Reflective and Scratch-resistant Coatings

What is the purpose of Mirroring and Scratch-resistant Coatings?

Reflective sunglasses often have a mirrored look. The lenses in these sunglasses have a reflective coating applied in a very thin, sparse layer - so thin that it's called a half-silvered surface.

The name "half-silvered" comes from the fact that the reflective molecules coat the glass so sparsely that only about half the molecules needed to make the glass an opaque mirror are applied. At the molecular level, there are reflective molecules speckled all over the glass in an even film but only half of the glass is covered. The half-silvered surface will reflect about half the light that strikes its surface, while letting the other half go straight through.

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.

The key problem with reflective sunglasses is that the coating is easily scratched. Apparently, sunglass manufacturers have not been able to successfully apply a scratch-resistant layer on top of the reflective coating. Therefore, the scratch-resistant coating is applied first to protect the lenses and the reflective coating is applied over it.

Scratch-resistant Coating

While glass is naturally scratch resistant, most plastics are not. To compensate, manufacturers have developed a variety of ways to apply optically clear hard films to the lens. Films are made of materials such as diamond-like carbon (DLC) and polycrystalline diamond. Through a process of ionization, a thin but extremely durable film is created on the surface of the lens. 

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.