【正文】 1 Renewable and Sustainable Energy Reviews Highbrightness LEDs— Energy efficient lighting sources and their potential in indoor plant cultivation ABSTRACT The rapid development of optoelectronic technology since mid1980 has significantly enhanced the brightness and efficiency of lightemitting diodes (LEDs). LEDs have long been proposed as a primary light source for spacebased plant research chamber or bioregenerative life support systems. The raising cost of energy also makes the use of LEDs in mercial crop culture imminent. With their energy efficiency, LEDs have opened new perspectives for optimizing the energy conversion and the nutrient supply both on and off Earth. The potentials of LED as an effective light source for indoor agriculturalproduction have been explored to a great extent. There are many researches that use LEDs to support plant growth in controlled environments such as plant tissue culture room and growth chamber. This paper provides a brief development history of LEDs and a broad base review on LED applications in indoor plant cultivation since 1990. Contents 1. Introduction 2. LED development. 3. Color ratios and photosynthesis 4. LEDs and indoor plant cultivation. . Plant tissue culture and growth . Space agriculture8 . Algaculture . Plant disease reduction 5. Intermittent and photoperiod lighting and energy saving 6. Conclusion 1. Introduction With impacts of climate change, issues such as more frequent and serious droughts, floods, and storms as well as pest and diseases are being more serious threats to agriculture. These threats along with shortage of food supply make people turn to indoor and urban farming (such as vertical farming) for help. With proper lighting, indoor agriculture eliminates weatherrelated crop failures due to droughts and floods to provide yearround crop production, which assist in supplying food in cities with surging populations and in areas of severe environmental conditions. The use of lightemitting diodes marks great advancements over existing indoor agricultural lighting. LEDs allow the control of spectral position and the adjustment of light intensity to simulate the changes of sunlight intensity during the day. They have the ability to produce high light levels with low radiant heat output 2 and maintain useful light output for years. LEDs do not contain electrodes and thus do not burn out like incandescent or fluorescent bulbs that must be periodically replaced. Not to mention that incandescent and fluorescent lamps consume a lot of electrical power while generating heat, which must be dispelled from closed environments such as spaceships and space stations. 2. LED development LED is a unique type of semiconductor diode. It consists of a chip of semiconductor material doped with impurities to create a p– n junction. Current flows easily from the pside (anode), to the nside (cathode), but not in the reverse direction. Electrons and holes flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon. The color (wavelength) of the light emitted depends on the band gap energy of the materials forming the p– n junction. The materials used for an LED have a direct band gap with energies corresponding to nearinfrared, visible or nearultraviolet light. The key structure of an LED consists of the die (or lightemitting semiconductor material), a lead frame where the die is placed, and the encapsulation which protects the die (Fig. 1). LED development began with infrared and red devices made with gallium arsenide. Advances in materials science have made possible the production of devices 3 with evershorter wavelengths, producing light in a variety of colors. reported that the first known lightemitting solid state diode was made in 1907 by H. J. Round. No practical use of Round’ s diode was made for several decades until the invention of the first practical LED by Nick Holonyak, Jr in 1962. His LEDs became mercially available inlate 1960s. These GaAsP LEDs bine three primary elements: gallium, arsenic and phosphorus to provide a 655nm red light with brightness levels of approximately 1– 10 mcd at 20mA. As the luminous intensity was low, these LEDs were only used in a few applications, primarily as indicators. Following GaAsP, GaP (gallium phosphide) red LEDs were developed. These device sex hibit very high quantum efficiencies at low currents. As LED technology progressed through the 1970s, additional colors and wavelengths became available. The most mon materials were GaP green and red, GaAsP orange, and high efficiency red and GaAsP yellow. The trend towards more practical applications (such as in calculators, digital watches, and test equipment) also began to develop. As the LED materials technology became more advanced, the light output was increased, and LEDs became bright enough to be used for illumination. In 1980s a new material, GaAlAs (gallium aluminum arsenide) was developed followed by a rapid growth in the use of LEDs. GaAlAs technology provides superior performance over previously available LEDs. The voltage requirement is lower, which results in a total power savings. LEDs could be easily pulsed or multiplexed and thus are suitable for variable message and outdoor signs. Along this development period, LEDs were also designed into bar code scanners, fiber optic data transmission systems, and medicalequipment. During this time, the improvements in crystal growth and optics design allow yellow, green and orange LEDs only a minor improvement in brightness and efficiency. The basic structure of the material remained relatively unchanged. As laser diodes with output in the visible spectrum started to mercialize in