Lenses of any material can be treated with coatings to give the lenses additional desired properties.
Photo chromic lenses
Coated with a layer of reflective material that greatly reduces the amount of heat entering the eyes. However, these lenses reduce visual acuity and are somewhat like looking through a two-way mirror. This kind of coating is purely cosmetic and frequently seen on glasses worn by state troopers. Mirror coatings come in a number of colours and are highly reflective. They usually are applied on sunglass-dark lenses. The persons looking at the wearer do not see the eyes of the wearer.
Ultraviolet protective lenses
Protect the eyes from ultraviolet sun rays.
Useful in protecting eyes from reflections when skiing or participating in water sports. They are available in plastic, glass, and high-index materials, but they are relatively expensive compared to other lenses.
Often referred to as hard coat lenses. They are applied to the front and back surfaces of lenses to protect against accidental scratching and to improve durability. Some high-quality lenses come from the factory with scratch coatings already applied. No coating can make lenses completely scratch proof.