Refraction vs. Reflection

As light encounters a lens three things happen. Some of the light passes through the lens, this is called refraction, some is absorbed and converted into heat energy (very small amounts) and the rest bounces off. When light bounces off the lens this 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.

But 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 a 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. And 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% and in High index lenses the transmission is increased 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 their trade mark color that they feel will either cosmetically enhance the lens or they will choose a color that the eye is less likely to pick up on.

The Future

I am sure that there are many readers who have in the past tried a/r coatings and were less than thrilled. They had a problem with the lenses smudging and the coating coming off. Within the past few years there have been may advancements in this field. New and better techniques have been employed to change the above problems. Wither or not this coating is needed truly depends on the patient. It is never necessary but if you really want to invest in a great pair of lenses that will optically perform at their best and are cosmetically appealing it would be a good choice.