# Multi-Azimuth Failure Mechanisms in Phosphor-Coated White LEDs by Current Aging Stresses
## 3 Results and Discussions  
### Photometric and Chromatic Analyses  
Figure 2 depicts optical decays of hppc-WLEDs, measured at 350 mA and 25 \({}^{\circ}\)C of the heat-sink temperature, for CSA-1, CSA-2, and CSA-3, respectively. As observed from Figure 2, significant optical decays occur in CSA-3 after aging for 3840 h, indicating that high current stresses will induce severe optical decays. Average optical decays after aging for 3840 h in three cases are estimated as 10.2%, 11.0%, and 15.2%, respectively.  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Instrument** & **Model** & **Characteristic** \\ \hline Spectrometer & Spectro-320e & Wavelength: 170–1700 nm \\ Integrating Sphere & ISP-500, ISP-150 & Sizes: 500 mm, 150 mm \\ Angular Distribution Analyzer & LEDGON 100 & Angle range: 360\({}^{\circ}\); Angle accuracy: 1\({}^{\circ}\) \\ Transient Thermal Tester (T3 Ster) & T3 Ster 2000/100 & Testing currents: 0–500 mA; Resolution: 0.1 \({}^{\circ}\)C \\ Electric Source Meter & Keithley 2611 & Maximum current: 1.5 A; Maximum voltage: 200 V \\ Temperature Controller & Keithley 2510 & Temperature range: 0–80 \({}^{\circ}\)C; Resolution: 0.001 \({}^{\circ}\)C \\ Field emission Scanning Electron & Sigma-HD & Magnifications: 10–1,000,000\(\times\); Resolution: 1.0 nm \\ Microscopy (FE-SEM) & & \\ Energy Dispersive Spectrometer & X-Max\({}^{\text{H}}\) & SDD detector: 50 mm\({}^{2}\); Accuracy: 5 nm \\ (EDS) & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Various items and their characteristics of instruments used in the experimental study.  
Figure 2: The normalized optical power versus aging time in three cases.  
To illustrate the change of the correlated color temperature (CCT) [26], we desire to introduce a parameter, \(\delta CCT\), which can be written as  
\[\delta CCT=CCT[after]-CCT[before], \tag{1}\]  
where \(CCT[before]\) and \(CCT[after]\) denote the CCT value before and after aging, respectively. Figure 3a plots the variation of CCT as a function of time. As observed, values of \(\delta\)CCT increase as time elapses, but with mild variations. Specifically, after aging for 3840 h, all values of \(\delta CCT\) in three cases lie within 200 K. Figure 3b-d show measured SPDs in three cases prior to and after aging. Both blue and yellow emissions decrease simultaneously after 3840 h aging. Optical decays of blue and yellow emissions are separately illustrated in these figures. In all three cases, severe optical degradations can be observed from yellow emissions relatively to blue counterparts. Especially in CSA-3, the optical decay reaches 12.2% for the blue emission and 17.1% for the yellow emission. For clear illustration, we calculate the ratio of the yellow optical power to the blue optical power, namely \(Y/B=P_{yellow}/P_{blue}\), which indirectly describes the phosphor-conversion efficiency (PCE) of hppc-WLEDs [27]. Values of \(Y/B\) are also shown in Figure 3b-d. Changes of \(Y/B\) are estimated as 2.0%, 4.3%, and 5.6% for CSA-1, CSA-2, and CSA-3, respectively. They imply that PCE slightly decreases after aging, because of the wavelength shift of blue LED chips after 3840 h aging [28]. The higher the aging current becomes, the more decrease PCE exhibits. Therefore, we conclude that the decrease in PCE during CSA tests leads to optical decays for these hppc-WLEDs.  
Figure 3: **(a)** The CCT variation versus aging time in three cases; SPDsof representative samples for three cases: **(b)** CSA-1; **(c)** CSA-2; and **(d)** CSA-3, respectively, prior to and after aging.