The characteristic feature of photochromic lenses is that they become steadily darker on increasing exposure to UV radiation and clear again when the radiation decreases. Photochromic lenses adapt to changing light conditions and hence provide optimum protection against glare in almost any situation. Due to the time required for the photochromic substances to react, however, protection cannot be guaranteed against sudden glare.
Photochromic lenses can be made of glass or plastic. Their darkening and clearing process is dependent on ambient temperature and on the intensity of the UV radiation to which they are exposed. The higher the intensity of the radiation and the lower the temperature, the darker the lens and the faster the darkening process.
This means that maximum darkening is achieved in wintry conditions in mountainous regions. The lenses become slightly less dark in summer heat. Maximum darkening is not possible in motor vehicles, as the windscreen absorbs UV radiation.