Anti-Reflective and Ultraviolet Coatings

What is 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.

Technologies used by Sunglasses

What are the Technologies used by Sunglasses?

Sunglasses use a variety of technologies to eliminate the problems with light that were discussed in a previous post. The different technologies currently in use are:

1. Tinting
2. Polarization
3. Photo-chromatic lenses
4. Mirroring
5. Scratch-resistant coating
6. Anti-reflective coating
7. UV coating

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.

1. 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.
2. Yellow or gold tints reduce the amount of blue light while allowing a larger percentage of other frequencies through. Since blue light tends to bounce and scatter off a lot of things, it can create a kind of glare known as blue haze. The yellow tint virtually eliminates the blue part of the spectrum and has the effect of making everything bright and sharp. That's why snow glasses are usually yellow. This tint really distorts color perception, which makes it inappropriate for any activity that relies on accurate color.
3. 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. In fact, Sun Tiger has a patent on a particular version of these called Blue Blockers. These sunglasses also distort colors similar to yellow lenses, but increase contrast and clarity.
4. 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.
5. 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.

Many manufacturers employ a process called constant density to tint the lenses. It is the oldest method of creating sunglasses and involves a glass or polycarbonate mixture with a uniform color throughout the material. The tint is built right into the lenses when they are created. Tinting can also 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.