There are three types of sunglasses:
Cosmetic sunglasses: these do not give significant protection against the sun and are worn as fashion accessories.
General purpose sunglasses: for reducing glare in bright light or in circumstances such as driving in daylight.
Special purpose sunglasses: for activities such as skiing or for people abnormally sensitive to glare.
Sunglasses give comfort in very bright conditions by reducing the total amount of light reaching the eye and, in particular, protect the eye from the damaging ultra-violet (UV) part of the spectrum. There are two types of UV - UVB and UVA.
UVB is radiation within a wavelength of 280nm to 315nm (nm = nanometre, which is one millionth of a millimetre!) This radiation is substantially absorbed at the surface of the cornea but can reach the retina. Excessive exposure can cause permanent damage to the cornea and conjunctiva, but this may not progress if further exposure is avoided. UVA (315nm - 380nm) radiation penetrates more deeply and can cause damage to the crystalline lens and retina. Permanent effects such as cataract development are seen as accelerations of the ageing process.
Which to Choose?
Always look for a CE, UV 400 or British Standard Mark. This ensures your sunglasses provide adequate UV protection. The British Standard sets performance levels for quality, strength, stability, design and manufacture as well as the amount of UV they let through. Purchasing sunglasses that don't conform to this standard is not advised.
Non UV absorbing lenses can do more harm than not wearing anything at all. Behind a tinted lens, the pupil opens wider allowing in more UV light than would happen normally and thus reduces the eyes' natural protection. Sunglasses sold under BS 2724 have a 'shade number'. Shade numbers relate to the amount of UV allowed through. The higher the number, the better the protection.
Does the colour of the lens make a difference?
Brown and grey are the most popular colours and green is one of the most effective. However, it is the 'shade number' that counts.
For safety choose plastic, toughened glass or laminated glass lenses.
Are there such things as 'sun contact lenses'?
Yes. This area is developing quickly. Ask your contact lens practitioner about the latest products available. Sports people involved in open-air activities may find these particularly interesting.
What about prescription lenses?
Both sunglasses and contact lenses are available to your normal optical prescription. Your practitioner will be pleased to advise you, but ensure that he or she knows that you wish to have UV protection built in.
Which sunglasses are best for driving?
The Highway Code states that tinted glasses should not be worn at night or in poor visibility. Sunglasses should not be used at night to stop headlamp glare. They should also be removed if driving from bright sun into a tunnel. Don't pick a very dark tint. A medium density is normally sufficient and it is safer as it transmits more light.
These are tinted darker at the top than at the bottom and give useful protection from bright overhead light, leaving a lighter area for map reading or seeing the dashboard.
These lenses darken on exposure to sunlight and should react efficiently in changing light conditions. They should not leave much tint present when the lens is not exposed to the sun.
These lenses reduce reflections from wet or polished road surfaces, but they reveal the stress patterns in the older types of toughened windscreens, which can be hazardous.
8 Top Tips for Buying the Best Sunglasses
1. UV Rays - Expensive sometimes means better, but not necessarily in the case of sunglasses. What really counts is the degree to which the lenses filter out harmful UV rays. Look out for glasses carrying the European Standard 'CE' Mark, UV 400 or British Standard Mark, these ensure that the sunglasses offer a safe level of UV protection.
2. Sunglasses for Driving - When buying sunglasses which will be worn for driving, make sure they are in the filter category range of 0-3. A lens carrying a filter category of 4 will be too dark for safe driving. Never wear sunglasses when driving at night or in poor light.
3. Lens Shade - Unless the glasses carry the British Standard Mark, do not confuse the shade of the lenses with their ability to filter UV rays. Dark sunglasses may still allow UV rays to enter the eye and can be MORE harmful than wearing no glasses at all, because they cause the pupil of the eye to dilate which allows more UV rays to enter. Therefore, when buying sunglasses with very dark lenses it is more important than ever to ensure they offer good UV protection. Sunglasses are marked with a filter category number from 0-4, where 4 is the darkest lens. Category 4 offers more comfort in bright sunlight as it avoids straining the eyes.
4. Filtering Blue Light - Ideally sunglasses will also absorb high energy visible radiation, known as blue light. This will enable the glasses to be worn for extended periods without tiring the eyes. It is recommended that no more than 95% of blue light should be filtered to avoid colour distortion.
5. Avoid Scratches - Scratched lenses will scatter the sun's light and could cause glare around the area of the scratch. Look after sunglasses by keeping them in a case and cleaning them with a mild detergent and water or a special lens cleaner. When drying lenses, do not use a paper towel, as this will scratch the lens. The solution is to use a good cloth, preferably one made of microfibre.
6. Contact Lenses with Protection - Contact lens wearers can now also enjoy the added protection of in-built UV protection. Contact lens practitioners will have details of all the latest products available.
7. Prescription Sunglasses - If you already wear spectacles, you can have sunglasses made to your prescription.
8. Have Fun! - Finally, have some fun with sunglasses. Designs are getting more flamboyant and adventurous, so make the most of the wide range available and add a real twist to summer dressing, but make sure that the lenses are big enough to protect the eyes from stray light. Sunglasses may be vital for protecting the eyes, but they are also great accessories for looking stylish and individual.