Fotochromische

Fotochromische, or photochromic, describes materials that change their optical properties in response to light. The effect is usually reversible: exposure to ultraviolet light triggers a color change that darkens the material, while removal of the light returns it toward its original clear state. This phenomenon is widely used in eyewear and, to a lesser extent, in architectural glazing and other coatings.

In ophthalmic applications, fotochromische lenzen (photochromic lenses) contain small molecules such as spiropyrans or naphthopyrans embedded in the lens polymer. When UV light penetrates the lens, these molecules undergo a reversible chemical transformation to a colored form that absorbs visible light, reducing transmission. Inorganic approaches exist as well, using metal-doped glass, but organic photochromic compounds are most common in consumer eyewear today.

Performance depends on several factors, including ambient UV intensity, temperature, and the specific chemistry of the photochromic system. Lenses typically darken more quickly in bright sun and may fade more slowly at lower temperatures. Indoor light or shade reduces darkening, and some lenses do not reach the darkness of dedicated sunglasses. The return to clear can take seconds to minutes, influenced by formulation and temperature. Over time, photochromic performance can degrade, potentially requiring replacement of the lenses.

Historically, photochromic materials were developed in the mid-20th century and became widely available in consumer eyewear through brands such as Transition Optical. Beyond lenses, fotochromische coatings or glass are used in certain windows and glazing applications to provide variable shading and glare control, though they are less common than in sunglasses.