Gear of the Week | Photochromatic glasses

The chemical function is elegant: tiny crystals of silver halides, simply put silver atoms that are firmly bonded to other atoms, are embedded in the glass mass. When exposed to UV light, which makes up a proportion of solar radiation, silver ions (i.e. a silver atom that is missing an electron) pick up an electron and react to form metallic silver. This metallic silver can absorb visible light, causing the glass to darken. If the UV source is removed, for example indoors or on a cloudy day, the metal in the crystal returns to its original compound and the glass becomes clearer. Thanks to this reversible principle, the glass darkens when the sun shines and automatically clears up again when UV light is absent.

Over time and with the need for lighter, shatterproof lenses, the material from which spectacle lenses are made has changed, and with it the technology. Nowadays, lenses for sunglasses and prescription glasses are usually made of plastics such as polycarbonate. Instead of silver halide, manufacturers now rely on other light-reactive molecules. These change their chemical structure under UV light and tint the glass, similar to the silver halide principle used in the past. When the UV light is removed, they return to their transparent form.

This infinite adaptability makes photochromic lenses reliable companions in changing light and weather conditions - ideal for mountain, ski and everyday use. It should be mentioned that the tinting process is slow and is significantly weaker behind a windshield because not enough UV light reaches it. The functionality also depends on the temperature. In cold weather, the reaction is slower and the lenses need more time to adapt to the new conditions.