The quality assessment and reliability improvement of high-power light-emitting diodes (LEDs), which are used in lighting applications, are important. Conventional lifetime testing for LEDs lasts approximately 6000 h. Accelerated life testing (ALT) to quickly assess LED quality and faults is essential for the mass production of LEDs. This paper conducted a systematic nondestructive fault analysis for the ALT of LEDs to effectively detect defect growth. A nondestructive z-scan analysis revealed the defects across the LED cross section (top-bottom); these defects were consistent with the results of destructive focused ion beam analysis. The results showed that parasitic series resistance degrades LED performance and that it ranges from 1.5 to $10.7 Ω$ under an aging current of 1 A applied for less than 50 h.

Multichip LED arrays are widely used for lighting to provide high luminance. Luminous efficacy, lifetime, and color temperature are highly dependent on the temperature at p-n junction. This paper investigated the effects of distance, number of chips, and driving current on the thermal resistance of LED module. Thermal resistance dramatically increased as the distance between LED chips decreased due to significant thermal spreading impedance for heat dissipation from junction to ambient. The parallel-resistance formula substantially underestimated the junction temperature of the LED modules due to significant thermal crowding effect. Thermal boundary can also rise junction temperature as the distance to the board edge decreased in both the two-chip and four-chip modules. Infrared results showed that chip temperatures were highly consistent with thermal resistance measurements under different driving currents.

Generally, automobiles are typically equipped with separate headlamp lanterns for generating low- and high-beam light. Compared with separate headlamp lanterns, a single headlamp producing both low- and high-beam light can be more compact and have less mechanical complexity. The single headlamp structure has become a main emphasis of research that manufacturers will continue to focus great efforts on in the future. A novel design of a single headlamp generating both low- and high-beam light is proposed in this study. The proposed headlamp consists of a compound ellipsoidal reflector, a baffle plate, a condenser lens, and LED array devices generating low- and high-beam light. The compound ellipsoidal reflector comprises a primary ellipsoidal reflector for generating low-beam light and a secondary ellipsoidal reflector for generating high-beam light. Monte Carlo ray tracing simulations were performed to confirm the optical characteristics of the proposed design. A prototype of the proposed headlamp was also fabricated and assembled to verify the design’s effectiveness. The simulated and measured illuminance distributions of the low-beam and high-beam light had the desired light patterns. Moreover, all the simulated and measured illuminances of each point and line met the ECE R112 regulation for low-beam and high-beam light. The proposed headlamp in this study is feasible for the application of single headlamp generating both low- and high-beam light.

Because of the high color rendering property, the two phosphors that convert into white light in a light-emitting diode are essential in the general lighting applications. Predicting the spectrum after two phosphors mix is challenging because of the mutual interaction between the absorption of long wavelength phosphor and the emission of short wavelength phosphor. To efficiently model the target spectrum, this paper provides a systematic method for simulating the concentration and proportion of two phosphors by using empirical equations. The simulated and experimental spectra overlapped effectively. The results show that the maximum differences are about 0.0046 for CIE 1931 color coordinates (x, y), about 59 K for correlated color temperature, and about five for color rendering index, respectively. The results show that this spectral optimization is an effective approach.

High-power light-emitting diodes (LEDs) are essential in general lighting applications. Because of the characteristics of semiconductors, the optical and electrical properties of LEDs vary with the operating temperature. To increase the reliability of high-power LEDs, this study conducted a shear stress test and examined thermal dissipation alongside electrical and optical properties. The simulated and experimental trends of the thermal distribution of the LED package overlapped effectively. Results revealed that changing the side glue height from 10% to 100% of die height, which consequently increases shear stress of 1693 g/mm2, reduces thermal resistance of 1 °C/W, and reduces luminous flux of 0.4 lm, can optimize LED reliability.