Narrow-bandwidth LEDs arrays as versatile, medium-power UV-C sources.

Light Emitting Diodes (LEDs) are a new class of UV-C ({200\mbox$--$280 ${nm}$) radiation source, which attracted the attention of specialists. UV-C LEDs can be organized in multi-element arrays, resulting in medium-power devices. They transform about $2%$ of the electrical power into radiation. This requires rapid dissipation of excess heat. At the ENEA Centre in Frascati, we assembled arrays of LEDs emitting radiation peaked at $277 {nm}$, with $11 {nm}$ FWHM bandwidth and $120^{\circ}$ full angle of emission. We have developed software that calculates the spatial distribution and average radiation value of UV-C intensity $vs.$ distance from the LED array with a given number of LEDs and their and relative distance. The results are in excellent agreement with the experimental data. Despite the aspect ratio $\approx 3$ of the array, the UV-C spot has an almost circular shape at any distance larger than the short side of the array. After a description of our LEDs arrays, we present the results of two irradiation applications: 1) enhancement of hormesis (innate defence mechanisms of plants against pathogens) in fruits and vegetables exposed to pathogens, and 2) low-dose inactivation of SARS-Cov-2 $in vitro$.