英语原文：
Life of LED-Based White Light Sources
The interest for using light-emitting diodes (LEDs) for display and illumination applications has been growing steadily over the past few years. The potential for long life and reduced energy use are two key attributes of this rapidly evolving technology that have generated so much interest for its use in the above mentioned applications. Traditionally, the lamp life of light sources commonly used in illumination applications is determined by subjecting them to a predetermined on/off cycle until half the number of light sources cease to produce light. Unlike these sources, LEDs rarely fail catastrophically; instead, their light output slowly degrades over time. Even if an LED is technically operating and producing light, at some point the amount of light produced by the LED will be insufficient for the intended application. Therefore, the life of an LED should be based on the amount of time that the device can produce sufficient light for the intended application,rather than complete failure. Based on this argument, a recent publication from an industry group defines the life of an LED device o
r system for use in general lighting applications as the operating time, in hours, for the light output to reach 70% of its initial value.
The most widely used white LEDs incorporate a layer of phosphor over a GaN-based, short-wavelength light emitter. Usually, the phosphor is embedded inside an epoxy resin that surrounds the LED die. Some portion of the short-wavelength radiation emitted by the LED is down-converted by the phosphor, and the combined radiation creates white light. Early white LEDs were packaged similar to the indicator-style colored LEDs, specifically 5 mm and SMD (surface mount devices). Although these products demonstrated the concept of a white light source, they did not produce sufficient light for display and illumination applications. Furthermore, these indicator-style white LEDs had a relatively short life, 5000–10 000h to reach 70% light level under normal operating conditions. To address the higher luminous flux requirements, manufacturers have started to commercialize high-power illuminator LEDs that are presently producing over one hundred times the flux compared to indicator-style white LEDs. The higher light output is achieved by using larger dies, higher drive currents,and improved heat extraction methods. In addition,some manufa
cturers are using better encapsulants to improve the life of white LEDs.
There are several studies that have investigated the aging mechanisms of GaN-based LEDs. During the 1990s,Barton et al. investigated the degradation of GaN-based blue LEDs and showed that light output reduction over time occurred primarily due to the yellowing of the epoxy surrounding the die. In 2001, Narendran et al. observed that indicator-style white LED packages degraded very rapidly, with the LEDs reaching the 50% light output level within 6000 h. In that same study, it was shown that the chromaticity values of the white LEDs shifted toward yellow over time, and it was speculated that the yellowing of the epoxy was the main cause for light output degradation. Therefore, based on past studies,the primary reason for the degradation of indicator-style white LED packages is the yellowing of the epoxy that is caused by excessive heat at the p-n-junction of the LED. Some of the newer illuminator-style white LEDs use encapsulant materials that have lower photodegradation characteristics,and therefore have a lower degradation rate. However, there are factors such as the degradation of the die attaché epoxy, discoloration of the metal reflectors and the lead wires, and degradation of the semiconducting element t
hat are influenced by heat, and these all contribute to the overall degradation of the white LED. Although the newer high-power white LEDs would have a lower degradation rate compared to the early indicator-style devices, it is the heat at the p-n-junction that most influences the degradation. The heat at the  p-n-junction is caused by the ambient temperature and the ohmic heating at the bandgap.
As stated earlier, long life is one key feature of LED technology that has attracted so many end-use communities. To benefit from the long-life feature, it is the final system that has to operate for a long time, not just the individual LED. As noted in past studies, heat at the p-n-junction is one of the key factors that determine the life of the white LED. Therefore, if systems are not properly designed with good thermal managemen techniques, even if they use long-life white LEDs the life of the final system would be short. Developing the relationship between junction temperature and life would be very useful for producing long-life systems.controller翻译中文
Although there are different methods available for estimating the junction temperature of LE
Ds, they are not very convenient,especially once the LEDs are integrated into a system . Furthermore, these methods are not direct; consequently, they are prone to erroneous results. Alternatively, it is much more convenient and direct to measure the heat at a location external to the LED package that is sufficiently close to the junction and where a temperature sensor can be directly attached. The temperature of this point should have a good relationship to the junction temperature. The point where a temperature sensor can be attached for this measurement could be the lead wire (cathode side) for the indicator-style LEDs and the board for high-power LEDs. Most manufacturers can recommend such a point,and we refer to this as the T-point in this manuscript.
Since white LEDs in the marketplace are packaged differently, their ability to transfer heat from the die to the surrounding environment is different from product to product. Therefore, it is reasonable to assume that different products have different degradation rates as a function of heat. A graph that shows the life of the LED as a function of T-point temperature is extremely useful for system manufacturers to build reliable, long-lasting systems. By knowing how much impact heat has on the degradation rate or life of the LED, the system
manufacturer can select components and drive parameters, including the amount of heat sink and drive current, for a product being designed for a given application.
Therefore, the objective of the study presented in this manuscript was to investigate the relationship between the T-point temperature and life of a white LED. A second objective was to understand the degradation rate of different high-power white LED products presently available in the marketplace.

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