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Principle of Photochromic Lenses
1. UV-Triggered Tinting: The lens contains light-sensitive materials like silver halide or spiropyran-based compounds, which undergo reversible chemical reactions when exposed to UV light. This causes the lens to darken, blocking intense light and absorbing UV radiation.
2. Transparency Restoration: In low or no UV conditions (e.g., indoors), the chemical structure reverts to its original state, restoring transparency for clear vision.
3. Self-Adjusting Mechanism: The entire process is automatic and dynamic, balancing optical clarity with light protection without manual intervention.

Summary: The lens achieves "darkening under light, transparency in darkness" through molecular structural changes in UV-responsive materials.