## 3 Results and Analysis  
### Lumen Maintenance Characteristics of OLED Panels and Luminaires  
To investigate the long-term reliability of OLED light panels and luminaires, we have measured the maintained lumen output characteristics. Maintained lumen output of OLED panels and luminaires after six thousand hours to their initial value is shown in Figure 2a,b, respectively. All OLED panels show about 80% of lumen maintenance and similar degradation characteristics even at six thousand hours of operation. In case of OLED luminaires Type E and F, lumen output degrades more steeply even at two thousand hours compared to OLED panels (type A, B, C, and D). This is because, in case of OLED luminaires, lumen degradation effect got worse by the effect of the control gear which is integrated in the luminaires and the overall luminaire design including thermal management. Moreover, the difference in the degradation pattern of OLED luminaires from one thousand hour to four thousand hour came from each unique design by the different luminaire manufactures. To evaluate the lumen maintenance of OLED panel and luminaires, lumen output data of three samples in each type were averaged. As the results, the averaged lumen maintenance of each OLED type at six thousand hours aging was evaluated in Table 3. As shown in Table 3, lumen outputs of all OLED panels were maintained to 80% after six thousands operation with rated condition. However, in case of OLED luminaires, maintained lumen output was 50% for type E and 40% for type F, respectively. For luminous efficacy, initial values of OLED panels were 50 lm/W for OLED panel type A, B, and C and 40 lm/W for OLED panel type D. After six thousand hours of operation, the luminous efficacy was decreased by amount of 10 lm/W. Figure 1: Experimental setup for operating OLED panels. (**a**) Integrating sphere measurement system for OLED products. (**b**) Operating conditions with fixed temperature in a chamber. International Electrotechnical Commission (IEC) technical committee 34 (title: lighting) has responsibility for safety and performance standard of OLED product for general lighting. Up to date, there is no international standard or industry standard, i.e., de facto standard for lifetime estimation of OLED product for general lighting. However, IEC 62922 (OLED panels for general lighting-Performance requirements) [17] standard, now in amendment stage, covers lifetime issue of OLED product for general lighting. In this amendment, process for lifetime evaluation is similar to that of LED packages for general lighting applications. To evaluate the lifetime of LED packages, there were many discussions over the couple of years. Then ANSI/IES TM-21: Projecting Long Term Lumen, Photon, and Radiant Flux Maintenance of LED Light Sources standard [18] and IEC 63013: 2017: LED packages--Long-term luminous and radiant flux maintenance projection standard [19] were developed and well adopted in the lighting industry. In these standard, exponential least squares curve-fit method was used with averaged normalized luminous flux data as shown in Equation (1). \[\Phi(t)=Bexp(-\alpha t) \tag{1}\]  
where:  
\(t\) = operating time in hours  
\(\Phi(t)\) = averaged normalized luminous flux output at time t  
\(B\) = projected initial constant derived by the least squares curve-fit  
\(\alpha\) = decay rate constant derived by the least squares curve-fit \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Type**} & **Maintained Lumen** & **Initial Luminous** & **Maintained Luminous** \\  & **Output at 6000 h [\%]** & **Efficacy [Im/W]** & **Efficacy at 6000 h [Im/W]** \\ \hline A & 79.60\% & 48.08 [Im/W] & 38.44 [Im/W] \\ B & 79.52\% & 47.16 [Im/W] & 37.88 [Im/W] \\ C & 78.57\% & 49.53 [Im/W] & 39.83 [Im/W] \\ D & 78.16\% & 39.64 [Im/W] & 31.58 [Im/W] \\ E & 50.47\% & 21.28 [Im/W] & 9.41 [Im/W] \\ F & 41.19\% & 22.64 [Im/W] & 8.14 [Im/W] \\ \hline \hline \end{tabular}
\end{table}