Sunglass Lenses
- Ultraviolet Coating
- Tinting
- Polarization
- Mirroring
- Anti-reflective Coating
- Choosing the Perfect Features for You
Sunglasses use a variety of technologies to eliminate the problems with light and glare.
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 which 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.
Tinting
The color of the tint determines the parts of the light spectrum that are absorbed by the lenses. Manufacturers use different colors to produce specific results.
- Gray tints are great all-purpose tints that reduce the overall amount of brightness with the least amount of color distortion. Gray lenses offer good protection against glare, making them a good choice for driving and general use.
- Amber and brownish tints are also good general purpose tints. They have the added benefit of reducing glare and have molecules that absorb higher frequency colors, such as blue, in addition to UV rays. There has been research that suggests that near-UV light frequencies such as blue and violet can contribute to the formation of cataracts over time.
- Green tints on lenses filter some blue light and reduce glare. Because green tints offer the highest contrast and greatest visual acuity of any tint, they are very popular.
- Purple and rose tints offer the best contrast of objects against a green or blue background. They make a good choice for hunting or water skiing.
Tinting can be accomplished by applying a coat of light-absorbing molecules to the surface of clear polycarbonate. The most common method for tinting polycarbonate lenses is to immerse the lenses in a special liquid containing the tinting material. The tint is slowly absorbed into the plastic. To make a darker tint, the lenses are simply left in the liquid longer.
Polarization
Light waves from the sun, or even from an artificial light source such as a light bulb, vibrate and radiate outward in all directions. Whether the light is transmitted, reflected, scattered or refracted, when its vibrations are aligned into one or more planes of direction, the light is said to be polarized. Polarization can occur either naturally or artificially. You can see an example of natural polarization every time you look at a lake. The reflected glare off the surface is the light that does not make it through the "filter" of the water, and is the reason why you often cannot see anything below the surface, even when the water is very clear.
Polarized filters are most commonly made of a chemical film applied to a transparent plastic or glass surface. The chemical compound used will typically be composed of molecules that naturally align in parallel relation to one another. When applied uniformly to the lens, the molecules create a microscopic filter that absorbs any light matching their alignment.
Most of the glare that causes you to wear sunglasses comes from horizontal surfaces, such as water or a highway. When light strikes a surface, the reflected waves are polarized to match the angle of that surface. So, a highly reflective horizontal surface, such as a lake, will produce a lot of horizontally polarized light. Therefore, the polarized lenses in sunglasses are fixed at an angle that only allows vertically polarized light to enter. You can see this for yourself by putting on a pair of polarized sunglasses and looking at a horizontal reflective surface, like the hood of a car. Slowly tilt your head to the right or left. You will notice that the glare off the surface brightens as you adjust the angle of your view.
A lot of sunglasses advertised as polarizing actually are not. There's a simple test you can perform before you buy them to make sure. Find a reflective surface, and hold the glasses so that you are viewing the surface through one of the lenses. Now slowly rotate the glasses to a 90-degree angle, and see if the reflective glare diminishes or increases. If the sunglasses are polarized, you will see a significant diminishing of the glare.
Mirroring
Reflective sunglasses often have a mirrored look. Often, the mirror coating is applied as a gradient that gradually changes shades from top to bottom. This provides additional protection from light coming from above while allowing more light to come in from below or straight ahead. What that means is that if you are driving, the sun's rays are blocked but you can see the dashboard. Sometimes the coating is bi-gradient, shading from mirrored at top and bottom to clear in the middle.
Anti-reflective Coating
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.
