acrylic based compounds similar to those found in artificial cosmetic fingernails. The coatings will adhere better when the surface is pretreated.
AntiReflective Coating and AntiReflective, AntiScratch Coatings
Wavelength cancellation theory
Lenses transmit less than 100% of light because of surface reflections. There are a number of ways of reducing these reflections.
For ophthalmic use the only way is to use wave interference techniques that make use of the fact that light is transmitted in waves. Interference relies on the principle of superposition, which basically states that two waves of light (or even two waves of water) will add together if they are coincident in the same space. If the peaks and troughs in the two waves coincide exactly, then a wave of double the size is the result.
However, if the peaks of one wave are aligned with the troughs of another, then they cancel each other out.
By putting a thin film on the surface of the lens, light waves from the reflections from the front and back of the film can be made to interfere so that they cancel out. In other words, if the reflected waves from the front and back of the coated lens are exactly out of phase with each other there will be no visible reflections.
This is achieved if the difference between the back and front is exactly half a wavelength and to achieve this the coating must be one quarter of a wavelength thick.
Since the coating must be only one quarter of a wavelength, it is incredibly fragile and susceptible to being damaged. So, all A/R coatings have some kind of anti scratch coating over them to create a “durable A/R coating”. Durable anti reflective coating is the most effective lens option available for improving overall visual performance of a spectacle lens. They reduce glare and reflections that can cause eye fatigue and they sharpen visual performance. A/R coatings decrease reflections other people see when they look at your glasses, thus creating a better cosmetic effect.
How are Reflection Free Lenses made?
Antireflection coatings are made up from extremely thin layers of different dielectric materials that are applied in a high vacuum onto both surfaces of the lenses. The quality of the A/R 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 A/R coating is the color of the residual 1% reflection of the lens – sometimes called the bloom. On modern broadband reflection free lenses it can be tuned to be either a soft green or blue without compromising the quality of the anti reflection properties. The A/R bloom should not be confused with a permanent lens tint. An A/R bloom is almost imperceptible and can only be seen when holding the lens up to sunlight or artificial light.
During the manufacturing process quartz is gasified and applied to the lens surface to create a ¼ wavelength coating and then an anti scratch acrylic is applied over it. This will create a basic A/R coating which are fairly inexpensive.
It turns out that the quartz layer is less flexible than the lens material and this causes cracking, crazing and peeling over time. It is analogous to painting a rubber ball with paint, letting it dry, then, squeezing the ball.
This is where the Crizal and Crizal-like coatings come in. Crizal puts a layer on the lens before applying the quartz to act like a shock absorber or buffer and makes the flexibility of the lens substrate material and the AR quartz coating compatible.
This is why not all lenses can have a Crizal coating. Essilor must certify the lens material to ensure this shock absorber treatment is effective before they will apply their coating on competitors lenses. The problem with Crizal type coatings is that they smudge terribly because they absorb water and they must be cleaned continuously which results in scratches.
Then along came the premium lens coatings in addition to A/R. This treatment creates a surface that water hates, so water beads up on it. The surface is extremely slippery. These properties are collectively referred to as oleophobic and have the benefit of being easy to clean. We clean less frequently so it is perceived as being more scratch resistant. The only problem is that it attracts static electricity which makes it attract dust particles which require cleaning.
So now we can treat the coating with a process that reduces static build up to reduce dust attraction to the surface. The surface requires less cleaning of the dry dust particles. These coatings must be applied in a vacuum and take hours of precessing. This is why premium lens coatings are more expensive and are perceived as one of the most scratch resistant A/R coatings.
A/R coating will improve visual performance and cosmesis on clear lenses. A/R will also improve visual performance of transition lenses. A/R will also improve visual performance and cut glare in sunglasses, including polarized sunglasses. The A/R is more important for the back surface of tinted lenses.
Think of an AR coated lens having up to 5 coats or treatments:
1) Buffer coat to make AR coat flexibility compatible with the lens substrate.
2) AR coat
3) Scratch resistant hardcoat
4) Oleophobic treatment
5) Anti static treatment