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Photochromic

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.

Photochromic, glass

Glass photochromic Lenses

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.

Plastic photochromic Lenses

It was only very recently that manufacturers succeeded in producing plastic lenses with photochromic properties comparable to those of glass lenses.

Organic photochromic lenses are produced from a special plastic which is optimised for the absorption of photochromic dyes. Unlike glass photochromic lenses, they are not given their photochromic properties until the semi-finished or finished stage. In this process – known as photochromisation – millions of photochromic molecules are incorporated in the front surface of the lens at a depth of approx. 0.15 mm.
The photochromic molecules are primarily indolino-spironaphthoxacins. These change their chemical structure when they are irradiated with UV and short-wave blue light. Much like a flower when exposed to sunlight, they unfold and cause the lens to change colour.
If the energy level of the UV radiation decreases or – as is the case when the wearer goes indoors – no UV is present at all, the molecules close again. The lens clears and adopts an attractive filter tint.
As the molecules are only found on the lens surface, the darkening process takes place evenly over the entire surface, irrespective of the dioptric power.

 

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