LED Lamps K-12 Projects and Experiments
See also LED Projects, Experiments and Background Information 
A LED lamp is a type of solid state lighting (SSL) that utilizes light-emitting diodes (LEDs) as a source of illumination rather than electrical filaments or gas. Solid state lighting (SSL) refers to a type of lighting that utilizes light-emitting diodes (LEDs), organic light-emitting diodes (OLED), or polymer light-emitting diodes (PLED) as sources of illumination rather than electrical filaments or gas. The term "solid state" refers to the fact that light in an LED is emitted from a solid object—a block of semiconductor—rather than from a vacuum or gas tube, as is the case in traditional incandescent light bulbs and fluorescent lamps. Unlike traditional lighting, however, SSL creates visible light with reduced heat generation or parasitic energy dissipation. In addition, its solid-state nature provides for greater resistance to shock, vibration, and wear, thereby increasing its lifespan significantly.
HistoryFor the past 150 years, lighting technology was mainly limited to incandescence and fluorescence. While derivative technologies such as high-intensity discharge lamps (HID) have emerged, none has achieved a luminous efficacy greater than 200 lm/W (for monochromatic low pressure sodium lamps), with incandescent lighting usually achieving an efficacy of less than 18 lm/W. With the advent of commercial LEDs in the 1960s, however, a new kind of lighting became available. LEDs can consume less electricity than conventional lighting and can produce less of the parasitic by-product heat. However, at present, commercial LED systems are not as efficient as fluorescent lighting. See luminous efficacy for a comparison. Initial LEDs were red in color, with yellow and orange variants following soon thereafter. To produce a white SSL device, however, a blue LED was needed, which was later discovered through materials science and extensive research and development. In 1993, Shuji Nakamura of Nichia Chemical Industries came up with a blue LED using gallium nitride (GaN). With this invention, it was now possible to create white light by combining the light of separate LEDs (red, green, and blue), or by placing a blue LED within a special package with an internal light conversion phosphor - some of the blue output becomes red and green with the result that the LED light emission appears white to the human eye. SSL has been described by the United States Department of Energy as a pivotal emerging technology that promises to alter lighting in the future. It is the first new lighting technology to emerge in over 40 years and, with its energy efficiencies and cost savings, has the potential to replace many existing fixtures. ModelsLED lamps (also called LED bars or Illuminators) come in different shapes, among them the light bulb shape with a large E27 Edison screw and MR16 shape with a bi-pin base. Other models might have a small Edison E14 fitting, GU5.3 (Bipin cap) or GU10 (bayonet socket). This includes low voltage (typically 12 V halogen-like) varieties and replacements for regular AC mains (120-240 V AC) lighting. Currently the latter are less widely available but this is changing rapidly. In LED lamps the luminous flux (in lumens), luminous intensity (in millicandelas) and the radiation angle (in degrees) are related. Technology overviewA single LED can produce only a limited amount of light, and only a single color at a time. To produce the white light necessary for SSL, light spanning the visible spectrum (red, green, and blue) must be generated in correct proportions. To achieve this effect, three approaches are used for generating white light with LEDs: wavelength conversion, color mixing, and most recently Homoepitaxial ZnSe. Wavelength conversion involves converting some or all of the LED’s output into visible wavelengths. Methods used to accomplish this feat include:
Color mixing involves utilizing multiple LEDs in a lamp and varying the intensity of each LED to produce white light. The lamp contains a minimum of two LEDs (blue and yellow), but can also have three (red, blue, and green) or four (red, blue, green, and yellow). As no phosphors are used, there is no energy lost in the conversion process, thereby exhibiting the potential for higher efficiency. Homoepitaxial ZnSe is a technology developed by Sumomito Electric where a LED is grown on a ZnSe substrate, which simultaneously produces blue light from the active region and yellow emission from the substrate. The resulting white light has a wavelength spectrum on par with UV LEDs. No phosphors are used, resulting in a higher efficiency white LED. To be considered SSL, however, a multitude of LEDs must be placed close together in a lamp to amplify their illuminating effects. This is because an individual LED produces an only limited amount of light, thereby limiting its effectiveness as a replacement light source. In the case where white LEDs are utilized in SSL, this is a relatively simple task, as all LEDs are of the same color and can be arranged in any fashion. When using the color-mixing method, however, it is more difficult to generate equivalent brightness when compared to using white LEDs in a similar lamp size. Furthermore, degradation of different LEDs at various times in a color-mixed lamp can lead to an uneven color output. Because of the inherent benefits and greater number of applications for white LED based SSL, most designs focus on utilizing them exclusively. Advantages of SSLTechnological comparisonSSL is intended to be a cost-effective yet high quality replacement for incandescent and fluorescent lamps. To better understand the technical merits of SSL, it is important to understand the technology behind the lamps it intends to replace.
Currently, however, there is no SSL on the market that can be offered as a true replacement for incandescent or fluorescent lamps, even though several manufacturers have gone forward with the introduction of such products. White LEDs produced today are too expensive to be considered affordable, and the lumens produced by the LEDs today are not as bright as traditional lighting. Future developments in LED technologies, however, are expected to address most of these issues. Based on research conducted by the Department of Energy (DOE) and the Optoelectronics Industry Development Association (OIDA), it is expected that by the year 2025, SSL will be the preferred method of illumination in homes and offices. The following chart, derived from information from Sandia National Laboratories, compares a perfected SSL device (to be released before 2025) with incandescent and fluorescent lights
BenefitsIn 2001, the United States consumed over 7.2 quads (7.2×1015 BTU = 7.6 EJ or 2.1 PW·h) of energy on lighting for commercial, residential, and industrial buildings. (US DOE). With America’s steady growth and limited resources, this continued rate of consumption is not sustainable. Recognizing the need for change, the DOE has set a goal to reduce electric lighting consumption 50% by 2025. SSL technologies are uniquely positioned to address this need, and at the same time
The U.S. Government, by way of the DOE and other agencies, has funded millions of dollars in research grants and projects relating to the development of a high quality yet affordable SSL. A major motive of funding such research, in addition to its environmental impact and energy savings potential, is to decrease dependence on foreign fossil fuels. ChallengesTechnological hurdlesThe current manufacturing process of white LEDs has not matured enough for them to be produced cost-effectively. There are multiple manufacturing hurdles that must be overcome. The process used to deposit the active semiconductor layers of the LED must be improved to increase yields and manufacturing throughput. Problems with phosphors, which are needed for their ability to emit a broader wavelength spectrum of light, have also been an issue. In particular, the inability to tune the absorption and emission, and inflexibility of form have been issues in taking advantage of the phosphors spectral capabilities. More apparent to the end user, however, is the low Color Rendering Index (CRI) of current LEDs. The CRI is widely used to measure how accurately a lighting source renders the color of objects. Sunlight and some incandescent lamps have CRI of 100, while fluorescent lamps have CRI ~80. The current generation of LEDs, which employs mostly blue LED chip + yellow phosphor, has a CRI around 70, which is much too low for widespread use in lighting particularly indoors. In order for SSL to effectively replace incandescent lamps, more research must be done on developing alternatives to the techniques currently used that address these concerns. Variations of CCT (color correlated temperature) at different viewing angles present another formidable obstacle against widespread use of white LED. It has been shown, that CCT variations can exceed 500 K, which is clearly noticeable by human observer, who is normally capable of distinguishing CCT differences of 50 to 100 K in range from 2000 K to 6000 K, which is the range of CCT variations of daylight. Adaptation hurdlesPotential pitfalls to the widespread adaptation of LED lamp devices include lighting fixture issues and general consumer resistance. Fixture issues can be overcome either by replacement of the fixture, or by modifying the LED lamp packaging so that it fits into existing sockets. The difficulty of retrofitting existing sockets has been overcome by some kinds of Fluorescent replacement LED lamps,and other retrofit LED bulbs currently available. LED lamp energy consumption is about 40% lower than fluorescent lights and 80% lower than incandescent at the same luminosity levels. LED lamps also have extremely long lifetimes that are at least 5 times that of the lamps they replace. It is the decrease in energy use and the increase in lifetime that is expected to reduce consumer resistance to LED lamps. Research and developmentIn order to further the development of SSL technology, the DOE has committed more than $50 million on over 45 applied research projects, including short- and long-term projects at large and small businesses, universities and national labs alike. Part of the department's goals include developing a better quality, lower cost, and highly efficient white LED. Other agencies and universities contributing to SSL development include:
FutureEvery 18 months the luminous intensity of LED lamps doubles. The expectation is that in 2010 the luminous intensity will be high enough to replace the extra long fluorescent lamps. See also
Further readingExternal links
This article is licensed under the GNU Free Documentation License. It uses material from Wikipedia Encyclopedia article "LED Lamp" |
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