The first report ever to be published on the subject of glass photochromic lenses appeared in 1964. ZEISS launched its first photochromic lens on the market in 1970.
Photochromic lenses gradually darken when exposed to an increasing intensity of short-wave light and return to their original state when the light is removed. They adapt automatically to changing light conditions and therefore provide optimum protection against glare in every situation.
Borosilicate glass is used to produce glass photochromic lenses. To obtain the photochromic properties, silver halides are added as photochromic substances to the melt. After cooling, the glass is bright blue and not yet photochromic. A heat treatment process at approx. 600 °C is required to create the photochromic properties. The duration and temperature of this process influence the colour of the glass, the speed of photochromic reaction and the degree of darkening.
Glass photochromic lenses are solid-tinted, i. e. the degree of darkening can vary slightly depending on the thickness of the lens, as the photochromic substances are evenly deposited over the entire thickness of the lens.