Photochromic lenses appear lighter indoors since they darken in sunlight and fade indoors to provide comfort according to the brightness of the environment. Polarizing lenses selectively absorb horizontally reflected glare from surfaces such as water or roadways to improve visibility. Contrast Enhancing lenses are designed to provide better visibility through contrast enhancement under certain conditions. Glass lenses are the most optically stable and scratch resistant lens material. Plastic lenses are the lightest weight lens material, some of which are coated to increase scratch resistance.
Polycarbonate lenses are the most impact resistant.
However, as described earlier, in hazardous situations such as when playing impact sports, one should wear lens-frame combinations or systems that are designed to protect under the situation of use, e.g. sports protective eyewear. How light or dark a lens should be depends upon the brightness of the environment and the individual likes and needs of the consumer wearing the lens. The most often used lenses are medium to dark. Medium to dark lenses are used for driving, picnicking, and general purpose sunglass use. The important aspect to remember in selecting the darkness of a lens is to let comfort and visibility in the expected use conditions, be your guide.