Sunglasses

The most important sunglass component is the lens!

There are various criteria high quality sunglasses have to comply with:

 

Ultra Violet (UV) Radiation Blockage

UV light exposure over time can cause permanent eye damage! For this reason, you must not compromise on the UV absorption properties of your sunglasses.

Sunglasses must block 95-100% of UVA and UVB radiation and at least 70% of visible light.

IMPORTANT! Even the best sunglasses cannot protect your eyes from certain intense light sources. Arc welding, tanning lights, snowfields or looking directly at the sun (especially during a solar eclipse) can damage your eyes! Looking at any of these light sources without adequate protection can cause a painful corneal condition called photokeratitis or even a permanent loss of vision! Do NOT rely upon sunglasses to protect your eyes under the above conditions. You MUST wear appropriate eye protection for these types of specialized activities or risk permanent damage to your eyes !


One has to distinguish between various terminologies and labels:


Darker doesn't necessarily mean greater UV protection, but darkness should be an important consideration for the activity during which the sunglasses are worn. E.g., you wouldn't wear light shades for snow skiing !
Both glass and plastic absorb some UV radiation, but UV absorption is greatly improved by the application of a chemical to the lens surface.
Maximum UV protection is ensured with wrap-around frames, which protect both the front and sides of your eyes from UV / light infiltration.
Sunglasses that do not adequately filter out UV light can actually cause more eye damage than wearing no sunglasses at all, because the pupils dilate, letting in more harmful UV rays.


Do NOT buy toy sunglasses for children that don't have the protection recommended above. Children are more prone to sun damage to the eyes as their pupils are usually larger than those of adults and the lenses of their eyes are clearer. Eye damage is cumulative, so start toddlers off early in life with the best UV protection!

 

Tints

A huge range of tints is available for sunglasses. The most popular ones are grey, green and brown.
With different degrees of UV-filter they are used for various activites, some of them are only used for a fashion statement.
Buy your sunglasses only at qualified Optometrists and insist on a comprehensive advice about the pros and cons of the desired tint.

 

Lens materials

The three most common lens materials in use today are crown glass, CR-39 plastic and polycarbonate plastic lenses.
All sunglasses must meet several impact standards. No lens is truly unbreakable, but plastic lenses are less likely to shatter than glass lenses when struck by a hard object, such as a stone. Most non-prescription sunglass lenses are plastic.

 

Coatings

The most common coatings on optical lenses are anti-reflective coating to reduce glare and blindings caused by reflections, the UV-protection and anti-scratch coatings (for plastic lenses) to extend the lifetime of the lens.

 

Polarization

Light has properties both as a particle and as an electromagnetic wave. Its electromagnetic wave properties allow it to become polarized. Since light waves have the ability to vibrate in multiple directions, it is possible to shut out some of these axes, thus producing polarized light. Light can be polarized via a variety of techniques.
Light waves from a natural light source, the sun, or from an artificial light source, such as an incandescent light bulb, vibrate and radiate outward in all directions. When their vibrations are aligned along the same polar plane, the light is said to be polarized. When not aligned, they are said to be randomly polarized.
Polarization can occur either naturally or artificially. An illustration of natural polarization is the reflected glare off the surface of a lake. The glare you see is reflected because it does not penetrate the "filter" of the water. This glare explains why it is difficult, if not impossible, to see anything below the surface, even when the water is very clear. Glare produces eyestrain, discomfort, and results in squinting. Sunglass tinting alone can not address the problem of glare. But polarized filters selectively absorb the reflected glare while passing, or transmitting useful light to your eyes. Polarization can be adjusted so the light passed is at a comfortable and useful light level. Polarization has been used in over one billion pairs of sunglasses over the last 50 years and its use remains widespread today. Unlike the earliest versions, today's versions also block out ultraviolet light and can selectively attenuate harmful blue light.
Polarized lenses are fabulous for many selected activities, but not for all. Some experts debate the appropriate use of polarized lenses for snow-covered surfaces. While they can reduce the intense glare from sunlight off snow, for downhill skiers they may not provide the "useful" glare and contrast the eye needs to distinguish smooth snow from ice patches or moguls.

 

Photochromic

Very popular is the use of photochromic lenses. These lenses tint according to the intensity of UV light and convert to sunglasses. They offer UV-protection and reduction of light-transmission to avoid blinding. The lenses are available as crown-glass as well as CR 39 (plastic) lenses. A very convenient way to use spectacles without carrying an extra pair of sunglasses, but there are also pros and cons.
Photochromic / photochromatic lenses - developed by Corning in the late 1960s and later popularized by Transitions' lenses. They have special additives (silver chloride, silver halide) which are reactive to UV rays in sunlight, causing the lenses to darken / lighten in proportion to the intensity of the rays. Some also react to temperature changes. Still commonly referred to generically as "transitions" lenses. Due, in part, to technology advances, their use is growing rapidly.
Early versions of photochromic shades were made out of glass and were relatively heavy, but today are available in various lightweight materials such as polycarbonate. Modern photochromics are superior to the older versions in uniformity of colour, and improved properties, e.g., darkening response in spaces where UV is filtered, such as behind the windshield of an automobile. A great everyday selection, as they can serve as both prescription and sunglasses in one, and can be worn indoors and out. They automatically protect against UV, but not all plastic photochromic lenses block 100% UVA and UVB radiation. Also, if frequently in/out of the sun, they may not change tint fast enough for the wearer's liking. Most of the darkening takes about 30 seconds, while the lightening can take up to five minutes.

 

Since the OZONE layer is becoming ever thinner,
it is imperative to wear Sun-protection eyewear,
especially in our sunny Namibia !!

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