有關(guān)led的畢業(yè)論文外文翻譯

1、<p>　　Renewable and Sustainable Energy Reviews</p><p>　　High-brightness LEDs—Energy efficient lighting sources and their potential in indoor plant cultivation</p><p><b>　　ABSTRACT<

2、/b></p><p>　　The rapid development of optoelectronic technology since mid-1980 has significantly enhanced the brightness and efficiency of light-emitting diodes (LEDs). LEDs have long been proposed as a p

3、rimary light source for space-based plant research chamber or bioregenerative life support systems. The raising cost of energy also makes the use of LEDs in commercial crop culture imminent. With their energy efficiency,

4、 LEDs have opened new perspectives for optimizing the energy conversion and the nutrient </p><p><b>　　Contents</b></p><p>　　1. Introduction </p><p>　　2. LED development.

5、</p><p>　　3. Color ratios and photosynthesis </p><p>　　4. LEDs and indoor plant cultivation.</p><p>　　4.1. Plant tissue culture and growth</p><p>　　4.2. Space agricultu

6、re8</p><p>　　4.3. Algaculture</p><p>　　4.4. Plant disease reduction</p><p>　　5. Intermittent and photoperiod lighting and energy saving</p><p>　　6. Conclusion</p>

7、<p>　　1. Introduction</p><p>　　With impacts of climate change, issues such as more frequent and serious droughts, floods, and storms as well as pest and diseases are becoming more serious threats to a

8、griculture. 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 weather-related crop fail

9、ures due to droughts and floods to provide year-round crop production, which assist in supplying food in cities</p><p>　　The use of light-emitting diodes marks great advancements over existing indoor agricul

10、tural lighting. LEDs allow the control of spectral composition 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 l

11、ow radiant heat output 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 replac</p><p>　　2.

12、LED development</p><p>　　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 p-side (ano

13、de), to the n-side (cathode), but not in the reverse direction.</p><p>　　Electrons and holes flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a low

14、er 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 dir

15、ect band gap with energies corresponding to near-infrared, visible or near-ultraviolet light.</p><p>　　The key structure of an LED consists of the die (or light-emitting semiconductor material), a lead frame

16、 where the die is placed, and the encapsulation which protects the die (Fig. 1).</p><p><b>　　Fig.1</b></p><p>　　LED development began with infrared and red devices made with gallium

17、arsenide. Advances in materials science have made possible the production of devices with ever-shorter wavelengths, producing light in a variety of colors. J.Margolin reported that the first known light-emitting solid st

18、ate 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 commerciall</p>

19、<p>　　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 vo

20、ltage 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 d

21、esigned into bar code scanners, fiber optic data transmission systems, an</p><p>　　As laser diodes with output in the visible spectrum started to commercialize in late 1980s, LED designers used similar techn

22、iques to produce high-brightness and high reliability LEDs. This led to the development of InGaAlP (indium gallium aluminum phosphide) visible light LEDs. Via adjusting the energy band gap InGaAlP material can have diffe

23、rent color output. Thus, green, yellow, orange and red LEDs could all be produced using the same basic technology. Also, light output degradation of InGaAlP</p><p>　　Shuji Nakamura at Nichia Chemical Industr

24、ies of Japan introduced blue LEDs in 1993. Blue LEDs have always been difficult to manufacture because of their high photon energies (>2.5 eV) and relatively low eye sensitivity. Also, the technology to fabricate thes

25、e LEDs is very different and less advanced than standard LED materials. But blue is one of the primary colors (the other two being red and green). Properly combining the red, green, and blue light is essential to produce

26、 white and full-color.</p><p>　　Compare to incandescent light’s 1000-h and fluorescent light’s 8000-h life span, LEDs have a very significantly longer life of 100,000 h. In addition to their long life, LEDs

27、have many advantages over conventional light source. These advantages include small size, specific wavelength, low thermal output, adjustable light intensity and quality, as well as high photoelectric conversion efficien

28、cy. Such advantages make LEDs perfect for supporting plant growth in controlled environment such as plant</p><p><b>　　Table 1</b></p><p>　　3. Color ratios and photosynthesis</p>

29、;<p>　　The chlorophyll molecules in plants initiate photosynthesis bycapturing light energy and converting it into chemical energy to help transforming water and carbon dioxide into the primary nutrient for living

30、 beings. The generalized equation for the photosynthetic process is given as:</p><p>　　CO2 + H2O—light—>（CH2O） + O2</p><p>　　where (CH2O) is the chemical energy building block for thesynthes

31、is of plant components.</p><p>　　Chlorophyll molecules absorb blue and red wavelengths most efficiently. The green and yellow wavelengths are reflected or transmitted and thus are not as important in the pho

32、tosynthetic</p><p>　　process. That means limit the amount of color given to the plants and still have them grow as well as with white light. So, there is no need to devote energy to green light when energy c

33、osts are a</p><p>　　concern, which is usually the case in space travel.</p><p>　　The LEDs enable researchers to eliminate other wavelengths found within normal white light, thus reducing the amo

34、unt of energy required to power the plant growth lamps. The plants grow normally and taste the same as those raised in white light. </p><p>　　Red and blue light best drive photosynthetic metabolism. These li

35、ght qualities are particularly efficient in improving the developmental characteristics associated with autotrophic growth habits. Nevertheless, photosynthetically inefficient light qualities also convey important enviro

36、nmental information to a developing plant. For example, far-red light reverses the effect of phytochromes, leading to changes in gene expression, plant architecture, and reproductive responses. In addition, photoper</

37、p><p>　　The superimposed pattern of luminescence spectrum of blue LED (450–470 nm) and that of red LED (650–665 nm) corresponds well to light absorption spectrum of carotenoids and chlorophyll. Various plant cu

38、ltivation experiments are possible when these twokinds of LED are used with the addition of far-red radiation (730–735 nm) as the light source. Along the line of the LED technology advancement, LEDs become a prominent li

39、ght source for intensive plant culture systems and photobiological researches.</p><p>　　Some of the pioneering researches are reviewed in the followings.</p><p>　　Bula et al. have shown that gro

40、wing lettuce with red LEDs in combination with blue tubular fluorescent lamp (TFL) is possible. Hoenecke et al. have verified the necessity of blue photons for lettuce seedlings production by using red LEDs with blue TFL

41、. As the price of both blue and red LEDs have dropped and the brightness increased significantly, the research findings have been able to be applied in commercial production. As reported by Agence France Press, Cosmo Pla

42、nt Co., in Fukuroi, Japan has</p><p>　　Tennessen et al. have compared photosynthesis from leaves of kudzu (Pueraria lobata) enclosed in a leaf chamber illuminated by LEDs versus by a xenon arc lamp. The resp

43、onses of photosynthesis to CO2 are similar under the LED and xenon arc lamps at equal photosynthetic irradiance. There is no statistical significant difference between the white light and red light measurements in high C

44、O2. Some leaves exhibited feedback inhibition of photosynthesis which is equally evident under irradiation of ei</p><p>　　Okamoto et al. have investigated the effects of different ratios of red and blue (red

45、/blue) photosynthetic photon flux density (PPFD) levels on the growth and morphogenesis of lettuce seedlings. They have found that the lettuce stem length decreases significantly with an increase in the blue PPFD. The re

46、search has also identified the respective PPFD ratio that (1) accelerates lettuce seedlings’ stem elongation, (2) maximizes the whole plant dry weight, (3) accelerates the growth of whole plants,</p><p>　　4.

47、 LEDs and indoor plant cultivation</p><p>　　4.1. Plant tissue culture and growth</p><p>　　Tissue culture (TC), used widely in plant science and a number of commercial applications, is the growth

48、 of plant tissues or cells within a controlled environment, an ideal growth environment that is free from the contamination of microorganisms and other contaminants. A controlled environment for PTC usually means filtere

49、d air, steady temperature, stable light sources, and specially formulated growth media (such as broth or agar). Micropropagation, a form of plant tissue culture (PTC), is used </p><p>　　1short-term testing o

50、f genetic constructions or regeneration oftrans genic plants,</p><p>　　2 cross breeding distantly related species and regeneration of the novel hybrid,</p><p>　　3 screening cells for advantageou

51、s characters (e.g. herbicidere sistance/tolerance),</p><p>　　4embryo rescue (i.e. to cross-pollinate distantly related specie sand then tissue culture there sulting embryo which would normally die),</p>

52、;<p>　　5 large-scale growth of plant cells in liquid culture inside bioreactors as a source of secondary products (like recombinant proteins used as biopharmaceuticals).</p><p>　　6production of double

53、d monoploid plants from haploid cultures to achieve homozygous lines more rapidly in breeding programs (usually by treatment with colchicine which causes doubling of the chromosome number).</p><p>　　Tissue c

54、ulture and growth room industries have long been using artificial light sources for production. These light sources include TFL, high pressure sodium lamp (HPS), metal halide lamp (MHL) and incandescent lamp, etc. Among

55、them, TFL has been the most popular in tissue culture and growth room industries. However, the use of TFL consumes 65% of the total electricity in a tissue culture lab. That is the highest non-labor costs. As a result, t

56、hese industries continuously seek for more efficient</p><p>　　Nhut et al. have cultured strawberry plantlets under different blue to red LED ratios as well as irradiation levels and compared its growth to th

57、at under plant growth fluorescent. The results suggest that a culture system using LED is advantageous for the micropropagation of strawberry plantlets. The study also demonstrates that the LED light source for in vitro

58、culture of plantlets contributes to an improved growth of the plants in acclimatization.</p><p>　　Brown et al. have measured the growth and dry matter partitioning of ‘Hungarian Wax’ pepper (Capsicum annuum

59、L.) plants grown under red LEDs compared with similar plants grown under red LEDs with supplemental blue or far-red radiation. Pepper biomass reduces when grown under red LEDs without blue wavelengths compared to plants

60、grown under supplemental blue fluorescent lamps. The addition of far-red radiation results in taller plants with greater stem mass than red LEDs alone. Fewer leaves develo</p><p>　　4.2. Space agriculture<

61、/p><p>　　Because re-supply is not an option, plants are the only options to generate enough food, water and oxygen to help make future explorers self-sufficient at space colonies on the moon, Mars or beyond. In

62、 order to use plants, there must be a light source. Standard light sources that used in homes and in greenhouses and in growth chambers for controlled agriculture here on Earth are not efficient enough for space travel.

63、While a human expedition outside Earth orbit still might be years away, the spa</p><p>　　Infrared LEDs that are used in remote controls devices have other uses. Johnson et al. have irradiated oat (Avena sati

64、va cv Seger) seedlings with infrared (IR) LED radiation passed through a visible-light-blocking filter. The irradiated seedlings exhibited differences in growth and gravitropic response when compared to seedlings grown i

65、n darkness at the same temperature. This suggests that the oat seedlings are able to detect IR LED radiation. These findings also expand the defined range of wave</p><p>　　Goins et al. grow wheat under red L

66、EDs and compare them to the wheat grown under (1) white fluorescent lamps and (2) red LEDs supplemented with blue light from blue fluorescent lamps. The results show that wheat grown under red LEDs alone displayed fewer

67、subtillers and a lower seed yield compared to those grown under white light. Wheat grown under red LEDs + 10% BF light had comparable shoot dry matter accumulation and seed yield relative to those grown under white light

68、. These results indicate </p><p>　　The research of Goins and his team continues in plant growth chambers the size of walk-in refrigerators with blue and red LEDs to grow salad plants such as lettuce and radi

69、shes. They hope the plant growth chamber would enable space station staff to grow and harvest salad greens, herbs and vegetables during typical fourmonth tours on the outpost .</p><p>　　4.3. Algaculture</

70、p><p>　　Algaculture, refers to the farming of species of algae, has been a great source for feedstock, bioplastics, pharmaceuticals, algae fuel, pollution control, as well as dyes and colorants. Algaculture als

71、o provides hopeful future food sources.</p><p>　　Algae can be grown in a photobioreactor (PBR), a bioreactor which incorporates some type of light source. A PBR is a closed system, as opposed to an open tank

72、 or pond. All essential nutrients must be introduced into the system to allow algae to grow and be cultivated. A PBR extends the growing season and allows growing more species. The device also allows the chosen species t

73、o stay dominant. A PBR can either be operated in ‘‘batch mode’’ or ‘‘continuous mode’’ in which a continuous stream of ste</p><p>　　When the algae grow and multiply, they become so dense that they block ligh

74、t from reaching deeper into the water. As a result, light only penetrates the top 7–10 cm of the water in most algalcultivation systems. Algae only need about 1/10 the amount of direct sunlight. So, direct sunlight is of

75、ten too strong for algae. A means of supplying light to algae at the right concentration is to place the light source in the system directly.</p><p>　　Matthijs et al. have used LEDs as the sole light source

76、in continuous culture of the green alga (Chlorella pyrenoidosa). The research found the light output of the LED panel in continuous operation sufficient to support maximal growth. Flash operation at 5-ps pulse ‘‘on’’ dur

77、ation between dark periods of up to 45 ps would still sustain near maximum growth. While longer dark periods tend to cut the growth rate, the light flux decrease resulting from such operation does not reduce the growth a

78、s mu</p><p>　　In order to take advantage of the biotechnological potential of algae, Lee and Palsson have calculated theoretical values of gas mass transfer requirements and light intensity requirements to s

79、upport high-density algal cultures for the 680 nm monochromatic red light from LED as a light source. They have also designed a prototype PBR based on these calculations. Using on-line ultra filtration to periodically pr

80、ovide fresh medium, these researchers have achieved a cell concentration of more than 2</p><p>　　Another research of algae via LEDs is conducted by Nedbal et al. Their research is a study of light fluctuatio

81、n effects on a variety of algae in dilute cultures using arrays of red LEDs to provide intermittent and equivalent continuous light in small-size (30 ml) bioreactors. The results endorse that the algae growth rates in ce

82、rtain calculated intermittent light can be higher than the growth rate in the equivalent continuous light. Yanagi and Okamoto has grown five spinach plants under the red </p><p>　　4.4. Plant disease reductio

83、n</p><p>　　Schuerger and Brown have used LED arrays with different spectral qualities to determine the effects of light on the development of tomato mosaic virus (ToMV) in peppers and powdery mildew on cucu

84、mbers. Their research concludes that spectral quality may alter plant disease development. Latter research regarding bacterial wilt on tomato has confirmed this conclusion and demonstrates that spectral quality may be us

85、eful as a component of an integrated pest management program for space-based ecologi</p><p>　　Miyashita et al. use red LEDs (peak wavelength: 660 nm) and white fluorescent lamps as light sources for potato p

86、lantlets growth in vitro. They found that shoot length and chlorophyll concentration of the plantlets increases with increasing 630– 690 nm red photon flux (R-PF) while there are no significant differences in dry weight

87、and leaf area of the plantlets with different R-PF levels. This means red light affects the morphology rather than the growth rate of potato plantlets in vitro. As a r</p><p>　　5. Intermittent and photoperio

88、d lighting and energy saving</p><p>　　Time constants for photosynthetic processes can be divided into three ranges: primary photochemistry, electron shuttling, and carbon metabolism. These three photosynthet

89、ic processes can be uncoupled by providing pulses of light within the appropriate range for each process. At high frequencies, pulsing light treatments can be used to separate the light reactions (light harvesting and ch

90、arge separation) from the dark reactions (electron shuttling) of photosynthetic electron transport. LEDs’ flexi</p><p>　　Tennessen et al. use LEDs to study the effects of light pulses (micro- to milli-second

91、) of intact tomato leaves. They found that when the equivalent of 50 mmol photons mp -2s-1 is provided during 1.5 ms pulses of 5000 mmol photons mp -2s-1 followed by 148.5 ms dark periods, photosynthesis is the same as i

92、n continuous 50 mmol photons mp -2s-1 . Data support the theory that photons in pulses of 100 ps or shorter are absorbed and stored in the reaction centers to be used in electron transport durin</p><p>　　Jao

93、 and Fang have investigated the effects of intermittent light on growth of potato plantlets in vitro. They also use conventional TFLs for the experiment to explore the electrical savings realized by adjusting the freque

94、ncy and duty ratio of LEDs. TFLs provide continuous fluctuating light at 60 Hz while LEDs provide nonfluctuating light and pulse light of the preset frequency and duty ratio. When the growth rate is the only concern, LED

95、s at 720 Hz (1.4 ms) and 50% duty ratio with 16-h light/8</p><p>　　6. Conclusions</p><p>　　The first sustained work with LEDs as a source of plant lighting occurred in the mid-1980s when a light

96、ing system for plant growth was designed for space shuttles and space stations for it is realized that people cannot go to the Moon, Mars, or beyond without first mastering the art of indoor farming on Earth. As the perf

97、ormance of LED continues to improve, these lighting systems progress from red only LED arrays using the limited components available to high-density, multi-color LED chip-on-boa</p><p>　　LEDs are the first l

98、ight source to provide the capability of true spectral composition control, allowing wavelengths to match to plant photoreceptors to optimize production as well as to influence plant morphology and composition. They are

99、easily integrated into digital control systems, facilitating complex lighting programs like varying spectral composition over the course of a photoperiod or with plant development stage. LEDs do not contain mercury. They