Thin film filter design and fabrication for human safe far-UVC lamp with excimer light sources

Dielectric Barrier Discharge (DBD) excimer lamps emitting at 222 nm are promising for sterilization due to strong antimicrobial activity and reduced risks to human health. Unlike conventional 254 nm mercury lamps, Far-UVC at 222 nm penetrates microorganisms but not living human cells, enabling safe use in occupied spaces. This study reports the design and fabrication of optical thin-film filters that selectively transmit 222 nm radiation while blocking harmful 254 nm light. Filters were designed with OpenFilters software and deposited on quartz substrates using 2-inch magnetron sputtering. The multilayer structure of Si3N4, Al, and SiO2 showed ~ 7% transmittance at 222 nm with complete blocking at 254 nm. Microbiological experiments were performed using a KrCl2 excimer lamp operated with and without the filter. For Escherichia coli, the filtered lamp achieved log reductions of up to 1.47 after 45 min, while the unfiltered lamp produced reductions exceeding 4 log under identical conditions. Similar trends were observed for Staphylococcus aureus. Although reduced microbial inactivation was observed with the filtered system, the effective suppression of hazardous wavelengths demonstrates its potential for safer Far-UVC disinfection. These results highlight the role of 222 nm band-pass filters in improving the safety and applicability of DBD excimer lamps for UV sterilization.