## 3 Results  
Figure 4 presents the changes in the \(OP\) of the radiation of the tested UVC LEDs over time after 1500 h of continuous operation. The \(OP\) degradation rate for each of the tested LEDs was different. The decrease in the measured \(OP\) for most of the tested LEDs was between 21% and 44%. In the case of one of the tested LEDs, catastrophic degradation occurred (LED4, multi-emitter chip); consequently, the \(OP\) drop was significantly higher (over 80%).  
The differences in the \(OP\) degradation rate may have been caused by several reasons, for example, differences in the internal structure of the tested chips, various current densities, and various thermal resistances of the chips. These data were not available in the manufacturers' datasets, but each affects the operation conditions of an LED and its junction temperature. Figure 5 presents the characteristics of the \(OP\) of the tested LEDs for two test temperatures before and after the degradation process. For each LED, we observed a significant change in the shape of the characteristic after the procedure time, which occurred with the decrease in the \(OP\).  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Pathogen**} & _Escherichia_ & _Pseudomonas_ & _Staphylococcus_ & _Bacillus_ & _Apergillus_ & **SARS-CoV-2** & **Rotavirus** \\  & _coli_ & _aeruginosa_ & _aurus_ & _subtilis_ & _niger_ & **SARS-CoV-2** & **Rotavirus** \\ \cline{2-8}  & **Bacteria** & **Bacteria** & **Bacteria** & **Bacteria** & **Fungi** & **Virus** & **Virus** \\ \hline D\({}_{\text{LP-Hg}}\) (J/m\({}^{2}\)) & 30 & 55 & 26 & 71 & 1320 & 18 & 81 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Doses of incident radiation \(D_{LP\_Hg}\) (for a wavelength of 253.7 nm) required for deactivation of 90% of selected pathogens in laboratory conditions [47].  
Figure 4: Changes in \(OP\) of tested UVC LEDs over time (\(OP\) of each LED relative to \(OP\) of this LED at time 0 h and temperature 60 \({}^{\circ}\)C).  
Figures 6 and 7 present the relative changes in the _SPD_ of the tested UVC LEDs due to the time of operation (Figure 6) and temperature (Figure 7). According to these results, the degradation process caused a change in the peak wavelength of \(\Delta\lambda=\pm 0.5\) nm (Figure 8a) and the change in the _FWHM_ of \(\Delta FWHM=\)\(<\)\(-0.5\) nm; 1.0 nm \(>\) (Figure 8b) depending on  
Figure 5: Characteristics of _OP_ as a function of the supply current of the tested UVC LEDs before and after the degradation process for two housing temperatures (20 \({}^{\circ}\)C and 60 \({}^{\circ}\)C) (_OP_ of each LED relative to _OP_ of this LED at time 0 h and temperature 20 \({}^{\circ}\)C).  
the UVC LED type. An increase in the temperature of the LED housing from 20 \({}^{\circ}\)C to 60 \({}^{\circ}\)C (Figure 7) caused an increase in the wavelength at the emission peak \(\Delta\lambda\) of 0.5-1.0 nm (Figure 8a) and the \(\Delta FWHM\) of 0.5-1.0 nm (Figure 8b). A more significant impact was observed regarding energy efficiency; an increase in the temperature from 20 \({}^{\circ}\)C to 60 \({}^{\circ}\)C resulted in a reduction in the \(OP\) of from 4 to 9% (Figures 5 and 7).  
Although the detected changes in the value of \(\lambda\) and the \(FWHM\) of the tested LEDs seem to be not very significant, they may affect the disinfection effectiveness due to the strong dependence of the pathogen sensitivity on the spectral distribution of the radiation (Figure 1).  
Figure 6: Relative SPDs of UVC LEDs measured before (solid line) and after degradation procedure (dotted line) (SPDs of LEDs before the degradation process are the reference); \(T=60\) \({}^{\circ}\)C, \(I_{LED}=I_{max}\), LP-Hg spectrum was added as a reference to illustrate the difference in the shape of SPD.  
Figure 7: Relative SPDs of UVC LEDs measured for emitter housing temperatures of \(T=20\) \({}^{\circ}\)C and \(60\) \({}^{\circ}\)C; \(t=1500\) h, \(I_{LED}=I_{max}\). LP-Hg spectrum was added as a reference to illustrate the difference in the shape of SPD.