Precise Temperature Mapping of GaN-Based LEDs by Quantitative Infrared Micro-Thermography

A method of measuring the precise temperature distribution of GaN-based light-emitting diodes (LEDs) by quantitative infrared micro-thermography is reported. To reduce the calibration error, the same measuring conditions were used for both calibration and thermal imaging; calibration was conducted on a highly emissive black-painted area on a dummy sapphire wafer loaded near the LED wafer on a thermoelectric cooler mount. We used infrared thermal radiation images of the black-painted area on the dummy wafer and an unbiased LED wafer at two different temperatures to determine the factors that degrade the accuracy of temperature measurement, i.e., the non-uniform response of the instrument, superimposed offset radiation, reflected radiation, and emissivity map of the LED surface. By correcting these factors from the measured infrared thermal radiation images of biased LEDs, we determined a precise absolute temperature image. Consequently, we could observe from where the local self-heat emerges and how it distributes on the emitting area of the LEDs. The experimental results demonstrated that highly localized self-heating and a remarkable temperature gradient, which are detrimental to LED performance and reliability, arise near the p-contact edge of the LED surface at high injection levels owing to the current crowding effect

THERMAL MODELING AND CHARACTERIZATION OF HIGH POWER DEVICES

Ph.DDOCTOR OF PHILOSOPH

The Study of Fire Spread on an Inclined Wooden Surface by Multiple Spectrum Imaging Systems and Diagnostical Techniques

Fire disaster, as an unavoidable threat around the world, caused millions of loss of living beings and properties. To minimise the damage that fire disaster causes, the prediction of fire spread is significant. Due to the burning of wood is a series of complicated processes, the study of fire spread along the wood surface is far from complete. To comprehensively understand the mechanism of wood combustion as well as the fire spread, a systematically study is necessary. With the development of digital cameras and computer science, the vision systems based on the digital camera become a useful tool for measurement and visualisation. In this work, the fire spread on inclined wooden rod surface is systematically studied based on the vison systems. An original designed imaging system that synchronises visible, schlieren, and thermal imaging systems is developed. The various diagnostical techniques, including the temperature measurements by two-colour method and thermal imaging, optical flow motion estimation, and selective enhancement technique are developed along with the imaging system. Rely on the imaging system that developed in this study, the burning ability, temperature of the wooden surface and flame, the dim blue flame and the invisible hot gas flow can be visualised simultaneously. The first contribution of this work is the developed pyrometers which are based on two methods: the first one is the two-colour method, which relies on the response ratio between two selected wavelengths. This method is used for measuring the soot flame temperature. The second pyrometer is based on thermal imaging, which uses a narrow band wavelength thermal image with known emissivity. It is used for reading the temperature of the wood surface. With the instruments developed in this work, the flame temperature and surface temperature of the burning wood can be monitored at the same time. Based on the imaging system and diagnostical techniques, the fire spread on wooden rods surface that inclined at various angles is investigated. It is first found in this work both the flame and fire plume would have geometry change at the inclination surface. Besides, with the help of the optical flow method, the minor fluctuate (<2mm/s) of the flame and hot flow can be detected and used to analyse the burning phenomena. Another finding is the essential role of underneath preheating on sustaining the burning and spreading the fire. The pyrolysis zone, as well as the preheating zone, have been illustrated and visualised by synchronising the blue flame with the schlieren image by the first time. Furthermore, with involving the thermal imaging system, the preheating length underneath the burning rod is calculated and shows a monotonically increase with the increasing angle. Moreover, this work introduces a novel method to measure the flame attachment phenomenon quantitatively by using enhanced thermal image. The effects of cross-wind on the burning of wood are investigated systematically with the imaging system. The main finding is that under the low speed of cross-wind, the burning and fire spread are enhanced, while they are decreased under the high speed of the wind. With the help of the multiple spectrum imaging, the mechanisms of the wood combustion under wind condition has been studied and visualised. This new imaging system with developed diagnostical techniques is a useful tool for investigating the burning of wood and fire propagation. The findings in this works could help enhance the understanding of the fire protection and optimise the strategy both in fire protection and fire extinction