There is a newcomer on the block that has gained considerable media attention: far-ultraviolet excimer lamps. Recent medical studies have indicated that, unlike 254 nm radiation, the 207 nm and 222-nm “far-UV” radiation emitted by excimer lamps is likely harmless (e.g., Buonanno et al. 2017, Welch et al. 2018). Excimer lamps have the same germicidal properties as mercury-vapor discharge lamps, but the shorter wavelength radiation cannot penetrate deeply enough into the outermost cells of the eyes and skin to disrupt their DNA.
This leads to the thought that we may be able to design UV-C germicidal systems using far-UV excimer lamps. Unlike mercury-vapor lamps and UV-C LEDs, there does not appear to be any significant photobiological risk (if their residual UV-C emissions are blocked), and so they could be deployed in direct view of the room occupants while disinfecting both the air and contaminated surfaces with their radiation.
Indeed, there are already companies advertising such products, although they do not appear to be commercially available as yet. This does not stop us, however, from asking the question: what does it take to design a UV-C disinfection system using far-UV radiation?