Sunglasses use a variety of technologies to eliminate the problems with light that were discussed in the previous section. The following sections discuss all of the different technologies currently in use:
- Photochromic lenses
- Scratch-resistant coating
- Anti-reflective coating
- 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.
- 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.
- 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. (Read Why is the sky blue? for more information on this effect.) 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.
- 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.
- 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.
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.
In the next few sections, we'll take a look at other sunglass technologies.