100% UV protective sunglass lenses are essential to prevent
damage to the eye from the sun's harmful UV (ultraviolet)
The reason UV is so dangerous is because, unlike the sun's
infrared and visible rays which manifest themselves as heat
and light, it is not easy to detect. Excessive exposure to UV
light can cause permanent damage to the eyes' surface or
internal structures resulting in corneal opacity or cataract.
Lenses that absorb UV don't necessarily need to be dark
although most are and a 100% UV absorbing filter can be
included in spectacles with only a slight cosmetic tint.
Alternatively light-sensitive (photochromic) lenses, which
change with the light, can also offer full UV protection even
when they are in their lighter mode.
For your own protection, however, only ever wear 'off the
shelf' sunglasses that display the appropriate CE mark and
guarantee to absorb all UV light up to 400nm.