## 1 Introduction
Despite that, due to their small size, they are increasingly being used to construct devices for inactivating microorganisms; a solution with a conventional UVC lamp is impossible or much more complex [31], e.g., in water purifiers [32], containers for the disinfection and sterilization of small items [32], and gas detection sensors [33]. Independently from the application, the efficiency and reliability of the deactivation of the pathogen depends on the stability of the _OP_ delivered to the disinfected area. The highest contribution to its instability comes from changes in the characteristics of the source of UVC radiation, which are caused by its degradation. The mechanisms of the degradation  
Figure 1: Relative spectral characteristics of the germicidal effectiveness of UV radiation according to [19; 20; 21] (average—average DIN and IESNA curves). of the \(OP\) have been the subject of many scientific studies. Among them, the primary concern is the gradual degradation related to the operation time. This phenomenon occurs with the successive loss of \(OP\) of the source and is also known for all types of sources for all wavelength ranges; for example, it is the reason for oversizing lighting installations to ensure at least the minimum maintained illuminance on a working plane during an assumed period of operation. Similarly, the sources of UVC radiation should be controlled adequately considering the decrease in the \(OP\) in order to maintain the required dose of radiation at the disinfected area. Regardless of the technology of the source of UVC radiation, the effects of the sources' degradation should be considered by the designers of disinfecting devices and systems. The degradation rate of optical radiation sources is assessed using standard methods [34; 35; 36], in which the essential part is conducting a degradation procedure called an accelerated life test (ALT). On this basis, the characteristics of changes in the emitted optical radiation in time can be recognized and predicted. For example, the lifetime of an LP-Hg lamp can be estimated as about 9000 h of operation (depending on the manufacturer), during which the decrease in the \(OP\) reaches about 30%. As this type of lamp has been known for many years, this knowledge is widely available, and the manufacturers provide the data. High-power UVC LEDs are still an emerging technology; therefore, there is a lack of manufacturer information and experimental data on the influence of the operation conditions on their performance and, consequently, on their disinfection process efficiency. Few papers considering the changes in the characteristics of UVC LEDs with an \(OP\) of over 10 mW have been published. In [37; 38], the authors showed that 50% of the \(OP\) of a 280 nm cooled LED emitter is lost after about 3000 h. The results of the research described in [3] indicate that the \(OP\) of a UVC LED with a wavelength of 262 nm operating at a constant current of 100 mA decreases by 42% after 250 h of operation [39]. The authors of some works have proven that a more substantial degradation of electroluminescence over time is observed for LEDs with an emission peak of 275-278 nm [40], while the opposite relationship was demonstrated in [41] for an LED with an emission peak of 275 nm. In none of these papers were the consequences for disinfection efficiency evaluated. Currently, UVC LEDs are increasingly being used in several applications. However, apart from reliability analyses and lifetime estimations [27; 40; 41; 42; 43; 44; 45], there has been no comparative analysis of the application reliability of commercially available UVC high-power LEDs in the scientific literature. Therefore, this study aims to experimentally assess the effect of degradation of currently available high-power UVC LEDs on their disinfection efficiency. Additionally, the focus on the consequences of selecting various wavelengths of radiation emitted by the LEDs is considered. This problem was not significant before the era of high-power UVC LEDs when only LP-Hg lamps were used.