FREQUENTLY ASKED QUESTIONS

 1.    What is solid-state lighting?

 2.    What is a semiconductor?

 3.    What is a semiconductor LED (light emitting diode)?

 4.    What is an organic LED (OLED)?

 5.    Where can I see LED lighting today?

 6.    How do you produce white light using LEDs?

 7.    How does solid-state lighting differ from conventional lighting?

 8.    What is the energy efficiency of solid-state lighting today? How does the energy efficiency compare with incandescent and fluorescent lamps?

 9.    What is the cost of solid-state lighting today? How does the cost compare with incandescent and fluorescent lamps?

10.    What is the quality of the white light using solid-state lighting today? How does it compare with incandescent and fluorescent lamps?

11.    How much energy can be saved with solid-state lighting? How much will these energy savings reduce CO₂ emissions?

12.    What will be required to make solid-state lighting competitive in the general- purpose lighting market?

13.    What about the benefits of white LED lighting for the developing countries?

14.    Is there enough gallium to light the world with GaN-based white LEDs?

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 or gas tube, as in the case of incandescent and fluorescent lighting.  There are two types of solid-state light emitters: inorganic light-emitting diodes (usually abbreviated LEDs) or organic light-emitting diodes (usually abbreviated OLEDs). (For a definition of solid-state, see the explanation from whatis.com.)

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 indium gallium phosphide (InGaP), which emits amber and red light, and indium gallium nitride (InGaN), which emits near-UV, blue and green light.

3.  What is a semiconductor LED (light emitting diode)?

    A light emitting diode (LED) is a semiconductor diode that emits light of one or more wavelengths (colors).  A diode is a device through which electrical current can pass in only one direction.  The electrical current injects positive and negative charge carriers which recombine to create light.  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 a source of electrical current.  The housing may incorporate one or many LEDs.  An LED is typically <1 mm² in size, or approximately the size of a grain of sand.  However, when encased in the housing, the finished product may be several millimeters or more across.

4.  What is an organic LED (OLED)?

    Because the vast majority of LEDs use inorganic semiconductors, the acronym LED normally refers to inorganic-semiconductor-based LEDs.  Some LEDs use organic semiconductors (carbon-based small molecules or polymers), and the acronym OLEDs refers to these organic-semiconductor-based LEDs.  They are similar to inorganic-semiconductor-based LEDs in that passing an electrical current through an OLED creates an excited state that can then produce light.  OLEDs are less expensive than LEDs, in part because they do not need to be crystalline (or "defect free").  Hence, their fabrication processes are more forgiving, and they can even be applied as large-area coatings on curved, flexible surfaces.   However, it is likely that OLEDs will be too fragile to sustain high electrical current density, hence their light output per unit area may be limited.  For these reasons, OLEDs may target applications compatible with broad-area light sources, while LEDs target applications compatible with small-area (point-like) light sources.  This FAQ sheet, and this SSL website as a whole, is concerned primarily with inorganic-semiconductor-based LEDs for lighting.

5.  Where can I see LED lighting today?

    The first LEDs were not very bright, and were used primarily as indicator lights on electronic devices.  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 (such as those in New York’s Time Square, or along the Las Vegas strip), channel lettering (replacements for neon-tube signage, architectural accent lighting, etc.).  LEDs emitting in the ultraviolet (UV) wavelengths are finding use in a wide range of environmental applications (e.g., water purification and biochemical detection), as well as in medical devices.

6.  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 approaches.

 1.    Wavelength Conversion.  This approach converts some, or all, of the LED output into visible wavelengths in the following ways:

·      Blue LED + yellow phosphor (the least expensive of today’s methods).  Some of the blue light from an LED is used to excite a phosphor which re-emits yellow light.  The yellow light mixes with some of the blue light leaking through, resulting in the appearance of white light.

·      Blue LED + several phosphors.  Similar to the above method, except that the blue light excites several phosphors, each of which emits a different color.  These different colors are mixed with some of the blue light leaking through, to make a white light with a broader, richer wavelength spectrum.  This gives a higher color-quality light than the above method, albeit at a slightly higher cost.

·      Ultraviolet (UV) LEDs + red, green and blue phosphors.   The UV light from an LED is used to excite several phosphors, each of which emits a different color.  These different colors mix to make a white light with the broadest and richest wavelength spectrum.  This gives the highest color-quality light, again albeit at a slightly higher cost.

 2.    Color Mixing. This method uses multiple LEDs in a single lamp, and mixes the light to produce white light.  Typically, the lamp contains at least two LEDs (blue and yellow) and sometimes three (red, blue, and green) or four (red, blue, green and yellow).  Because no phosphors are used, no losses associated with wavelength conversion are incurred; hence this approach has the potential for the highest efficiency.

7.  How does solid-state 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 energy is emitted as invisible infrared light (or 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.  Small LEDs have lifetimes up to 100,000 hours .

8.  What is the energy efficiency of solid-state lighting today? How does the energy efficiency compare with incandescent and fluorescent lamps?

    Light output is commonly measured in lumens — a convolution 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 the light output (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 85 lumens/Watt.  These lighting technologies are very mature and their luminous efficacies have not improved much in many years.  Today’s white LEDs, at around 30 lumens/Watt, have luminous efficacies that are already better than those of incandescent lamps.  Moreover, it is believed possible to increase the luminous efficacies of LEDs to as high as 150-200 lumens/Watt (over 10X and 2X better than incandescent and fluorescent lamps, respectively!), with further improvements in the underlying materials and device properties and design.

9.  What is the cost of solid-state lighting today? How does the cost compare with incandescent and fluorescent lamps?

    The life ownership cost of lighting is the cost of lighting over the lifetime of the light source, including both the purchase and operating costs of lighting.  Current white LEDs have life ownership costs roughly 2x higher than incandescent lamps, and roughly 10x higher than fluorescent lamps.  Part of the promise of solid-state lighting is its potentially longer lifetimes and greater efficiencies, which could ultimately lead to life ownership costs that are 1/10 and 1/2 those of incandescent and fluorescent lighting, respectively.  A more detailed discussion of life ownership cost can be found in the recent U.S.-Department-of-Energy-sponsored OIDA Roadmap entitled " Light Emitting Diodes (LEDs) for General Illumination Update 2002" (pdf - 1.7MB).

10.  What is the quality of the white light using solid-state lighting today? 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 (CRI) is an imperfect, but widely used quantitative measure of the ability of the lighting source to accurately render the color of objects.  Natural sunlight and incandescent lamps have CRIs of 100 while fluorescent lamps have CRIs between 70 and 85.  The type and quality of lighting required depends on the application.  Current white LEDs have CRIs around 70, suitable for flashlights and outdoor lights; future white LEDs will likely have values above 80, suitable for use in commercial spaces, offices, and homes.

11.  How much energy can be saved with solid-state lighting? 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:

·     Decrease by 50% the global amount of electricity used for 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).

·     Free over 250 GW of electric generating capacity for other uses, saving about $100B in construction costs.

·     Reduce projected 2025 global carbon emissions by about 300 Mtons/year.

12.  What will be required to make solid-state 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.  A recent U.S.-Department-of-Energy-sponsored OIDA Roadmap entitled " Light Emitting Diodes(LEDs) for General Illumination Update 2002" (pdf - 1.7MB) outlines many of the technical challenges and approaches that will be required for LEDs to be economically competitive with conventional lighting.

13.  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 degrade their environment and cost 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 efficient lighting technology that can be implemented in small increments and that 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.

14.  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. GaN is part gallium (not a plentiful mineral) and part nitrogen (plentiful, in the atmosphere). Even though gallium is not plentiful, if white LEDs achieve the efficiencies envisioned in the 2002 OIDA Roadmap (see above), there is still enough gallium to light the world with LEDs. A rough estimate of the amount of gallium necessary to do so is 50 tons/year.

Worldwide gallium production capacity in the year 2003 was about 200 tons/year, with actual production about 100 tons/year (see 2003 USGS Minerals Yearbook -- Gallium). Hence, the additional use of gallium for white LED lighting could be easily accommodated by the current production capacity. In addition, gallium is available as a byproduct of bauxite refining, with known reserves of bauxite enabling the potential production of about 1.1 million tons of gallium (see 2003 USGS Gallium mineral commodity summary ). Thus, there is more than enough of this natural resource to accommodate the needs of GaN-based LEDs in lighting well into the foreseeable future.

The rough estimate of the need for 50 tons/year of gallium is based on the following assumptions and calculations:

·     Lighting Needs: By 2012, we can project (roughly) that there will be about 40 Teralumens (1 Tlm = 10¹² lumens) of installed lighting. If we assume that 20% of this is replaced every year, then about 8 Tlm of new lighting would need to be manufactured every year.

·     Efficacy of LED Lighting: If we achieve LED lighting with luminous efficacies of 150 lumens/watt and input power densities to the LED chip of 500 Watts/cm², we will need to produce 10,000 m² of LED chips per year.

·     Gallium Necessary to Produce LEDs: The actual volume of GaN will depend on whether the LED chips use GaN only to create thin layers grown on wafers made of other materials (e.g., sapphire, or Al2O3), or use GaN to create the wafers themselves. Creating wafers would use the most gallium. If these wafers are 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 gallium in that volume of GaN would be roughly 25 tons.

·     Efficiency of Gallium Usage in LED Chips: Then, assuming the original gallium mined and produced is utilized in the GaN-based LED chips with an efficiency of 50%, the mass of gallium that would be needed would be double that, or 50 tons.

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