## 4 Discussion  
The results of the measurements of the optical power and spectral parameters of the tested LEDs (Figures 4-8) showed that in order to ensure the stability of the \(OP\) of UVC LEDs in disinfection systems, the changes to the specific operating conditions (\(I,T\)) as well as their impact on the degradation process resulting in a decrease in the electroluminescence (\(OP\)) over time must be known. The results of our tests indicated that the time of operation affects the characteristics of high-power UVC LEDs. As predicted, similarly to LEDs operating in other spectral ranges, the supply current during the tests, the housing and junction temperatures, and the structure of the emitter are also important.  
According to our results, the degradation of the \(OP\) of the tested high-power UVC LEDs was the strongest during the initial 100 h of operation, after which the process was slower (Figure 4). This observation confirms the results available in the literature [40]. The analysis of the influence of the degradation on the spectral parameters of the radiation of the UVC LEDs showed that the changes in the value of \(\lambda\) and the _FWHM_ were not more significant than \(+/-0.5\) nm and could be considered insignificant. On the contrary, the increase in the LED housing temperature affected its spectral parameters enough to be evident in terms of the disinfection reliability. According to our calculations and including the germicidal effectiveness curves (Figure 1), we may state that even such minor modifications to the value of \(\lambda\) of UVC radiation, as obtained in our tests, may affect the exposure time required to achieve a certain level of disinfection. A more significant impact was observed in terms of the thermal changes in \(OP\). According to our research, the decrease in the \(OP\) was the most substantial effect of the time-related degradation of UVC LEDs. Consequently, it had the highest impact on the required disinfection time \(t_{\text{dis}}\) (Figures 11b and 12b), which was inversely proportional to the changes in the relative germicidal effectiveness \(RGE\) (Figure 9b). A similar relationship should be considered to include the effect of the temperature on the \(OP\).  
An LP-Hg lamp (currently the most popular germicidal lamp) was chosen as a reference with the assumption that a constant irradiance was maintained. This result means that when the LED1 emitting peak wavelength at 271 nm was used, a higher effectiveness is obtained, and therefore, a shorter disinfection time could be applied to achieve the same effect. This was caused by the difference in the peak wavelength of the emitted optical radiation and the position of this wavelength concerning the peaks of the germicidal curves in Figure 1. The peak of LED1 was the nearest to the peaks of these curves, and therefore, it may be recognized as the most efficient if the spectrum is considered. When comparing the \(RGE\) characteristics for monochromatic wavelengths and the real radiation of LED and LP-Hg sources with actual spectral distributions, it should be emphasized that the differences were noticeable and reached up to 10%, as shown in Figure 2 versus Figure 9a.