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Solid State Lighting
Time:2012-11-26
 

 

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 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 virtually no heat or parasitic energy dissipation. In addition, its solid-state nature provides for greater resistance to shock, vibration, and wear, thereby increasing its lifespan significantly.


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 be a very disruptive technology in the marketplace as well.

 

History

For 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 have achieved energy efficiencies exceeding 25%, with incandescent lighting achieving an efficiency of less than 2%. With the advent of commercial LEDs in the 1960s, however, the door for the most radical and exciting form of lighting technology had opened. Unlike conventional lighting, LEDs consume less electricity and have largely avoided the parasitic by-products of its predecessors: heat.

 

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. Advances in materials science and extensive research and development on the subject did just that. 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 creating white LEDs themselves by means of doping. SSL could now become a commercial viability.


Technology Overview

A 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:

 

   Blue LED & yellow phosphor -Considered the least expensive method for producing white light. Blue light from an LED is used to excite a phosphor which then re-emits yellow light. This balanced mixing of yellow and blue lights results in the appearance of white light.

 

   Blue LED & several phosphor -Similar to the process involved with yellow phosphors, except that each excited phosphor re-emits a different color. Similarly, the resulting light is combined with the originating blue light to create white light. The resulting light, however, has a richer and broader wavelength spectrum and produces a higher color-quality light, albeit at an increased cost.

 

   Ultraviolet (UV) LED & red, green, & blue phosphors -The UV light is used to excite the different phosphors, which are doped at measured amounts. The colors are mixed resulting in a white light with the richest and broadest wavelength spectrum.

 

    Blue LED & quantum dots - A process by which a thin layer of nanocrystal particles containing 33 or 34 pairs of atoms, primarily cadmium and selenium, are coated on top of the LED. The blue light excites the quantum dots, resulting in a white light with a wavelength spectrum similar to UV LEDs.


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 Blue LED is an LED developed by Sumomito Electric where a homoepitaxial ZnSe blue 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. Here also 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 SSL

Technological Comparison

SSL 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.

 

   Incandescent lamps (light bulbs) create light by running electricity through a thin filament, thereby heating the filament to a very high temperature and producing visible light. The incandescing process, however, is considered highly inefficient, as over 98% of its energy is emitted as invisible infrared light (or heat). Incandescent lamps, however, are relatively inexpensive to produce. The typical lifespan of an incandescent lamp is around 1,000 hours.

 

    Fluorescent lamps (light bulbs) work by passing electricity through mercury vapor, which in turn produces ultraviolet light. The ultraviolet light is then absorbed by a phosphorous coating inside the lamp, causing it to glow, or fluoresce. While the heat generated by fluorescent lamps is much less than its incandescent counterpart, efficiencies are still lost in generating the ultraviolet light and converting this light into visible light. In addition, mercury is detrimental to health, and should the lamp break, exposure to the substance can be hazardous. Fluorescent lamps are typically five to six times the cost of incandescent lamps, but have life spans around 10,000 hours.

 

     SSL achieves its purpose by grouping smaller LEDs in an orderly fashion, thereby creating a unified beam. The SSL can be comprised of multiple white LEDs, or from ones that are color-mixed-where LEDs of different colors are mixed to produce white light. The inherent advantages and disadvantages of SSL are the same as those of an LED. Advantages include:

        - High durability - no filament or tube to break
        - Long life span - LEDs last approximately 100,000 hours
        - Low power consumption - reduces overall electricity bill
        - Flexible application - small size of LEDs can lead to unique lighting devices
        - Low heat generation -very little parasitic energy loss

 

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, will 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

Technology

SSL

Incandescent

Fluorescent

Luminous Efficacy (lm/W)

200

16

85

Lifetime (khr)

>100

1

10

Flux (lm/lamp)

1,500

1,200

3,400

Input Power (W/lamp)

7.5

75

40

Lumens Cost ($/klm)

< 2

0.4

1.5

Lamp Cost ($/lamp)

<3

0.5

5

Color Rendering Index (CRI)

>80

95

75

Benefits

A 2001 study conducted by the DOE indicated that over 7.2 Quads (quadrillion British Thermal Units -BTUs) were used that year to provide lighting for commercial, residential, and industrial buildings and stationary fixtures in the USA. 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

 

    reduce CO2 emissions, thereby positively affecting the greenhouse effect

   decrease by 50% the global amount of electricity used for lighting

    provide higher quality lighting

   decrease by 10% the total global consumption of electricity (projected to be about 1.8 TW-hr/year, or $120B/year, by the year 2025)

   reduce projected 2025 global carbon emissions by about 300 Mtons/year

      create new industries and jobs  

 

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.

 

Challenges

Technological Hurdles

The current manufacturing process of white LEDs has not matured enough to be produced cost-effectively. Among the manufacturing hurdles to overcome include improving the processes used to deposit the active semiconductor layers of the LED, thereby increasing yields and throughput as well as decreasing costs. Problems with phosphors and their ability to emit a broader wavelength spectrum light have also been an issue. In particular, the untunability of absorption and emission, and inflexibility of form in phosphors have been issues in their 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 incandescent lamps have CRI of 100, while fluorescent lamps have CRI >75. 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 500K, which is clearly noticeable by human observer, who is normally capable of distinguishing CCT differences of 50 to 100K in range from 2000K to 6000K, which is the range of CCT variations of daylight.
Adaptation Hurdles

 

Potential pitfalls to the widespread adaptation of SSL devices include lighting fixture issues and general consumer resistance. Fixture issues can be overcome either by replacement of the fixture, or a modified SSL device that would fit into the socket. With the ubiquity of SSL, it is believed that any customer resistance will be dissipated over time.


Research and Development

In 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:

   Sandia National Laboratories 

    Rensselaer Polytechnic Institute 

   National Electrical Manufacturers Association  

 

Future

Solid State Lighting has the potential to be one of the most disruptive technologies to come to market to date. If and when the technological hurdles that are present today are overcome, it will provide long-term benefits both environmentally and economically. Various research organizations and government labs are currently working towards finding the ideal white LED, which would usher in a new era in the world of lighting.


Reference

Wikipedia the free encyclopedia

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