## 1 Introduction  
The SARS-CoV-2 virus pandemic has highlighted the importance of biosecurity in everyday life [1; 2; 3]. Facing global health problems and the anticipated challenges related to the development of cities, as well as forecasting subsequent pandemics, increased importance has been placed on preventing the transmission of pathogens [4]. For many years, various methods of deactivating pathogens have been used, including primarily active chemical substances of high toxicity [5], electrostatic disinfection technologies [6], and UV radiation [4; 7]. The last one is currently being developed in many areas, including the disinfection of water [8; 9], air [10], surfaces [11], skin [12], food products [13; 14], and crops [15; 16]. The main advantages of ultraviolet radiation disinfection are its fast action, low cost, the possibility of full automation, and the lack of risks associated with using chemicals containing chlorine compounds [17]. The availability of effective and safe devices plays a crucial role in developing new technological solutions based on applying ultraviolet radiation. Due to the effects of ultraviolet radiation on living organisms, non-germicidal bands (UVA 315-400 nm) and actinic bands (UVB 280-315 nm, UVC 200-280 nm, and VUV 100-200 nm) are distinguished [18]. The UVB and UVC radiation bands cover the DNA and RNA absorption spectrum, which enables the elimination of viruses, bacteria, and fungi [18]. Still, not every radiation wavelength in this range is characterized by the same germicidal effectiveness [19; 20; 21; 22]. Even a slight change in the wavelength of UV radiation used for disinfection results in a significant modification to its germicidal effectiveness (seeFigure 1), e.g., between wavelengths of approximately 265 nm and 255 nm or 275 nm, its decrease is about 10-20% [23]. The modern germicidal effectiveness curves used are the IESNA and DIN curves, published in [24]. Although the older Gates curve, proposed in 1930, is not very popular nowadays, it is still sometimes mentioned in research as it was elaborated based on experiments with multiple species, in contrast to the other curves [25]. According to DIN, the maximum inactivation efficiency equal to 1 is for the wavelength range from 265 nm to 267.5 nm, and for IESNA, it is only for 265 nm [26]. Currently, the most popular sources of UVC radiation for disinfection are low-pressure mercury lamps (LP-Hg) [27] emitting narrowband radiation cumulated around 253.7 nm. The germicidal effectiveness of this wavelength is between 60%, according to [19], and 80% [20,21]. This source of UVC radiation is well characterized, and the maintenance of its operation can be easily predicted. However, due to the harmfulness of mercury, the UN announced the desire to eliminate mercury-containing products, including their significant reduction after 2020. For this reason, UVC LEDs have been developed as the most promising technology to replace LP-Hg lamps. UVC LEDs are characterized by an emission spectrum comparable to that of an LP-Hg lamp--a typical half-width of the spectrum (_FWHM_) in the range of 250-280 nm is 10-15 nm [28]. Their significant advantages are their low energy consumption and the availability of radiation sources with a maximum emission at different wavelengths, including those close to the maximum spectral germicidal effectiveness (_RGE_) (Figure 1). In addition, unlike LP-Hg lamps, when the power supply is turned on, they emit radiation at the full predicted power, and during their operation, it can be easily adjusted electronically; therefore, they are suitable for smart applications. Although the thermal emission per 1 W of electric power in UVC LEDs is higher than that from LP-Hg lamps, it is easier to manage the heat during operation, ensuring thermal stability [29]. Among the disadvantages of UVC LEDs, the relatively high price and the low optical power (_OP_) of single chips (currently up to about 100 mW), as well as energy efficiency (currently 5-10%) [30], should be listed.