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.
ZEISS scientists and engineers have responded to consumer expectations on self-tinting lenses by refining this technology to create PhotoFusion. The lenses represent a genuine improvement in comfort for the wearer.