Mechanism of Light Stabilizers

The mechanism of a light stabilizer depends on its structure and type. Some shield or reflect UV radiation, or absorb it and convert it into harmless heat; some quench the excited states of UV-excited molecules or groups, returning them to the ground state and thereby ruling out or slowing photo-oxidation-reduction reactions; others capture the free radicals generated by photo-oxidation-reduction, blocking the radical chain reactions that age a product and protecting it from UV damage. 
  In industry, effective prevention and retardation of photoageing is mostly achieved by combining two or more anti-ageing agents with different mechanisms, because each anti-ageing agent, especially light absorbers, has its own UV absorption band. Composite formulations such as benzophenone + benzotriazole + hindered amine (HALS) can achieve the optimal effect unattainable by a single light stabilizer.  
The UV absorbers added to sunscreens work on the same principle: they scatter or absorb the UV radiation reaching the skin surface, so that the skin is spared or receives less UV damage.