## 2 Materials and Methods
Throughout the measurements, the ambient temperature was maintained at 20 \({}^{\circ}\)C.  
The measurements included a study of the characteristics of the \(OP\) and \(SPD\) (\(\lambda\) and \(FWHM\)) as a function of the current, \(I\), and the housing temperature, \(T\), before and after the degradation process lasting 1500 h of continuous operation with a DC power supply. During this period, the LEDs were never switched off. To perform such measurements, a dedicated and secure stand with micrometric control of the position of the LEDs against measuring tools was constructed. The measuring stand was based on a basic layout for irradiance and radiant intensity measurement (photodetector next to the radiation source at a constant distance). The field of view of the photodetector was limited by a dedicated shielding element to exclude the effect of other LEDs, and, based on the changes in the irradiance, the changes in the \(OP\) were concluded as relative changes in the \(OP\) following the relative changes in the irradiance. The \(OP\) and \(SPD\) measurements were performed for 20 \({}^{\circ}\)C and 60 \({}^{\circ}\)C housing temperatures and were stabilized by a controlled Peltier-based thermal system using a Cooltronic TC2812 Peltier cell controller, Pt100 temperature sensors, and two Peltier cells with a power of 89 W each. The measurements were carried out by setting the temperature of the LED housing, \(T\), starting from the lowest (20 \({}^{\circ}\)C), and then the value of the supply current was set, starting from \(I_{\text{max}}\) and reducing it in 10% steps. The LED emitters were supplied separately with direct current with the maximum intensity specified by the manufacturer for the housing temperature of \(T=60\)\({}^{\circ}\)C (Table 1). The \(OP\) was measured with an accuracy of \(\pm 7\%\) (Thorlabs PMD100D power meter with an S120 VC UV extended Si photodiode), while the \(SPD\) was captured with a cooled Stellarnet Silver fiberoptic spectrometer. Based on the measurement results in the second step, the changes in the \(OP\), \(\lambda\), and \(FWHM\) due to the degradation of the tested LEDs were assessed, and their impact on the \(t_{\text{dis}}\) values for the selected microorganisms was analyzed. The results were compared to the times required for the same process conducted using a non-degraded LP-Hg lamp. For this \begin{table}
\begin{tabular}{c c c c c c} \hline LED & \begin{tabular}{c} **Absolute Maximum** \\ **Current I\({}_{\text{max}}\)** \\ **(mA)** \\ **\end{tabular} & \begin{tabular}{c} **OP at Current** \\ **(mW @ mA)** \\ **\end{tabular} & \begin{tabular}{c} **Peak Wavelength** \\ **(nm)** \\ **\end{tabular} & \
\begin{tabular}{c} **FWHM** \\ **(nm)** \\ **\end{tabular} &
\begin{tabular}{c} **Current During** \\ **(mA)** \\ **\end{tabular} \\  & * & ** & ** & ** \\ \hline
1 & 150 & 10.5 @ 100 & 271 & 13 & 90 \\ \hline
2 & 150 & 52–72 @ 150 & 277 & 14 & 150 \\ \hline
3 & 150 & 32 @ 100 & 277 & 14 & 105 \\ \hline
4 & 450 & 100 @ 450 & 278 & 11 & 350 \\ \hline
5 & 500 & 50 @ 350 & 280 & 11 & 350 \\ \hline
6 & 500 & 55 @ 350 & 280 & 11 & 350 \\ \hline \end{tabular}
\end{table} 
Table 1: Basic data of tested UVC LEDs. Figure 3: View of LED chips.