How Sunglasses Work

By: Jeff Tyson

Anti-reflective and Ultraviolet Coatings

A common problem with sunglasses is called back-glare. This is light that hits the back of the lenses and bounces into the eyes. The purpose of an anti-reflective (AR) coating is to reduce these reflections off the lenses.

Similar to a scratch-resistant coating, AR is made of a very hard, thin film that is layered on the lens. It is made of material that has an index of refraction that is somewhere between air and glass. This causes the intensity of the light reflected from the inner surface and the light reflected from the outer surface of the film to be nearly equal. When applied in a thickness of about a quarter of light's wavelength, the two reflections from each side of the film basically cancel each other out through destructive interference, minimizing the glare you see. AR coatings are also applied to the front of prescription eyewear and some sunglasses to eliminate the "hot spot" glare that reflects off the lens.


Ultraviolet Coating

Several of the most serious eye problems can be linked to one cause: UV light. UV is often separated into two categories based on the frequency and wavelength of the light: UV-A and UV-B.

As a natural protection mechanism, the cornea of your eye absorbs all of the UV-B and most of the UV-A light. But some of the UV-A light reaches the lens of the eye, and over time this absorption can lead to cataracts. The small amount of UV-A that gets past your cornea and reaches the retina can eventually lead to macular degeneration, the leading cause of blindness in people older than age 65. Intense and prolonged exposure to UV radiation can cause either cancer of the eye or photokeratitis, which is basically a sunburn on your retina. Because it occurs most often when a person is outside on bright winter day, with sunlight glaring off the snow, this condition is commonly known as snow blindness.

A good UV coating on your sunglasses can eliminate UV radiation, and you should check to make sure that your sunglasses filter out 100 percent of both types of UV rays. There should be a statement on the label telling you how much UV protection the sunglasses have. You want 100-percent protection.