1. What is solid-state lighting?
Solid-state lighting is a new technology that has the potential to far exceed the energy efficiencies of incandescent and fluorescent lighting. Solid-state lighting uses light-emitting diodes or "LEDs" for illumination, the same kind of practical and inexpensive devices that provide the letters on your clock radio. The term "solid-state" refers to the fact that the light in an LED is emitted from a solid object - a block of semiconductor - rather than from a vacuum tube, as in the case of incandescent and fluorescent. There are two types of solid-state light emitters: inorganic light-emitting diodes (LEDs) or organic light-emitting diodes (OLEDs) for white-light illumination.
2. What is a semiconductor?
A semiconductor is a substance whose electrical conductivity can be altered through variations in temperature, applied fields, concentration of impurities, etc. The most common semiconductor is Silicon, which is used predominantly for electronic applications (where electrical currents and voltages are the main inputs and outputs). For optoelectronic applications (where light is one of the inputs or outputs) other semiconductors must be used, including GaAs, InP and GaN. For inorganic LEDs the most common semiconductors are: InGaN, which emits near-UV, blue and green light; and InGaP, which emits amber and red light.
3. What is a semiconductor LED (Light Emitting Diode)?
A light emitting diode (LED) is a small semiconductor device that emits light in one or more wavelengths (colors). A diode is a device with two electrodes through which a current can be passed in only one direction. The diode is attached to an electrical circuit and encased in a plastic, epoxy, resin or ceramic housing. The housing usually consists of some sort of covering over the device as well as some means of attaching the LED to an electrical source. The housing may incorporate one or many LEDs. A LED is typically 0.1 mm to 1mm in size, or approximately the size of a grain of sand. However, when encased in the housing, the finished product maybe several mm or more across.
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4. Where can I see LED lighting today?
Today's high-brightness LEDs can be found in a wide number of consumer applications. These include backlighting for color displays in personal electronics (e.g., cell phones), automotive interior and exterior lighting, traffic signals, large-area outdoor displays, channel lettering (replacement for neon-tube signage, architectural accent lighting, etc.). LEDs emitting in the ultraviolet (UV) wavelengths are finding use in a wide range of environmental detectors and sensors, as well as in medical devices.
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5. How do you produce white light using LEDs?
An individual LED produces a single color of light. To produce white light, light spanning the visible spectrum (red, green and blue) must be generated in the correct proportions. Methods to generate white light using LEDs can be broadly classified into two methods.
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a)
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Phosphor Method - converts some, or all, of the LED output into visible wavelengths in the following ways: |
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Blue LED + yellow phosphor (Most commercial white LEDs use this method). Some of the blue light from the LED excites the phosphor to emit yellow light, and then the rest of the blue light is mixed with the yellow light to make white light. |
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Blue LED + several phosphors (enhances the method above). |
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Ultraviolet (UV) LEDs + red, green and blue phosphors. |
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b)
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Multiple LED Method - uses multiple LEDs in one housing. Typically, the housing contains at least two LEDs (blue and yellow) and sometimes three LEDs (red, blue, and green or red, blue and yellow). No phosphor is used. When the LED is illuminated, the red, blue, and green/yellow light combine to produce white light. This last method can also use red, green and blue, in which case, many colors including white can be produced; with yellow added as a fourth color, the widest range of whites are produced.
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6. How does LED based lighting differ from conventional lighting?
Incandescent lamps (light bulbs) create light by heating a thin filament to a very high temperature. Incandescent lamps have low efficiencies because most (over 90%) of the electricity gets emitted as heat. A fluorescent lamp produces ultraviolet light when electricity is passed through a mercury vapor, causing the phosphor coating inside the fluorescent tube to glow or fluoresce. There are efficiency losses in generating the ultraviolet light, and in converting the ultraviolet light into visible light. Incandescent lamps typically have short lifetimes (around 1,000 hours) due to the high temperatures of the filaments, while fluorescent lamps have moderate lifetimes (around 10,000 hours) that are limited by the electrodes for the discharge. LEDs, on the other hand, use semiconductors that are more efficient, more rugged, more durable, and can be controlled (for example, dimmed) more easily. LEDs have lifetimes up to 100,000 hours.
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7. What is the energy efficiency of LED based lighting today and how does the energy efficiency compare with incandescent and fluorescent lamps?
Light output is commonly measured in lumens, which is the product of the radiated power and the sensitivity of the human eye. A 60-Watt incandescent bulb produces about 850 lumens. The efficiency of lighting (luminous efficacy) is generally described as the amount of light (lumens) produced per unit of input electrical power (Watts) or lumens/Watt. An incandescent lamp wastes most of its power as heat, with the result that its luminous efficacy is only around 15 lumens/Watt. A fluorescent lamp is much better at roughly 70-85 lumens/Watt. These lighting technologies are very mature and have not improved much in luminous efficacy in many years. Today?s white LEDs have luminous efficacies that are already better than incandescent lamps at around 50 lumens/Watt. However, the luminous efficacy of LEDs can potentially be increased to 200 lumens/Watt (over 10X better than incandescent and 3.5X better than fluorescent lamps. Current state of the art is 70+ lumens/watt for white LED clusters. It should be noted that CFL or compact fluorescent lamps are 40-50 lumens/Watt.
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Light Source
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Efficiency/Efficacy
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Lifetime
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Incandescent bulb
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16 lumens/watt
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1000 hours
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Fluorescent lamp
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85 lumens/watt
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10,000 hours
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Today?s white LEDs
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50+ lumens/watt
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20,000 hours -100,000 hours
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Future white LEDs
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up to 200 lumens/watt (5-10 years)
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100,000+ hours
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8. What is the cost of solid-state LED lighting today? How does the cost compare with incandescent and fluorescent lamps?
Current white LEDs are over 10-20 times more expensive in $ per lumen than incandescent or fluorescent lamps. Part of the promise of solid-state lighting is that it can achieve much longer lifetimes and greater efficiency, which will help reduce its overall cost of ownership. Nevertheless, it will still be necessary to continue to reduce the LED cost. Part of this problem is the cost of a 20 watt incandescent bulb is nearly the same material cost as a 100 watt bulb. An LED bulb's material cost increases substantially as the wattage is increased making the cost comparison skewed for higher wattage bulbs.
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9. What is the quality of the white light using solid-state lighting today and how does it compare with incandescent and fluorescent lamps?
The quality of a lighting source is judged by its ability to reproduce the appearance of an object as if illuminated by "true" white light. The color-rendering index (Ra) is a quantitative measure the ability of the lighting source to accurately render the color of objects. High quality incandescent lamps have a CRI near 100 while fluorescent lamps are available with Ra values between 70 and 85. The type and quality of lighting required is dependent upon the application. Current white LEDs have Ra around 70, but the next generation of white LEDs will have values above 80, suitable for wide use in commercial spaces, offices, and homes.
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10. How much energy can be saved with solid-state lighting and how much will these energy savings reduce CO2 emissions?
A little over one-third of all primary energy is used for generation of electricity, and a little over one-fifth of all electricity is used for lighting. Hence, around one-fifteenth of all energy is used for lighting in the United States. Doubling the average luminous efficacy of white lighting through the use of solid-state lighting would potentially:
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Decrease by 50% the global amount of electricity used for lighting. |
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Decrease by 10% the total global consumption of electricity. |
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Global reductions of 1,100 Billion k Wh/year of electricity, or $100B/year. |
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Free over 125 GW of electric generating capacity for other uses, saving about $50B in construction costs. |
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Reduce global carbon emissions by 200M tons/year. |
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11. What will be required to make LED lighting competitive in the general-purpose lighting market?
The performance of solid-state lighting will need to be substantially improved. For example, improvements in materials and devices, and the physics that underlies them, are needed to improve the luminous efficacy and the color rendering quality of the white light. The cost of solid-state lighting will also need to be substantially reduced. This will require improvements in the manufacturing processes and materials. For example, improvements are needed in the processes used to deposit the active semiconductor layers of the LED in order to improve yields, increase throughput, and reduce overall capital and operating costs. The recently completed OIDA roadmap entitled "Light Emitting Diodes (LEDs) for General Purpose Illumination Update 2002" outlines many of the technical challenges and approaches that will be required for LEDs to be economically competitive with conventional lighting.
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12. What about the benefits of white LED lighting for the developing countries?
There are approximately 2 billion people without access to electricity. These people use traditional fuels (e.g., kerosene or bio-mass) that degrades their environment and costs over 1500 times more per lumen-hour than the conventional lighting using electricity in developed countries. Solid-state lighting can be highly beneficial to developing countries by providing a more efficient lighting technology that can be implemented in small increments and works well with small, micro-power systems (e.g., solar photovoltaic, small hydroelectric generators, etc.). Organizations like Light Up The World (http://www.lutw.org) and Solar-Electric Light Fund (http://www.self.org) are helping to promote lighting in the developing world using energy-efficient lighting.
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13. Is there enough Gallium to light the world with GaN-based white LEDs?
Gallium nitride (GaN) is the semiconductor material of choice for solid-state lighting, and is half gallium and half nitrogen. Nitrogen is plentiful (e.g., in the atmosphere), but gallium is not. Still, there is enough gallium to light the world with GaN-based white LEDs. A rough estimate of the amount of gallium necessary is forty-eight tons per year. Though this appears to be a very large quantity, it is much smaller than the amount of gallium already consumed in the world today. Gallium consumption in 2000 was about 200 tons. The use of gallium for white LED lighting would increase gallium consumption by about 25% over this amount.
The rough estimate of forty-eight tons per year is based on the following argument. By 2012, we can project (roughly) that there will be about 40 Teralumens (1 Tlm = 1012 lumens) of installed lighting. If we assume that at most 20% of this is replaced every year, then about 8 Tlm of new lighting will need to be manufactured every year. If we achieve LED lighting with outputs of 150 lumens/watt and input power densities of 500 Watts/cm², we will need to produce 10,000 m² of gallium nitride (GaN) per year. That is enough GaN to cover two football fields. The actual volume of GaN will depend on whether the GaN is used only to create thin layers grown on wafers made of other materials (e.g., sapphire, or Al2O3), or to create the wafers themselves. In the worst case, GaN will be used to create the wafers. If these wafers were approximately 400 microns thick, then the volume of GaN necessary to produce those wafers in a given year would be approximately 4 million cm³, and the mass of Ga in that volume of GaN would be roughly 24 tons. Assuming a usage efficiency of 50%, the mass of Ga that would be needed would be 48 tons.
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