Hollywood mythmakers can create or change a movie star’s image with a particular pair of sunglasses. Models, musicians and movie stars popularize trendy shades and out-of-sight styles. But are your sunglasses fashionable and functional? With the right ingredients, sunglasses can protect your eyes against ultraviolet (UV) radiation, bright visible light and glare. The color, darkness and fashionable design of sunglasses has little to do with UV protection. Many countries set standards for UV absorption in sunglasses. Special lens treatments can add polarizing filters, mirrored coatings or special tints to sunglasses that can reduce glare, reflect bright light, enhance contrast or make it easier to see on hazy or overcast days.
Choosing sunglasses
Each year, over 300 million pairs of sunglasses are distributed in the United States alone. When purchasing sunglasses, people are looking for more than a great look. They want to cut down their exposure to bright light, harmful ultraviolet (UV) light and glare.
UV hazards
Ozone depletion in the upper atmosphere has increased the amount of ultraviolet (UV) radiation that reaches Earth. For every 1% of ozone depletion, exposure to UVB rays increases by 2%. Exposure to UV light is greater at higher altitudes. Up to 85% of UV rays are reflected off snow and light-colored sand.
UVA, the sun’s “aging” rays, contribute to early wrinkling and cataracts. Closest to visible light, they travel year-round through glass, water, clouds and some clothing. UVB, the sun’s “burning” rays, cause eyelid and other skin cancers and photokeratitis (sunburn of the eye). These rays are more intense in summer months. Experts also think that UV overexposure may contribute to macular degeneration (damage to central retina).
All sunglasses filter of UV light to some degree, but not all sunglasses offer the same UV protection (see “Standards for sunglasses”). For the best protection, choose sunglasses that block UVA and UVB rays between 290 and 400 nanometers (nm).
The color or shade of sunglasses has nothing to do with their ability to block UV light [see “Shades and tints”]. UV protection is recommended for adults and children who spend time outdoors.
Glare
Bright flashes off water or shiny surfaces can subject the eyes to 10 to 12 times more light than that needed to see. Glare can be painful and dangerously distracting during driving or sporting activities.
To be certain sunglasses will block glare, try them on in front of a mirror. If you can see your eyes, the sunglasses are probably not dark enough to block glare.
General tips
For comfortable vision on sunny days, sunglasses should block 75 to 90% of visible light. Sunglasses that absorb at least 75% of visible light shield the eyes against blue light, part of the visible-light spectrum. It’s difficult to focus in blue light, so people often squint to see. Blue light can distort colors.
Sunglasses should have an overall good fit for sharp and comfortable vision, which is not the case if worn over your head! In doing so, your sunglasses loosen and your visual comfort is compromised. Wraparound or side-screen models block UV light that can skirt the sides, tops and bottoms of regular frames, but they may increase optical distortion.
To test optical quality, put on the sunglasses then look at a vertical edge or line (e.g., door frame or floor tiles). Move your head back and forth, sweeping your eyes across the lens. If you notice any wiggle in the line, an optical defect in the lens may distort your vision.
The lens tint should be uniform, not darker or lighter in spots. With gradient lenses, the tint should lighten gradually and uniformly from top to bottom.
If you intend to wear sunglasses while driving, check the label to ensure that they meet traffic-signal recognition standards. Some shades of sunglasses distort the true color of road signals, especially those with special UV blocking abilities. These sunglasses should not be worn while driving.
All sunglasses must be impact resistant, but they are not shatterproof. They are not tough enough to act as protective eyewear for high-impact sports or industrial safety purposes.
Sunglasses are not recommended for night driving. They cannot protect against intense light sources, such as sun lamps, lasers, welding torches or solar eclipses.
What is polarization?
Conventional sunglasses reduce visible light but offer little protection against glare. Since 1936, polarized lenses have controlled glare by absorbing light that scatters in all but one direction – the vertical plane. Polarization has nothing to do with ultraviolet UV protection, but most polarized lenses contain UV-blocking chemicals.
Because they cut glare, polarized lenses are favored by golfers, fishers and outdoors enthusiasts. They are recommended for drivers and people who complain of glare from computer screens.
Polarized filters are added to glass lenses by a special lamination process. Hard-resin and high-index plastic lenses are polarized during molding. Polarizing films are added to polycarbonate lenses while they are still in liquid form.
Polarizing filters are available for bifocals, progressive, photochromic and single-vision lenses. Some common filter colors are:
- light gray, which transmits 35 to 43% of visible light.
- medium gray, which is sometimes used with photochromic and mirrored lenses
- dark gray, which transmits 14 to 25% of visible light
- light brown or tan, which transmits 27 to 29% of visible light
- dark brown, which transmits 18 to 27% of visible light
- yellow, which transmits 68 to 71% of visible light
- amber, a high-definition filter that inhibits blue light
- neutral gray or brown, for indoor or computer use, transmitting 60% of visible light
Mirrored coatings
A mirrored coating reflects high-intensity light to reduce glare. It lessens the amount of visible light that reaches the eye, but it does not block UV light.
To make a mirrored coating, a thin layer of vaporized metal is bonded to the lens surface in a vacuum chamber. Colorful coatings – silver, blue, red-brown or orange-gold – are primarily for fashion, not eye protection. Metallized coatings, which often use chromium, are less durable than high-performance dielectric coatings, which use titanium or quartz. Mirrored coatings tend to darken sunglasses uniformly.
Photochromic lenses
Photochromic lenses are a good compromise for people who don’t want to switch between prescription glasses and sunglasses. These light-sensitive lenses automatically darken within about 30 seconds in bright light. Once indoors, they lighten within 5 minutes.
General purpose sunglasses transmit from 8 to 40% of light. Most people choose sunglasses in the 15 to 25% range. When outdoors, most photochromic lenses fall within this range, but the amount of light transmission varies from one manufacturer to another. The darker photochromic lenses transmit from 12% (outdoors) to 75% (indoors) of light. Lighter brands may transmit 35% (outdoors) to 85% (indoors) of light. To find a comfortable match of dark and light shades, people should try on several brands.
Although acceptable for everyday use, photochromic lenses are not suitable for high-glare sports, such as boating or skiing.
Shades and tints
The darkness (shade) and color (tint) of sunglasses are poor guides to ultraviolet (UV) protection. UV-absorbing chemicals are added to glass, plastic and polycarbonate lenses. They are usually colorless. Even clear lenses, when treated, will block UV light.
Lenses come in different tints and shades. Light to medium shades are good for daily wear. For bright conditions and outdoor sports, darker shades are more appropriate.
With gradient tinting, the lens shade gradually lightens from top to bottom or top to middle. It protects the eyes from glare when looking skyward but allows clear vision below. Double-gradient lenses are dark on the top and bottom but lighter in the middle. Useful for reflecting water or snow glare, they are not recommended for driving, because they dim the dashboard.
Prescription sunglasses can be customized to fit your needs. Like UV-absorbing chemicals, tints are added to sunglass lenses during manufacturing. Along with tints, manufacturers have developed light-filtering chemicals (filters) that absorb specific sections of the light spectrum. They can combine tints and filters to create a sunglass dye for specific activities, e.g., golf.
Tints filter light differently. Most block 70 to 90% of light transmission. Some tints filter certain wavelengths of visible light, while transmitting others. For that reason, different tints may enhance or distort colors and affect contrast. Some tints, such as pink, blue or purple, are purely cosmetic or fashion colors that are poor light blockers. The best way to determine lens tint is to hold the sunglasses at arm’s length against a white background.
Handy tint guide
Gray: allows true color perception; popular neutral color; good general purpose tint; does not enhance contrast; good tint for golf, cycling or running.
Green: allows true color perception; popular neutral color; good general purpose tint; fair contrast in low light; reduces eye strain in bright light.
Brown: better in hazy sun; enhances contrast; good tint for high-glare sports, such as skiing, fishing or sailing.
Amber: blocks blue light; brightens cloudy, hazy or foggy days; world looks yellow or orange; excellent for contrast; minimizes eye strain; good tint for sportsmen, pilots, skiers.
Yellow: excellent depth perception and contrast in low light; ski industry standard for overcast days; good tint for trap shooting.
Vermilion: excellent depth perception in low light; contrasts objects against blue and green backgrounds; good tint for skiing or sportsmen.
The impact of ultraviolet (UV) light on eye health is a worldwide concern. Several countries have set voluntary or, in the case of Australia, mandatory standards for sunglasses in an effort to protect their citizens’ eyes.
Not all sunglasses block the same amount of ultraviolet (UV) radiation. Since lens tint (color) has nothing to do with a UV-blocking ability, it is difficult to tell how much protection is offered by different pairs of sunglasses. Consumers should check the product sticker or tag or the manufacturer’s brochure to find out how much UV and visible light will be blocked by sunglasses and to ensure that sunglasses are safe for driving, because some tints can block the true colors of traffic signals.
Several regulatory agencies have set standards for UV protection. The most widely recognized are ANSI (The American National Standards Institute), CSA (Canadian Standards Association), CEN (European standard), BSI (British Standard) and AS (Australian Standard).
ANSI
ANSI is one of two U.S. organizations to set standards and labeling for sunglasses. The other is the Sunglass Association of America in conjunction with the U.S. Food & Drug Administration (FDA). Both standards and labeling programs are voluntary.
ANSI sets requirements for cosmetic quality, refractive properties (e.g., distortion or blur) and impact resistance. ANSI standard Z80.3-1996 divides sunglasses into three groups:
1. cosmetic: sunglasses that block at least 70% of UVB and up to 60% UVA
2. general purpose: sunglasses that block at least 95% of UVB and a minimum of 60% UVA
3. special purpose: sunglasses that block at least 99% of UVB and 60% UVA
In the USA, unless sunglasses meet standards of the International Standards Organization (ISO-14889) or ANSI Z80.3-1996 section 4.6.3, they must carry a caution that reads: “not recommended for use while driving.”
CSA
The Canadian standard for non-prescription sunglasses (CSA Z94.5-1995) is similar to the ANSI standard. This standard also provides guidelines for color transmission, has special requirements for different kind of lenses (e.g., polarizing to photosensitive) and tests for impact resistance.
CEN
European standards classify sunglasses into four groups based on their ability to block UV rays: weak, medium, strong and intense. Sunglasses in the latter group are not recommended for driving.
BSI
British standards are similar to but slightly more stringent than European or American standards for UV blockage.
AS
Sunglasses that do not met Australian Standards (AS) 1067.1-1990 and 1067.2-1990 cannot be sold in Australia. Like ANSI and other standards, they ensure that sunglasses provide adequate UV protection and identify lenses that interfere with the ability to recognize the colors of traffic lights or other roadway signals.
For more information
http://www.sunglassassociation.com
UV protection: http://www.aoanet.org
Sources
Barker FM. Does the ANSI Z80.3 nonprescription sunglass and fashion eyewear standard go far enough? Optometry and Vision Science 1990; v. 6, no. 6, p. 431-4
Hovis JK, Cranton D, Chou BR. Tinted lenses and the ANSI standards for traffic signal transmittances. Optometry and Vision Science 1991; v. 68 no. 9, p. 750-5
http://www.fda.gov/cdrh/ode/sunglass.pdf
http://www.hc-sc.gc.ca/ehp/ehd/general/iyh/sunglass.htm
http://www.accc.gov.au
