## 2 Materials and Methods  
Disinfection is a process that depends on the effective delivery of a necessary amount of energy to a surface, which is determined by the dose of energy required to inactivate the microorganisms at the level of 90% (in the air or at the surface). It can be expressed by the radiant energy (in J) delivered to a specific area or its surface density (radiant exposure) (in J/m\({}^{2}\)) [24]. These values depend on the environmental conditions (e.g., relative humidity) and the type of microorganism. For LP-Hg lamps, they typically stay within the range from 20 J/m\({}^{2}\) to 200 J/m\({}^{2}\)[24]. Technically, this dose of energy, \(D\), can be delivered by generating a specific value of irradiance, \(E\) (in mW/cm\({}^{2}\) or W/m\({}^{2}\)), on the disinfected surface and keeping it constant during the disinfection procedure lasting for time \(t_{dis}\) [s]:  
\[D=E\cdot t_{dis}, \tag{1}\]The delivered energy destroys the molecular structure of the microorganisms, but different wavelengths of optical radiation are characterized by different levels of germicidal effectiveness (Figures 1 and 2). For this reason, to predict the germicidal efficiency of the procedure, the \(OP\) of the radiation used for disinfection should be corrected to obtain the value of the effective power:  
\[OP_{g}=\int_{0}^{\infty}OP(\lambda)\cdot S_{g}(\lambda)d\lambda, \tag{2}\]  
\[S_{g}(\lambda)=\frac{S_{gIESNA}(\lambda)+S_{g,DIN}(\lambda)}{2} \tag{3}\]  
where \(OP_{g}\)--the germicidal weighted optical power (W), \(OP(\lambda)\)--the spectral power distribution (_SPD_) of optical radiation, and \(S_{g}(\lambda)\)--the relative spectral sensitivity of the microorganism, which is equal to the average (Equation (3)) spectral germicidal effectiveness (black curve from Figure 1). The closer the radiation emission wavelength is to the maximum of the germicidal effectiveness curve (Figures 1 and 2), the higher its expected effectiveness, and thus, it is possible to reduce the working time or the power density of the radiation reaching the surface. This, in turn, affects the system's geometry and the number and power of the radiation sources used. Sources manufactured with semiconductor technology (LEDs) are considered sources whose operation is affected by the temperature. Therefore, a necessary stage in constructing these disinfection systems is to determine the operating temperature of the UVC LED. The temperature changes may affect the \(SPD\) of the emitted optical radiation at various stages of LED operation. The changes in the \(SPD\) may include a reduction in the \(OP\), a shift in the emission spectrum \(\Delta\lambda\), and a change in the half-width of the spectrum, \(\Delta FWHM\). Consequently, a change in the germicidal weighted power, \(OP_{g}\), of the source and in the disinfection time \(t_{dis}\) required to achieve its deactivation at the assumed level may occur. Therefore, it is essential to know how their SPD changes over time to predict the changes in the disinfection efficiency and the required time of the procedure caused by the degradation of the UVC LED emitters. We conducted research according to the following procedure to obtain data for the analysis. In the first step, the electro-optical characteristics of 6 different UVC LEDs were measured. For this study, emitters of various designs (Figure 3) with an emission in the spectral range of 270-280 nm and an \(OP\) in the 10-100 mW range were selected (Table 1). Based on the obtained results, the functional effects of their degradation were assessed. Figure 2: Relative germicidal effectiveness of UV radiation for selected monochromatic wavelengths (average GDI—average RGE of Gates DIN and IESNA curves; average DI—average of DIN and IESNA curves) [19; 20; 21]. The measurements were carried out in an air-conditioned optical darkroom.