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Sunglasses - A Glossary of Terms

 

ANSI - American National Standards Institute, a body that develops voluntary standards for sunglasses and many other products. Many sunglasses advertise these standards:


ANSI Z80.3 - As applied to special-purpose sunglasses, the Z80.3 standard effectively assures (among many other things) that the lenses screen 99 to 100 percent of harmful UV rays and permit acceptable color recognititon. (In other words, you can tell a green light from a red light.) Standards for cosmetic sunglasses are less stringent. Anyone serious about eye protection should avoid cosmetic sunglasses. All standards apply to the lenses only, not to the sunglasses themselves, because fit and design vary greatly.


ANSI Z.87.1 - Assures a very high degree of protection against impacts.


Base curve - The degree of wrap in sunglass lenses. Base curves of 6 and 8 are most common in sunglasses. Those with a base curve of 6 are gently curved; “wrap” shades are typically 8-base or 9-base (or in between), and provide excellent peripheral protection. Because it is difficult to grind prescriptions in wrapped lenses, prescription sunglasses are generally 6-base or flatter.




Base tint - The color you see when you look at the back side of sunglasses. Base tint is not a coating. It is a dye embedded in the lens. Coatings on the front side may make the lenses appear different from the front. For the wearer, it is the base tint as seen from the back side that counts. The quality and effect of base tints vary from maker to maker. In general, though, these are the qualities of common base tints:

Brown and copper: Sharpens contrast while still permitting accurate color perception. Ideal for moderate to bright conditions and changeable cloud cover. Filters out diffused blue light, allowing for comfort in bright but cloudy conditions.



Gray and green:  Color-neutral and soothing to the eye. Ideal for bright conditions. Can be a bit flat.

Rose:  Sharpens details and allows for good depth perception, but the view is, well, a bit rosy.

Yellow:  Enhances depth perception in low-light conditions while permitting excellent detection of detail. Popular with hunters, skiers, and fishermen. Filters short-wavelength blue light, so it effectively reduces “optical noise” from haze and mist. Not for prolonged use in bright light.


Coatings - Surface lens treatments that achieve special and cosmetic purposes. Common coatings include:

Anti-reflective: When applied to both surfaces of a lens, can reduce distracting reflections that are particularly common with curved lenses.

Hydrophobic:  Sheds water and sweat to reduce spotting on lens surfaces.

Mirrored:  Largely a cosmetic coating popular with law enforcement personnel in the South (or at least their movie counterparts), but can reduce glare and bright light.

Scratch-resistant:  Also known as hard coating, helps polycarbonate, CR-39, and other plastic lenses resist scratching.


Nosepad:  Where the rubber meets the nose. Soft, nonslip (hyydrophilic) materials are best for sweaty pursuits. Adjustable nosepads suit a variety of faces. Fixed nosepads better work from the get-go, or you’ll never be happy.


Optical clarity (acuity) - The ability of a lens to deliver a sharp image to the eye. High-quality sunglasses are tapered and finely polished so that the lenses focus the image properly on your eyes. Each lens is subtly thicker in the middle and tapered to the outside so that incoming light waves reach your eye at the same time. The result: no eyestrain, and a crystal-sharp image. They prevent aberration, which is the failure of a mirror or lens to bring light rays to a single focal point, producing a defective image.

Optical clarity is easier to achieve with glass than with polycarbonate or other plastic lenses. But with any material, manufacturers of high-quality sunglasses go to the effort and expense to achieve optimal optical clarity. This is a big reason why good sunglasses are so sharp, and cheap ones will ultimately strain your eyes.


Lens materials - Lenses are either glass or some sort of plastic. No lens material is a guarantee of quality. Bad glass lenses are much worse than quality CR-39 lenses. These are the most common materials:

Polycarbonate:  A type of plastic lens material that is highly shatterproof. More subject to scratching than glass, but quality PC lenses are hard-coated to make them more scratch-resistant. The material of choice for extreme conditions where high-velocity impact is a possibility.

CR-39:  The original high-quality plastic lens material. More subject to scratching than glass, but impressively scratch-resistant on its own, and often hard-coated to make it even more scratch-resistant.

Glass:
  Optical-quality glass is the material of choice for superb clarity and resistance to scratching. Glass has liabilities, though. Glass lenses are heavier than plastic lenses, cannot be curved as radically as plastic, and are not shatterproof. Glass is a great choice for everyday street wear, but not for action sports.


Frame material - Proprietary names and hybrid materials can confuse the issue. Most frames, though are essentially either:

Nylon (aka grilamid, zyl, acetate, etc.): Durable and flexible.

Metal: Lightweight and easily adjusted, but subject to inadvertent bending and breakage.


Photochromic (photosensitive)
- Photochromic lenses adjust the density of their tint as light conditions change. They permit more light transmission in darker conditions and less light in bright conditions. Photochromic technology has improved greatly, but photochromic lenses still take several minutes to respond fully. Be aware that photochromic lenses respond to ultraviolet radiation. They are therefore not the best choice for driving, because car windshields screen most UV rays. Inside a car, photochromic sunglasses may not be dark enough for your driving comfort.


Polarization - A filter applied to sunglasses to reduce or eliminate glare—that is, rays bouncing off reflective surfaces such as water, ice, or metal. Reflected light rays become concentrated on the horizontal plane, which is why they have a stabbing effect on the eye. A polarizing filter is a thin sheet of film with horizontally aligned molecules. It works like a set of mini blinds to filter out 99 percent of those rays. In quality sunglasses, the polarizing film is embedded between layers of lens material to remain safe from scratching or peeling.


Ultraviolet (UV) light - Invisible light rays that can damage your eyes and skin. Longterm exposure to UVA and UVB rays is associated with cataracts, macular degeneration, cancer, benign growths on the eye’s surface, and so-called snow blindness (photokeratitis). The effect of UV radiation is cumulative, so it is important to protect your eyes from an early age. Quality sunglasses block 99 to 100 percent of UV rays.


Visible light transmission (VLT) - The amount of visible light that sunglasses transmit to the eye. Sunglasses typically block 70 to 90 percent of visible light—that is, VLT of 10 to 30. Lenses with VLT darker than 10 are for extreme situations such as high-altitude snow and glacier travel. Lenses with VLT lighter than 30 will not be comfortable in bright sunlight. Remember, your eyes work best when they are fully dilated, so darker is better—but lenses that are too dark for the situation may prevent you from seeing what you need to see. The VLT comfort zone for most situations is between 12 and 18. Too much exposure to visible light can contribute to poor night vision.