Fig. 1. Representative emission spectra of the investigated 310 nm UV LEDs normalized to the emission peak. The spectra were measured at 100 mA at a heat sink temperature of 25°C before aging and after aging experiment 3 (Table 1).

Fig. 2. Optical power over time of LEDs run at different current densities. The values are normalized to the initial value and averaged over 15 LEDs. For all currents, the junction temperature was kept at (90±5)°C.

Fig. 3. Operation time at which the optical power has reduced to 70% (t70%) of the initial value versus current density.

Fig. 4. Mean normalized optical power of the LEDs versus the product of the operation time and the cube of the current density.

Fig. 5. Mean normalized optical power (see Fig. 2) versus logarithmic time scale. For different current densities, the characteristic times τ are indicated. Also shown are the logarithmic functions (dashed lines) from Eq. (2) and the extended logarithmic function (solid line) from Eq. (4).