a photosterilization reactor for sterilization of a liquid or a gas using ultraviolet light is disclosed that overcomes many of the shortcomings of current ultraviolet sterilization systems and improves the efficiencies of the sterilization process . the photosterilization reactor has an optically coupled flow path for maximizing the cross - sectional interaction between the ultraviolet radiation and the liquid or gas in the flow path , a reactor vessel for the efficient entrapment , reflection and use of the generated and unabsorbed ultraviolet radiation , and operational improvements to the ultraviolet light source , thermal and mechanical shielding of the ultraviolet light source , and electrical safety improvements . the photosterilization reactor may further contain a novel fluoroscopic pathogen detector that provides for real time monitoring of pathogens in the liquid or gas flow path internal to the reactor .