In order to provide your eyes with adequate protection against potentially harmful ultraviolet (UV) rays, sunglasses should say “99-100% UV absorbent” or “UV 400” on their labels. Beyond that, there are several features unrelated to UV protection that may also prove helpful. For instance, polarized lenses cut down on glare caused by light reflected off water and ice. Mirrored lenses can also be helpful in very bright conditions. Photochromic lenses, which lighten and darken in accordance with the ambient light, make it easier to go indoors with your sunglasses still on. Gradient lenses are shaded darker at the top than bottom, which helps shield light from above while making it easier to see the horizon and below.
Photochromic lenses are well suited to individuals who wear prescription lenses and do not want to switch to prescription sunglasses when they go outdoors.