Muqing Liu

Study on methodology of LED's luminous flux measurement with integrating sphere

Errors are introduced when using traditional methods for measuring the total luminous flux of LEDs since an LED is quite different from traditional light sources in terms of physical size, flux level, spectrum and spatial distribution. This paper uses commercial lighting simulation software named Tracepro to simulate the self-absorption effect when using traditional integrating sphere methods to measure the total luminous flux of LEDs and then presents a modified method for the measurement. The LED under investigation or a specially designed narrow beam standard lamp is placed on the interior wall of the sphere in our method. The results show that the measurement method presented here can lead to better precision in the evaluation of the total luminous flux of LEDs.

Dependence of reliability of GaN LEDs on their junction temperatures and ideal factors

The relationship between the reliability of GaN LEDs and their junction temperatures and ideal factors is investigated. 20 groups of both blue and white GaN LEDs are tested. Their ideal factors and junction temperatures under 700mA operating current are measured. The measurement methods are introduced. After the measurement, 700mA high current accelerated life test is carried out on the LEDs. Analysis results show that the reliability of the LEDs is strongly dependent on their junction temperatures and ideal factors. For most of the unreliable LEDs with their 50% ALT life less than 400 hours, their ideal factors are higher than 10, or the junction temperatures of the blue LEDs under 700mA are higher than 130°C, and the junction temperatures of the white LEDs under 700mA are higher than 120°C.

Uncertainty analysis in lifetime measurement for white light emitting diodes

Lifetime is one of the most important characteristics of white LEDs for the solid state lighting industry and end users. The measurement uncertainties should be controlled well to ensure consistent measurement results. This paper gives uncertainty analysis in the measurement for the L50 lifetime of white LEDs. The exponential model is assumed for LEDs’ light output degradation, and an Eyring model is used for accelerated life test. The influences of photometric measurement instruments, measurement duration and interval, junction temperature, input current, current accelerating index and activation energy are analysed. The analysis method introduced in this paper can be referenced for other related analysis, and the results are important to the practices in LED lifetime measurement.