Requirements/Specifications and standards for correct quality sunglasses
1) They should absorb sun radiation and wavelengths needed to provide proper and relaxed vision.
2) The materials used in the sun lenses should be optically processed homogenous and unspoiled as in the case of vision lenses.
3) Their durability under pressure should comply with international standards.
4) The materials used in the frames of sunglasses should not cause irritation to the skin/ allergies and generally should be compatible with human skin.
The color in some lenses, in relation to the extent of their darkness, define the suitability (or non-suitability) of the lenses, depending on the environmental conditions (cloudiness, sunshine, surfaces which reflect with high intensity). One aspect which requires serious consideration is the fact that quality standards of sunglasses may decline with the passage of time (2 to 2.5 years).
Special mention should be made regarding sunglasses with polarized filters that diminish blurriness caused by vertical reflection of sun rays against surfaces like water, asphalt, window screens etc. As a result, they are suitable for driving as well as usage in countryside or at the sea etc.
Consult your optometrist who is responsible for providing information regarding the safe usage of sunglasses. Do not trust your vision to amateurs and mere street sellers. Protect the most valuable human sense; protect your vision.