Special photosensitive molecules are applied during the coating of the lens in the ZEISS production area. Self-tinting lenses use a chemical reaction that was originally discovered in the 1960s and has been continuously enhanced ever since.
The principle behind it all is very simple. When sunlight hits the lens, it increases the surface area of these molecules which makes the lenses darken. The stronger the sunlight or UV light, the darker the lens gets. This means that the right tint intensity is always achieved for the level of light. When the level of UV rays drops, the molecules 'fold' again and the lens becomes clearer. The speed at which the molecules act is a good indicator of the quality of self-tinting lenses.