Author: Gen Chen Zhe
Abstract: In view of the development and development of light-emitting diodes ( LEDs ) for lighting in recent years, the development status and characteristics of LEDs are briefly introduced. Some key technical problems that need to be solved in the development of LEDs for lighting are discussed. Such lighting systems will find wide application.
LED development status
A light-emitting diode (Light Emitting Diode, abbreviated as LED) is a semiconductor light-emitting device that converts electrical energy into light energy and is a solid-state light source. LEDs were introduced in the early 1960s. At that time, the LEDs were mainly red, the luminous efficiency was very low, and the luminous flux was very small. They could only be used as indicator lights and instrument displays. With the continuous improvement of the die material, structure and packaging technology, the variety of LED colors has increased, and the light efficiency has been greatly improved. The current red Led Light effect can reach 100lm/W, and the green LED can reach 50lm/W. The luminous flux can reach tens of lumens. Especially in recent years, the development of high-efficiency, high-brightness white LEDs has made LED applications in the field of lighting possible. In the more than 100 years since Edison invented incandescent lamps, electric light source illumination has undergone three important development stages. The representative light sources in these three stages are incandescent lamps, fluorescent lamps and high-intensity discharge lamps. It is now widely believed that LEDs will be expected to develop into fourth-generation light sources, namely semiconductor lighting .
1 Characteristics of semiconductor light-emitting diodes
The LED is a semiconductor device that realizes electro-optical conversion through a PN junction. Its characteristics are:
1.1 Energy saving, no environmental pollution
LEDs consume less energy and will become a new type of energy-saving lighting source as technology advances. At present, the luminous efficacy of white LED has reached 25lm/W, which exceeds the level of ordinary incandescent lamps, and some people predict the current development speed of LED technology. By 2005, the luminous efficacy of white LED can reach 50lm/W, and by 2010- In 2015, the efficacy of white LEDs is likely to reach 150 to 200 lm / W, far exceeding the efficacy of all current lighting sources. In addition, fluorescent lamps, mercury lamps and other light sources that are widely used today contain mercury that is harmful to human health. The production process of these light sources and discarded lamps cause environmental pollution. LEDs do not have these problems and are a "clean" light source.
1.2 Long life
Generally speaking, the life of an ordinary incandescent lamp is about 1000h, and the life of a fluorescent lamp or a metal halide lamp is not more than 10,000 hours, and the service life of the LED can be as long as tens of thousands of hours.
1.3 Strong structure
The LED is a solid-state light source encapsulated with epoxy resin. The structure has no fragile components such as glass bubbles and filaments. It is an all-solid structure and can withstand vibration and impact without causing damage.
In addition, LED as a light source for illumination has some valuable advantages, such as: the illuminant is close to the point source, which is convenient for luminaire design ; the illuminating response time is short; it is easy to be made into a thin luminaire, saving installation space and the like.
In summary, LED is a light source that meets the requirements of green lighting. The concept of "green lighting" refers to the use of scientific lighting design, the use of high efficiency, long life, safety and stable performance of lighting electrical products (electric light source, lighting electrical accessories, lamps, wiring materials, and dimming control equipment And light control devices) to improve the conditions and quality of people's work, study and life, thus creating an efficient, comfortable, safe, economic and beneficial environment and fully embody the lighting of modern civilization.
2 The key to developing LEDs for lighting
Led Lighting is currently only available for special occasions. The widely used illumination source should be white LED, but the white LED light developed at home and abroad is not high enough, the power is not big enough, and the price is more expensive. If it is required to completely replace the currently widely used incandescent or fluorescent lamps, there are still some technical problems to be solved.
(1) First of all, the problem of luminous efficiency. The most important way to improve the luminous efficiency of LEDs is to improve the structure and manufacturing process of semiconductor luminescent materials and LED chips . Since this part of the work requires a solid theoretical research foundation and advanced semiconductor process equipment, it is not easy to carry out research work in this area.
(2) High power problems. As illumination, the luminous flux of a single LED output must be large enough. To increase the luminous flux of the LED, sufficient electrical power must first be injected. However, the temperature rise of the LED chip should not be too high, otherwise the performance, especially the service life, will be greatly affected. Obviously, LED devices and luminaires with larger input power must have a good heat dissipation structure in addition to larger chips. Now some famous foreign companies have designed and developed some special LED device structures, and have achieved good results.
(3) Since the light emitted by a typical LED is relatively concentrated in a small solid angle range, the reflector in a general luminaire is no longer a necessary optical component, and a lens is often used as a collimating optical component. For example, a parallel lens is produced by a convex lens or a Fresnel lens. Then, the beam is re-diffused and deflected with a pincushion lens, a wedge prism, or the like to produce a light distribution that satisfies the requirements of various illumination lamps. This requires a unique secondary optical system design for the luminaires of LED illuminators.
(4) Since LED lighting needs to be composed of multiple LED tubes, its parameter dispersion is also a technical problem. In addition to pre-selection, classification, and consistency as much as possible, it is necessary to design a reasonable luminaire structure (including LED arrangement and position layout) and study appropriate driving circuits to prevent occasional energy concentration and burn some LEDs.
(5) In addition, since a plurality of LEDs constitute a lighting fixture, the LEDs are connected in parallel and in series. In the process of use, as long as there is a short circuit or open circuit of the LED, it will cause the whole small piece or the whole LED to be extinguished, which will affect the lighting effect. To this end, simple and inexpensive protection circuits must be studied to minimize this adverse effect.
(6) In view of the high requirements of the LED for heat dissipation conditions, if the junction temperature of the die exceeds the standard limit value, the irreversible light intensity will be attenuated. Therefore, in addition to sufficient heat dissipation measures when using, there must be reasonable circuit design and circuit layout, try to keep the LED in good working condition and give full play to its long life advantage.
3 Conclusion
Although there are still a series of technical key issues to be solved in the development of LED lighting systems, with the rapid improvement of the light-emitting efficiency of LED chips, the packaging of LEDs and the further improvement of the manufacturing process of lamps, some economic and technical obstacles will be Gradually eliminated. It is believed that in the near future, such lighting systems can be widely used in many fields.
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