fotocromische

Fotocromische materials are substances whose optical properties change in response to light, most commonly a transition from clear to dark under ultraviolet exposure and a return to transparency when the light diminishes. The change is reversible, allowing the material to adapt to changing light conditions automatically.

Most photochromic systems in consumer products are organic compounds, such as spiropyrans or spirooxazines, that undergo photoisomerization. UV light induces a colorless form to transform into a colored, more planar form; when the light is removed, the molecule relaxes back to its original state. In some inorganic glasses, light changes the arrangement of silver halide grains, producing a temporary darkening.

The best-known application is in ophthalmic lenses and sunglasses that darken outdoors and return indoors. Photochromic materials are also used in architectural glazing, textiles, inks, and cosmetics, though to varying degrees of effectiveness.

Performance depends on temperature, light intensity, and exposure duration. Higher temperatures can slow darkening and accelerate fading; indoor lighting generally does not trigger full darkening. Durability varies with the chemical class and formulation and, with repeated cycling, some materials may show gradual fading or reduced darkness.

Historically, commercial photochromic lenses appeared in the mid-20th century and have since evolved toward faster switching, lighter coloration, and broader spectral response.