Anti-Reflective Coating on Glasses: Are They Worth the Money?

woman-wearing-anti-reflective-glasses

While purchasing new eyewear you may on occasion have been asked whether or not you would like an anti-reflective (a/r) coating on your lenses. What exactly is an a/r coating? Do you need it? How this coating works and what it does is a fascinating subject.

Refraction vs Reflection

As light encounters a lens, three things happen:

  • Some of the light passes through the lens (this is called refraction),
  • some of the light is absorbed and converted into heat energy (very small amounts), and
  • some of the light bounces off.
reflection-vs-refraction-illustration

Light bouncing off the lens creates glare. Glare or reflections can be annoying and in some severe cases can decrease visual acuity. 

There are some cases in which these reflections can be amplified. A lens with a higher index (a thinner lens) is a much denser material and provides more resistance to light, creating more reflections as well as a lens that has a flatter front curve. 

Some prescriptions require the front of the lens to be very flat. This flat surface can act a bit like a mirror bouncing light and creating troublesome reflections.  Even sunglasses need an a/r coating. The back of the lens acts like a dark concave mirror, which is why the coatings are often applied to the back of the lens.

How Do Anti-Reflective Coatings Work?

How exactly do these coatings work? This is where things get a bit technical. Anti-reflective coatings rely on a principle called destructive interference. Several layers of coatings, such as magnesium fluoride, are placed atop one another each half a wavelength thick. As a wavelength of light hits the lens, it can bounce back the light out of phase, thus eliminating the reflection. 

In turn, there is another action that happens. This is called constructive interference. This phenomenon increases light transmission. For example, light transmission in a conventional plastic lens is increased from 91% to 99.5%. In high-index lenses, the transmission is increased by 10 to 12%.

Now, the name anti-reflective is a bit misleading. No coating has been introduced that will create 100% light transmission. Often, you will see a colored reflection off of one of these coatings. Each company has a trademark color that they feel will either cosmetically enhance the lens. Typically, it is a color that the eye is less likely to pick up on.

The Future of Anti-Reflective Coatings

If you’re like most eyeglasses wearers, then you likely have tried a/r coatings and were less than thrilled. You might have had a problem with the lenses smudging or the coating coming off. 

The good news is that within the past few years, there have been many advancements in this field. New and better techniques have been employed to change the aforementioned problems. 

However, whether or not an a/r coating is needed truly depends on the patient. It is never strictly necessary. But if you want to invest in a great pair of lenses that will optically perform at their best and are cosmetically appealing, a lens with an a/r coating is a good option.