11 Aug 2008
Common Sunglass Questions Answered
1. Is it safe to buy sunglasses without a prescription?
If you do not need corrective lenses, and you do not have a medical condition which would require special eyewear, the answer is absolutely yes!
2. What is UV?
Ultraviolet Radiation (UV) consists of the short, invisible rays from the sun that reach the earth's surface. The sun's rays contain three bands of UV: UVA, UVB and UVC.
UVC rays are absorbed by the upper atmosphere and do not reach the earth's surface and, therefore, are not relevant to sunglass performance.
UVB rays are responsible for sunburn of the skin. Environmental UVB in concert with cold wind and snow microcrystals has the potential to cause photokeratitis (snow blindness), a rare but painful temporary disorder of the cornea. Photokeratitis generally corrects itself within 12 to 48 hours.
It is generally agreed that there are many factors that might be associated with cataracts, a gradual clouding of the lens of the eye. Some factors that have been associated with cataracts are age, gender, ethnic origin, genetic factors, smoking, nutrition, use of alcohol, microwave radiation, and diabetes. Although not all scientists agree, there is some research that suggests an association with daily exposure to UVB in very bright sunlight over a period of many years.
UVA rays are mostly absorbed within the lens of the human eye. While there are no documented disorders of the human eye from UVA, it remains a much studied and debated topic.
As a result, sunglass standards place limits on UVB and UVA for sunglass lenses.
3. Does pupil dilation from wearing sunglasses cause UV problems for the eye?
The small amount of pupil dilation in the eye, due to the wearing of sunglasses, is not known to cause any problems. All sunglasses absorb UV more strongly than visible light. They reduce the UV more than if you were to stand in the shade, since the shade more or less evenly attenuates all wavelengths, visible and UV, while sunglasses preferentially attenuate shorter wavelengths i.e., UV wavelengths. Therefore, sunglasses actually provide substantially more UV reduction than the shade. Wearing a pair of sunglasses does not increase your exposure to UV radiation nor does standing in the shade.
This idea is a myth!
4. Should sunglasses be worn for driving?
Sunglasses are the favorite "necessary accessory" worn by drivers to reduce glare and increase comfort during driving. Under bright conditions sunglasses are very helpful to drivers to reduce the brightness and glare, thereby allowing better vision under those driving conditions.
5. Can sunglasses be worn at night?
Sunglasses should not be worn at night. They are not meant to be worn after dark. "Sun" glasses are for daytime use. The SAA recommends against wearing sunglasses during night driving.
So-called night driving glasses are generally amber tinted eyewear meant to reduce the glare of oncoming headlights. While they may make the driver feel more comfortable, they also reduce the wearers visibility of the darker portions of the roadway.
6. How many sunglass lens options are there?
There are many lens treatments, lens performance types and lens materials, many of which are combined in concert to make the lens options available today.
A) Sunglass lens treatments are many and varied. Tints vary from light to medium to dark and very dark. There are:
Solid tinted lenses are evenly tinted across the entire lens to cut glare from all directions.
Gradient tinted lenses are darker at the top and lighter at the bottom to cut overhead glare and provide clearer vision straight ahead and when looking down.
Double gradient lenses are darker at the top and bottom and lighter in the center to cut overhead and reflected glare from the ground and provides clearer vision and straight ahead.
Mirror coated lenses reflect light across a wide spectrum including the infra red or heat rays (IR) and may tend to be somewhat cooler to wear.
B) Some Sunglass lens performance types are:
Fixed Tint lenses where you get what you see.
Photochromic lenses appear lighter indoors since they darken in sunlight and fade indoors to provide comfort according to the brightness of the environment.
Polarizing lenses selectively absorb horizontally reflected glare from surfaces such as water or roadways to improve visibility.
Contrast Enhancing lenses are designed to provide better visibility through contrast enhancement under certain conditions.
lnfra Red ( IR) attenuating lenses reduce the sunlight heat rays passing through the lenses to the eyes for comfort.
C) Lens materials in use are:
Sunglass lens materials are designed and manufactured to meet the requirements of ANSI standards for UV absorption, flammability and impact resistance.
Glass lenses are the most optically stable and scratch resistant lens material.
Plastic lenses are the lightest weight lens material, some of which are coated to increase scratch resistance. Polycarbonate lenses are the most impact resistant. However, as described earlier, in hazardous situations such as when playing impact sports, one should wear lens-frame combinations or systems that are designed to protect under the situation of use, e.g. sports protective eyewear.
Laminated lenses are combinations of glass and/or plastic to achieve a specific performance.
7. How much light should a lens transmit?
How light or dark a lens should be depends upon the brightness of the environment and the individual likes and needs of the consumer wearing the lens. Generally speaking, in and around town, light or medium tint sunglass are worn; while on the beach, or skiing, darker sunglasses are worn. The most often used lenses are medium to dark.
Medium to dark lenses are used for driving, picnicking, and general purpose sunglass use. The important aspect to remember in selecting the darkness of a lens is to let comfort and visibility in the expected use conditions, be your guide. A caution, however, extremely dark lenses (below 8% visible transmittance) should not be used for driving.
8. Can Bluelight in the normal environment cause retinal problems?
Bluelight in extremely high intensity, such as staring directly at the sun, especially through binoculars or a telescope, can cause retinal disorders. No one should stare at the sun under any conditions. That is why, before a solar eclipse, eyecare professionals issue warnings through the media and describe methods to safely view the eclipse. Concerning senile macular degeneration or other age related diseases, it has not been shown that the bluelight in the normal sunlit environment is associated with any retinal disorder.
9 . Do sunglasses have regulations?
Sunglasses are regulated by the Food and Drug Administration (FDA) as medical devices. Domestic manufacturers and importers of sunglasses must register with the FDA, and are subject to numerous reporting requirements and good manufacturing practice inspections. The lenses must be impact resistant (not shatterproof) and the frames must be non-flammable. All sunglasses must be labeled with the name and address of the manufacturer or import-distributor and imported sunglasses must be marked with the country of origin.