The anti-reflective (AR) coating is a transparent thin film structure consisting of alternative layers of contrasting refractive index. Thin film is a special branch of optics that considers the physics of light passing through materials that are close to the wavelength of visible light in thickness (about 500 nanometers–500 billionth of a meter). When light passes through film, it is bent by the material of the film, both on entry and when it leaves the film. When it passes through film of this thin-ness light exhibits exceptional properties called thin-film interference. This is the kind of optical effect you can see in oil slicks which produce rainbow colors in sunlight.
The AR coating on optical glass (found on optical instruments, camera lenses, as well as eyeglasses) have alternative thin film layers chosen in such a way that the interference patterns (the rainbow colors) will cancel each other. Light reflecting from the surface will be cancelled out by “destructive interference.” Light transmitted through the lens will be enhanced by “constructive interference.” The coatings are carefully macro-engineered to enhance transmission of the light and reduce reflections in just the right range of colors. For instance, coatings can be chosen to reduce ultraviolet (short wavelength light) or infrared (long wavelength red).
Lenses with AR coatings look better because the glass looks more invisible to people looking at the wearer. They can be effective at reducing glare from bright lights because they can let more light pass through the lens and increase apparent contrast. To the eyeglass wearer, the effect of AR glasses may be slight during the day, but AR coatings can significantly improve vision at night when bright light glares on the surface of the glass. The coatings largely prevent reflections that hit the glass on the inside of the glasses (light behind the wearer). The benefit is stronger for someone looking at the glasses from a bright area, because with the AR coating, there will be much less reflection from the surface of the glass.
AR coatings are composed of a clear magnesium fluoride crystalline layer.
- The coating can be sprayed onto the lens (either on the outside or both sides). Then, exposing the lens to high heat for an extended time liquefies the crystals and bonds the coating to the lens.
- The coating can also be applied using a vacuum application process. Then the glasses are baked under intense heat to crystallize and harden the solution, binding it to the glasses.
- Some glasses have the AR coating installed by embedding the crystals in the glass when the glasses are made.
If the coating is externally applied, it can be removed by an optician if the glasses become discolored or the coating is scratched.
There are many sources from which you can buy AR coatings on the glasses aftermarket. There appears to be a disparity between the precision of engineered microscopically thin crystalline layers and products that are purchased over the counter. Many people who look for over-the-counter coatings complain that the vast majority of opticians will not install the coatings. One writer a few years ago posted
“I had to locate an optical engineer who runs a highly specialized shop that would apply an AR coating…”
Applying anti-reflective coating on my glasses after purchase is not an ordinary process, but some optical shops will do it.
A User reports that
“Someone…walked into Lenscrafters or one of the other major glasses chains and got it done for $40 or $50. I asked my local eye doctor who has his own lab and he was willing to do it as a personal favor for about the same price.”
The consensus is that most glasses retailers will not add AR coatings in-house. One writer says,
“I even looked up the only AR coating fabrication company in my state and they turned me down as well.”
AR coatings are precisely defined, layered molecule thin coatings that reduce light reflection from the surface of glasses. They make the glasses more attractive by making your eyes look clearer to people around you. They may reduce some glare to make your glasses more comfortable. The coatings are usually installed on the glasses at purchase. After purchase installation of a coating my be difficult to find, but not impossible. There is a variety of coatings and coating chemistries but only a few have established aftermarkets to purchase the coatings.