Photochromic

Photochromic describes materials that reversibly change color or opacity in response to light, most often ultraviolet radiation. When exposed to activating light, the material undergoes a chemical or structural change that increases light absorption, producing a visible color or darkening. When the activating light is removed or the material is exposed to heat or to visible light, it gradually returns to its original, lighter state. The effect is designed to be reversible and repeatable, with performance depending on the specific chemistry and environmental conditions.

Several classes of photochromic systems are used in commercial products. Organic photochromic compounds, such as spiropyrans, spirooxazines, and diarylethenes, change between a colorless or lightly colored form and a colored isomer through reversible isomerization or cyclization. In many eyewear lenses, a different approach is used: silver halide microcrystals dispersed in a polymer matrix darken under UV exposure and fade in visible light or with heat. This inorganic mechanism is common in self-tensing sunglasses and can be engineered for rapid darkening and fading.

Applications span eyewear lenses, architectural glass, automotive glazings, textiles, and inks. Advantages include automatic tinting in bright light and potential energy savings from reduced need for active shading. Limitations can include slower fading in low temperatures, gradual fatigue after repeated cycles, and reduced performance with extreme heat or prolonged UV exposure. Research continues to improve color stability, response speed, and durability across diverse environments.