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
- Tints
- Lens materials
- Coatings
- Polarization
- Photochromic
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:
- "UV absorption up to 400nm" is the same as 100% UV absorption.
- "Meets ANSI UV Requirements" or "special purpose" indicates the sunglasses block at least 99% of UV rays.
- Sunglasses labeled "cosmetic" block about 70% of UV rays.
- Labels that claim"Provides UV Protection" without specifying exactly what percentage of UV rays are blocked.
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.
- Gray the best, most popular, all-purpose colour. Virtually no colour distortion; depth perception is unaffected. Offers good protection against glare. Excellent absorption of UV and infrared light.
- Green similar performance to gray. Filters some "blue light", light at the violet-blue end of the spectrum often found in fog, haze or low light environments. Reduces glare. Green offers the highest contrast of any tint. Very popular.
- Brown / amber good general purpose tint, but increases colour distortion. Depth perception is enhanced. Particularly effective in absorbing harmful "blue light", in addition to UV rays. Blue, or Blu Blockers are a patented version of sunglasses that utilize this "blue light" property inherent in brown/amber tints. Most popular colour for driving, but also favourable for boating, skiing, and general use.
- Yellow / gold depth perception is enhanced, but so is colour distortion. Reduces the amount of "blue light", tending to make images bright and sharp. Good for snow activities. A special coating must be applied to yellow lenses to absorb infrared light.
- Rose / purple offer the best contrast of objects against a green or blue background, so they are good for hunting or water skiing. Often selected as a stylistic preference and has a softer, warmer feel over a long period of time.
- Blue claimed by some to be good for playing tennis, as blue lenses let in blue and green light and may allow you to see a tennis ball better. Lets in more damaging "blue light", so some professionals advise against this colour. Often a stylistic preference.
- Single-gradient a uniform decrease in the tint of a lens, generally from the top (darker tint) to the bottom (lighter tint) of the lens. Useful for driving because they don't dim your view of the dashboard, but not as good on snow or at the beach.
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.
- Crown glass soda-lime glass used for lenses and prisms; is ground and polished to exacting and consistent standards to virtually eliminate distortion. It is highly scratch resisitant, but is heavier and is more vulnerable to breakage on impact than its non-glass counterparts. Superior optical quality.
- CR-39 plastic made from a hard resin with excellent optical quality. Good impact resistance. Strong and lightweight.
- Polycarbonate is a synthetic material, lighter and more impact resistant than its glass and plastic counterparts, but with slightly reduced optical quality. Although a tough material, it scratches relatively easily, so look for those lenses with scratch resistant coatings. Polycarbonate lenses are a must for your sunglasses if you participate in potentially eye hazardous work or sports. Also an excellent choice for active children.
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.
- UV protection - is obtained primarily by the application of a chemical to the surface of the sunglasses.
- Scratch-resistance - clear, hard films are applied to plastic lenses to improve durability.
- Anti-reflective - a common problem with sunglasses is back glare, or light hitting the back of the lens and reflecting into the eyes. An anti-reflective coating is applied to the lenses to reduce this glare.
- Reflective / Mirror - a mirror coating is applied to the outside surface of the lenses to reflect light.
- Mirroring today is commonly applied in a half-silvered, single gradient fashion. This means that the mirroring is not only translucent, but decreases in silvering from the top to the bottom of the lens, reflecting more light from above, while allowing light from below to enter. Often a stylistic preference (looks tough, authoritative, intimidating and mysterious, like law enforcement). Popular for world poker and other activities in reflecting overhead light and concealing your eyes. Best for intense glare conditions like snow and water. Note, the reflective properties of mirrored sunglasses tend to cause your nose to sunburn faster than with no mirroring, as it receives both direct and reflected light. Another issue is the mirror coating is prone to scratching, reducing aesthetics and performance.
- Water sheeting - a property of shedding water in sheets, rather than beads or drops, for improved visibility in wet conditions (rain, motorcycling, waterfalls, lakes, oceans, etc.). Achieved through a combination of very smooth surface and application of a special lens coating.
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.