Sunglasses

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 progess 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 BS2724. This 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 BS2724 have a 'shade number'. Shade numbers relate to the amount of UV allowed through. The higher the number, the better the protection.

Safety
For safety, always choose plastic, toughened glass or laminated glass lenses. Check sunglasses by holding them at arm's length and looking through each lens separately at a vertical line. If the line distorts or wavers when the lens is rotated slightly, the lens is imperfect.