Novel bandpass filter for far UV-C emitting radiation sources

Disinfection with far UV-C radiation (<230 nm) is an effective method to inactivate harmful microorganisms like the SARS-CoV2 virus. Due to the stronger absorption than regular UV-C radiation (254 nm) and hence limited penetration into human tissues, it has the promise of enabling disinfection in occupied spaces. The best far-UV sources so far are discharge lamps based on the KrCl* excimer discharge peaking at 222 nm, however they produce longer wavelength radiation as a by-product. In current KrCl* excimer lamps usually a dichroic filter is used to suppress these undesired longer wavelengths. A phosphor-based filter is an alternative which is cheaper and easier to apply. This paper describes the results of our exploration of this opportunity. Various compounds were synthesized and characterized to find a replacement for the dichroic filter. It was found that Bi³⁺-doped ortho-borates with the pseudo-vaterite crystal structure exhibit the best absorption spectrum i.e. high transmission around 222 nm and strong absorption in the 235-280 nm range. Y_0.24Lu_0.75Bi_0.01BO₃ showed the best absorption spectrum in the UV-C. To suppress the unwanted Bi³⁺ emission (UV-B), the excitation energy can be transferred to a co-dopant. Ho³⁺ turned out to be the best co-dopant, and Ho_0.24Lu_0.75Bi_0.01BO₃ appeared to be the best overall candidate for the phosphor filter material. A suitable formulation for a coating suspension containing this material was found, and quite homogeneous coatings were achieved. The efficiency of these filter layers was investigated and the results in terms of exposure limit increase i.e. gain factor vs. no filter were compared with the dichroic filter. We achieved a gain factor for the Ho³⁺ containing sample of up to 2.33, i.e. not as good as that of the dichroic filter (∼4.6), but a very relevant improvement, making Ho_0.24Lu_0.75Bi_0.01BO₃ an interesting material for a cost-effective filter for KrCl* far UV-C lamps.