Parts of an LED. Although not
directly labeled, the flat bottom surfaces
of the anvil and post embedded inside the epoxy act as anchors, to
prevent the conductors from being forcefully pulled out from mechanical
strain or vibration.
A
light-emitting diode (
LED) is a
semiconductor light source.
[4]
LEDs are used as indicator lamps in many devices and are increasingly
used for other lighting. Appearing as practical electronic components in
1962,
[5] early LEDs emitted low-intensity red light, but modern versions are available across the
visible,
ultraviolet, and
infrared wavelengths, with very high brightness.
When a light-emitting diode is switched on,
electrons are able to recombine with electron holes within the device, releasing energy in the form of
photons. This effect is called
electroluminescence
and the color of the light (corresponding to the energy of the photon)
is determined by the energy gap of the semiconductor. An LED is often
small in area (less than 1 mm
2), and integrated optical components may be used to shape its
radiation pattern.
[6]
LEDs present many advantages over incandescent light sources including
lower energy consumption, longer lifetime, improved physical robustness,
smaller size, and faster switching. However, LEDs powerful enough for
room lighting are relatively expensive and require more precise current
and heat management than compact
fluorescent lamp sources of comparable output.
Light-emitting diodes are used in applications as diverse as
aviation lighting,
automotive lighting, advertising, general lighting, and
traffic signals.
LEDs have allowed new text, video displays, and sensors to be
developed, while their high switching rates are also useful in advanced
communications technology. Infrared LEDs are also used in the remote
control units of many commercial products including televisions, DVD
players and other domestic appliances. LEDs are also used in
seven-segment display.
History
Discoveries and early devices
Green electroluminescence from a point contact on a crystal of
SiC recreates
H. J. Round's original experiment from 1907.
Electroluminescence as a phenomenon was discovered in 1907 by the British experimenter
H. J. Round of
Marconi Labs, using a crystal of
silicon carbide and a
cat's-whisker detector.
[7][8] Russian
Oleg Vladimirovich Losev reported creation of the first LED in 1927.
[9][10]
His research was distributed in Russian, German and British scientific
journals, but no practical use was made of the discovery for several
decades.
[11][12] Rubin Braunstein
[13] of the
Radio Corporation of America reported on infrared emission from
gallium arsenide (GaAs) and other semiconductor alloys in 1955.
[14] Braunstein observed infrared emission generated by simple diode structures using
gallium antimonide (GaSb), GaAs,
indium phosphide (InP), and
silicon-germanium (SiGe) alloys at room temperature and at 77 kelvins.
In 1961 American experimenters Robert Biard and Gary Pittman, working at
Texas Instruments,
[15] found that GaAs emitted infrared radiation when electric current was applied and received the patent for the infrared LED.
The first practical visible-spectrum (red) LED was developed in 1962 by
Nick Holonyak, Jr., while working at
General Electric Company.
[5] Holonyak first reported this breakthrough in the journal Applied Physics Letters on the December 1, 1962.
[16] Holonyak is seen as the "father of the light-emitting diode".
[17] M. George Craford,
[18]
a former graduate student of Holonyak, invented the first yellow LED
and improved the brightness of red and red-orange LEDs by a factor of
ten in 1972.
[19]
In 1976, T. P. Pearsall created the first high-brightness,
high-efficiency LEDs for optical fiber telecommunications by inventing
new semiconductor materials specifically adapted to optical fiber
transmission wavelengths.
[20]
Commercial development
The first commercial LEDs were commonly used as replacements for
incandescent and
neon indicator lamps, and in
seven-segment displays,
[21]
first in expensive equipment such as laboratory and electronics test
equipment, then later in such appliances as TVs, radios, telephones,
calculators, and even watches (see list of
signal uses). Until 1968, visible and infrared LEDs were extremely costly, in the order of
US$200 per unit, and so had little practical use.
[3] The
Monsanto Company
was the first organization to mass-produce visible LEDs, using gallium
arsenide phosphide (GaAsP) in 1968 to produce red LEDs suitable for
indicators.
[3] Hewlett Packard
(HP) introduced LEDs in 1968, initially using GaAsP supplied by
Monsanto. These red LEDs were bright enough only for use as indicators,
as the light output was not enough to illuminate an area. Readouts in
calculators were so small that plastic lenses were built over each digit
to make them legible. Later, other colors grew widely available and
also appeared in appliances and equipment. In the 1970s commercially
successful LED devices at less than five cents each were produced by
Fairchild Optoelectronics. These devices employed compound semiconductor
chips fabricated with the
planar process invented by Dr. Jean Hoerni at
Fairchild Semiconductor.
[22]
The combination of planar processing for chip fabrication and
innovative packaging methods enabled the team at Fairchild led by
optoelectronics pioneer Thomas Brandt to achieve the needed cost
reductions.
[23] These methods continue to be used by LED producers.
[24]
LED display of a
TI-30 scientific calculator (ca. 1978), which uses plastic lenses to increase the visible digit size
As LED materials technology grew more advanced, light output rose,
while maintaining efficiency and reliability at acceptable levels. The
invention and development of the high-power white-light LED led to use
for illumination, which is fast replacing incandescent and fluorescent
lighting
[25][26] (see list of
illumination applications).
Most LEDs were made in the very common 5 mm T1¾ and 3 mm T1 packages,
but with rising power output, it has grown increasingly necessary to
shed excess heat to maintain reliability,
[27] so more complex packages have been adapted for efficient heat dissipation. Packages for state-of-the-art
high-power LEDs bear little resemblance to early LEDs.
The blue and white LED
Illustration of
Haitz's law. Light output per LED as a function of production year; note the logarithmic scale on the vertical axis
The first high-brightness blue LED was demonstrated by
Shuji Nakamura of
Nichia Corporation in 1994 and was based on
InGaN.
[28] Its development built on critical developments in
GaN nucleation on sapphire substrates and the demonstration of
p-type doping of GaN, developed by
Isamu Akasaki and H. Amano in
Nagoya.
[citation needed] In 1995,
Alberto Barbieri at the
Cardiff University
Laboratory (GB) investigated the efficiency and reliability of
high-brightness LEDs and demonstrated a "transparent contact" LED using
indium tin oxide (ITO) on (AlGaInP/GaAs). The existence of blue LEDs and high-efficiency LEDs quickly led to the development of the first
white LED, which employed a
Y3Al5O12:Ce, or "
YAG", phosphor coating to mix (
down-converted yellow light with blue to produce light that appears white. Nakamura was awarded the 2006
Millennium Technology Prize for his invention.
[29]
The development of LED technology has caused their efficiency and light output to
rise exponentially, with a doubling occurring approximately every 36 months since the 1960s, in a way similar to
Moore's law.
This trend is generally attributed to the parallel development of other
semiconductor technologies and advances in optics and material science,
and has been called
Haitz's law after Dr. Roland Haitz.
[30]
In 2001
[31] and 2002,
[32] processes for growing
gallium nitride (GaN) LEDs on
silicon were successfully demonstrated. In January 2012,
Osram demonstrated high-power InGaN LEDs grown on Silicon substrates commercially.
[33] It has been speculated that the use of six-inch silicon wafers instead of two-inch
sapphire wafers and
epitaxy manufacturing processes could reduce production costs by up to 90%.
[34]
Technology
The inner workings of an LED
I-V diagram for a
diode. An LED will begin to emit light when the on-
voltage is exceeded. Typical on voltages are 2–3
volts.
Physics
The LED consists of a chip of semiconducting material
doped with impurities to create a
p-n junction. As in other diodes, current flows easily from the p-side, or
anode, to the n-side, or
cathode, but not in the reverse direction. Charge-carriers—
electrons and
holes—flow into the junction from
electrodes with different voltages. When an electron meets a hole, it falls into a lower
energy level, and releases
energy in the form of a
photon.
The
wavelength of the light emitted, and thus its color depends on the
band gap energy of the materials forming the
p-n junction. In
silicon or
germanium diodes, the electrons and holes recombine by a
non-radiative transition, which produces no optical emission, because these are
indirect band gap materials. The materials used for the LED have a
direct band gap with energies corresponding to near-infrared, visible, or near-ultraviolet light.
LED development began with infrared and red devices made with
gallium arsenide. Advances in
materials science have enabled making devices with ever-shorter wavelengths, emitting light in a variety of colors.
LEDs are usually built on an n-type substrate, with an electrode
attached to the p-type layer deposited on its surface. P-type
substrates, while less common, occur as well. Many commercial LEDs,
especially GaN/InGaN, also use
sapphire substrate.
Most materials used for LED production have very high
refractive indices. This means that much light will be reflected back into the material at the material/air surface interface. Thus,
light extraction in LEDs is an important aspect of LED production, subject to much research and development.
Refractive index
Idealized example of light emission cones in a semiconductor, for a
single point-source emission zone. The left illustration is for a fully
translucent wafer, while the right illustration shows the half-cones
formed when the bottom layer is fully opaque. The light is actually
emitted equally in all directions from the point-source, so the areas
between the cones shows the large amount of trapped light energy that is
wasted as heat.
[35]
The light emission cones of a real LED wafer are far more complex than a
single point-source light emission. The light emission zone is
typically a two-dimensional plane between the wafers. Every atom across
this plane has an individual set of emission cones. Drawing the billions
of overlapping cones is impossible, so this is a simplified diagram
showing the extents of all the emission cones combined. The larger side
cones are clipped to show the interior features and reduce image
complexity; they would extend to the opposite edges of the
two-dimensional emission plane.
Bare uncoated semiconductors such as
silicon exhibit a very high
refractive index
relative to open air, which prevents passage of photons at sharp angles
relative to the air-contacting surface of the semiconductor. This
property affects both the light-emission efficiency of LEDs as well as
the light-absorption efficiency of
photovoltaic cells. The refractive index of silicon is 3.96 (590 nm),
[36] while air is 1.0002926.
[37]
In general, a flat-surface uncoated LED semiconductor chip will emit
light only perpendicular to the semiconductor's surface, and a few
degrees to the side, in a cone shape referred to as the
light cone,
cone of light,
[38] or the
escape cone.
[35] The maximum
angle of incidence is referred to as the
critical angle.
When this angle is exceeded, photons no longer penetrate the
semiconductor but are instead reflected both internally inside the
semiconductor crystal and externally off the surface of the crystal as
if it were a
mirror.
[35]
Internal reflections
can escape through other crystalline faces, if the incidence angle is
low enough and the crystal is sufficiently transparent to not re-absorb
the photon emission. But for a simple square LED with 90-degree angled
surfaces on all sides, the faces all act as equal angle mirrors. In this
case the light can not escape and is lost as waste heat in the crystal.
[35]
A convoluted chip surface with angled
facets similar to a jewel or
fresnel lens
can increase light output by allowing light to be emitted perpendicular
to the chip surface while far to the sides of the photon emission
point.
[39]
The ideal shape of a semiconductor with maximum light output would be a
microsphere
with the photon emission occurring at the exact center, with electrodes
penetrating to the center to contact at the emission point. All light
rays emanating from the center would be perpendicular to the entire
surface of the sphere, resulting in no internal reflections. A
hemispherical semiconductor would also work, with the flat back-surface
serving as a mirror to back-scattered photons.
[40]
Transition coatings
After doping the
wafer, people at the
fabrication plant cut the wafer apart into individual
die. Each die is commonly called a chip.
Many LED semiconductor chips are encapsulated or
potted in clear or colored molded plastic shells. The plastic shell has three purposes:
- Mounting the semiconductor chip in devices is easier to accomplish.
- The tiny fragile electrical wiring is physically supported and protected from damage.
- The plastic acts as a refractive intermediary between the relatively high-index semiconductor and low-index open air.[41]
The third feature helps to boost the light emission from the
semiconductor by acting as a diffusing lens, allowing light to be
emitted at a much higher angle of incidence from the light cone than the
bare chip is able to emit alone.
Efficiency and operational parameters
Typical indicator LEDs are designed to operate with no more than 30–60
milliwatts (mW) of electrical power. Around 1999,
Philips Lumileds introduced power LEDs capable of continuous use at one
watt.
These LEDs used much larger semiconductor die sizes to handle the large
power inputs. Also, the semiconductor dies were mounted onto metal
slugs to allow for heat removal from the LED die. LED power densities up
to 300W/cm2 have been achieved.
[42]
One of the key advantages of LED-based lighting sources is high
luminous efficiency.
White LEDs quickly matched and overtook the efficacy of standard
incandescent lighting systems. In 2002, Lumileds made five-watt LEDs
available with a
luminous efficacy of 18–22 lumens per watt (lm/W). For comparison, a conventional
incandescent light bulb of 60–100 watts emits around 15 lm/W, and standard
fluorescent lights emit up to 100 lm/W. A recurring problem is that efficacy falls sharply with rising current. This effect is known as
droop and effectively limits the light output of a given LED, raising heating more than light output for higher current.
[43][44][45]
As of 2012, the Lumiled catalog gives the following as the best efficacy for each color:
[46]
|
Color |
Wavelength range (nm) |
Typical efficacy (lm/W) |
|
Red |
620 < λ < 645 |
72 |
|
Red-orange |
610 < λ < 620 |
98 |
|
Green |
520 < λ < 550 |
93 |
|
Cyan |
490 < λ < 520 |
75 |
|
Blue |
460 < λ < 490 |
37 |
In September 2003, a new type of blue LED was demonstrated by the company
Cree Inc. to provide 24 mW at 20
milliamperes
(mA). This produced a commercially packaged white light giving 65 lm/W
at 20 mA, becoming the brightest white LED commercially available at the
time, and more than four times as efficient as standard incandescents.
In 2006, they demonstrated a prototype with a record white LED luminous
efficacy of 131 lm/W at 20 mA.
Nichia Corporation has developed a white LED with luminous efficacy of 150 lm/W at a forward current of 20 mA.
[47]
Cree's XLamp XM-L LEDs, commercially available in 2011, produce 100
lumens per watt at their full power of 10 watts, and up to 160
lumens/watt at around 2 watts input power. In 2012, Cree announced a
white LED giving 254 lumens per watt.
[48]
Practical general lighting needs high-power LEDs, of one watt or
more. Typical operating currents for such devices begin at 350 mA.
Note that these efficiencies are for the LED chip only, held at low
temperature in a lab. Lighting works at higher temperature and with
drive circuit losses, so efficiencies are much lower.
United States Department of Energy (DOE) testing of commercial LED lamps designed to replace incandescent lamps or
CFLs showed that average efficacy was still about 46 lm/W in 2009 (tested performance ranged from 17 lm/W to 79 lm/W).
[49]
Cree issued a press release on February 3, 2010 about a laboratory
prototype LED achieving 208 lumens per watt at room temperature. The
correlated
color temperature was reported to be 4579 K.
[50]
In December 2012 Cree issued another press release announcing
commercial availability of 200 lumens per watt LED at room temperature.
[51]
Lifetime and failure
Solid-state devices such as LEDs are subject to very limited
wear and tear
if operated at low currents and at low temperatures. Many of the LEDs
made in the 1970s and 1980s are still in service today. Typical
lifetimes quoted are 25,000 to 100,000 hours, but heat and current
settings can extend or shorten this time significantly.
[52]
The most common symptom of LED (and
diode laser)
failure is the gradual lowering of light output and loss of efficiency.
Sudden failures, although rare, can occur as well. Early red LEDs were
notable for their short service life. With the development of high-power
LEDs the devices are subjected to higher
junction temperatures
and higher current densities than traditional devices. This causes
stress on the material and may cause early light-output degradation. To
quantitatively classify useful lifetime in a standardized manner it has
been suggested to use the terms L70 and L50, which is the time it will
take a given LED to reach 70% and 50% light output respectively.
[53]
Like other lighting devices, LED performance is temperature
dependent. Most manufacturers' published ratings of LEDs are for an
operating temperature of 25 °C. LEDs used outdoors, such as traffic
signals or in-pavement signal lights, and that are utilized in climates
where the temperature within the luminaire gets very hot, could result
in low signal intensities or even failure.
[54]
LED light output rises at lower temperatures, leveling off, depending on type, at around −30 °C.
[citation needed] Thus, LED technology may be a good replacement in uses such as supermarket freezer lighting
[55][56][57]
and will last longer than other technologies. Because LEDs emit less
heat than incandescent bulbs, they are an energy-efficient technology
for uses such as in freezers and refrigerators. However, because they
emit little heat, ice and snow may build up on the LED luminaire in
colder climates.
[54]
Similarly, this lack of waste heat generation has been observed to
sometimes cause significant problems with street traffic signals and
airport runway lighting in snow-prone areas. In response to this
problem, some LED lighting systems have been designed with an added
heating circuit at the expense of reduced overall electrical efficiency
of the system; additionally, research has been done to develop heat sink
technologies that will transfer heat produced within the junction to
appropriate areas of the luminaire.
[58]
Colors and materials
Conventional LEDs are made from a variety of inorganic
semiconductor materials. The following table shows the available colors with wavelength range, voltage drop and material:
Ultraviolet and blue LEDs
Current bright blue LEDs are based on the wide
band gap semiconductors GaN (
gallium nitride) and
InGaN
(indium gallium nitride). They can be added to existing red and green
LEDs to produce the impression of white light. Modules combining the
three colors are used in big
video screens and in adjustable-color fixtures.
The first blue LEDs using gallium nitride were made in 1971 by Jacques Pankove at
RCA Laboratories.
[67]
These devices had too little light output to be of practical use and
research into gallium nitride devices slowed. In August 1989, Cree Inc.
introduced the first commercially available blue LED based on the
indirect bandgap semiconductor, silicon carbide.
[68] SiC LEDs had very low efficiency, no more than about 0.03%, but did emit in the blue portion of the visible light spectrum.
In the late 1980s, key breakthroughs in GaN
epitaxial growth and
p-type doping
[69]
ushered in the modern era of GaN-based optoelectronic devices. Building
upon this foundation, in 1993 high-brightness blue LEDs were
demonstrated.
[70] High-brightness blue LEDs invented by
Shuji Nakamura of
Nichia Corporation using gallium nitride revolutionized LED lighting, making high-power light sources practical.
By the late 1990s, blue LEDs had become widely available. They have an active region consisting of one or more InGaN
quantum wells
sandwiched between thicker layers of GaN, called cladding layers. By
varying the relative In/Ga fraction in the InGaN quantum wells, the
light emission can in theory be varied from violet to amber.
Aluminium gallium nitride
(AlGaN) of varying Al/Ga fraction can be used to manufacture the
cladding and quantum well layers for ultraviolet LEDs, but these devices
have not yet reached the level of efficiency and technological maturity
of InGaN/GaN blue/green devices. If un-alloyed GaN is used in this case
to form the active quantum well layers, the device will emit
near-ultraviolet light with a peak wavelength centred around 365 nm.
Green LEDs manufactured from the InGaN/GaN system are far more efficient
and brighter than green LEDs produced with non-nitride material
systems, but practical devices still exhibit efficiency too low for
high-brightness applications.
With nitrides containing aluminium, most often
AlGaN and
AlGaInN,
even shorter wavelengths are achievable. Ultraviolet LEDs in a range of
wavelengths are becoming available on the market. Near-UV emitters at
wavelengths around 375–395 nm are already cheap and often encountered,
for example, as
black light lamp replacements for inspection of anti-
counterfeiting
UV watermarks in some documents and paper currencies.
Shorter-wavelength diodes, while substantially more expensive, are
commercially available for wavelengths down to 247 nm.
[71] As the photosensitivity of microorganisms approximately matches the absorption spectrum of
DNA,
with a peak at about 260 nm, UV LED emitting at 250–270 nm are to be
expected in prospective disinfection and sterilization devices. Recent
research has shown that commercially available UVA LEDs (365 nm) are
already effective disinfection and sterilization devices.
[72]
Deep-UV wavelengths were obtained in laboratories using
aluminium nitride (210 nm),
[63] boron nitride (215 nm)
[61][62] and
diamond (235 nm).
[60]
White light
There are two primary ways of producing white light-emitting diodes (WLEDs), LEDs that generate high-intensity
white light. One is to use individual LEDs that emit three
primary colors[73]—red,
green, and blue—and then mix all the colors to form white light. The
other is to use a phosphor material to convert monochromatic light from a
blue or UV LED to broad-spectrum white light, much in the same way a
fluorescent light bulb works.
Because of
metamerism, it is possible to have quite different spectra that appear white.
RGB systems
Combined spectral curves for blue, yellow-green, and high-brightness red solid-state semiconductor LEDs.
FWHM spectral bandwidth is approximately 24–27 nm for all three colors.
White light can be formed by mixing differently colored lights; the most common method is to use
red, green, and blue
(RGB). Hence the method is called multi-color white LEDs (sometimes
referred to as RGB LEDs). Because these need electronic circuits to
control the blending and
diffusion
of different colors, and because the individual color LEDs typically
have slightly different emission patterns (leading to variation of the
color depending on direction) even if they are made as a single unit,
these are seldom used to produce white lighting. Nevertheless, this
method is particularly interesting in many uses because of the
flexibility of mixing different colors,
[74] and, in principle, this mechanism also has higher quantum efficiency in producing white light.
There are several types of multi-color white LEDs:
di-,
tri-, and
tetrachromatic white LEDs. Several key factors that play among these different methods, include color stability,
color rendering capability, and
luminous efficacy.
Often, higher efficiency will mean lower color rendering, presenting a
trade-off between the luminous efficiency and color rendering. For
example, the dichromatic white LEDs have the best luminous efficacy (120
lm/W), but the lowest color rendering capability. However, although
tetrachromatic
white LEDs have excellent color rendering capability, they often have
poor luminous efficiency. Trichromatic white LEDs are in between, having
both good luminous efficacy (>70 lm/W) and fair color rendering
capability.
One of the challenges is the development of more efficient green
LEDs. The theoretical maximum for green LEDs is 683 lumens per watt but
today few green LEDs exceed even 100 lumens per watt. The blue and red
LEDs get closer to their theoretical limits.
Multi-color LEDs offer not merely another means to form white light but a new means to form light of different colors. Most
perceivable colors
can be formed by mixing different amounts of three primary colors. This
allows precise dynamic color control. As more effort is devoted to
investigating this method, multi-color LEDs should have profound
influence on the fundamental method that we use to produce and control
light color. However, before this type of LED can play a role on the
market, several technical problems must be solved. These include that
this type of LED's emission power
decays exponentially with rising temperature,
[75]
resulting in a substantial change in color stability. Such problems
inhibit and may preclude industrial use. Thus, many new package designs
aimed at solving this problem have been proposed and their results are
now being reproduced by researchers and scientists.
Phosphor-based LEDs
Spectrum of a “white” LED clearly showing blue light directly emitted by
the GaN-based LED (peak at about 465 nm) and the more broadband
Stokes-shifted light emitted by the Ce
3+:YAG phosphor, which emits at roughly 500–700 nm.
This method involves
coating LEDs of one color (mostly blue LEDs made of InGaN) with
phosphors of different colors to form white light; the resultant LEDs are called
phosphor-based white LEDs.
[76] A fraction of the blue light undergoes the
Stokes shift
being transformed from shorter wavelengths to longer. Depending on the
color of the original LED, phosphors of different colors can be
employed. If several phosphor layers of distinct colors are applied, the
emitted spectrum is broadened, effectively raising the
color rendering index (CRI) value of a given LED.
[77]
Phosphor-based LED efficiency losses are due to the heat loss from
the Stokes shift and also other phosphor-related degradation issues.
Their efficiencies compared to normal LEDs depend on the spectral
distribution of the resultant light output and the original wavelength
of the LED itself. For example, the efficiency of a typical YAG yellow
phosphor based white LED ranges from 3 to 5 times the efficiency of the
original blue LED because of the greater
luminous efficacy
of yellow compared to blue light. Due to the simplicity of
manufacturing the phosphor method is still the most popular method for
making high-intensity white LEDs. The design and production of a light
source or light fixture using a monochrome emitter with phosphor
conversion is simpler and cheaper than a complex
RGB system, and the majority of high-intensity white LEDs presently on the market are manufactured using phosphor light conversion.
Among the challenges being faced to improve the efficiency of
LED-based white light sources is the development of more efficient
phosphors. Today the most efficient yellow phosphor is still the YAG
phosphor, with less than 10% Stoke shift loss. Losses attributable to
internal optical losses due to re-absorption in the LED chip and in the
LED packaging itself account typically for another 10% to 30% of
efficiency loss. Currently, in the area of phosphor LED development,
much effort is being spent on optimizing these devices to higher light
output and higher operation temperatures. For instance, the efficiency
can be raised by adapting better package design or by using a more
suitable type of phosphor. Conformal coating process is frequently used
to address the issue of varying phosphor thickness.
The phosphor-based white LEDs encapsulate InGaN blue LEDs inside phosphor coated epoxy. A common yellow phosphor material is
cerium-
doped yttrium aluminium garnet (Ce
3+:YAG).
White LEDs can also be made by
coating near-
ultraviolet (NUV) LEDs with a mixture of high-efficiency
europium-based
phosphors that emit red and blue, plus copper and aluminium-doped zinc
sulfide (ZnS:Cu, Al) that emits green. This is a method analogous to the
way
fluorescent lamps work. This method is less efficient than blue LEDs with YAG:Ce phosphor, as the
Stokes shift
is larger, so more energy is converted to heat, but yields light with
better spectral characteristics, which render color better. Due to the
higher radiative output of the ultraviolet LEDs than of the blue ones,
both methods offer comparable brightness. A concern is that UV light may
leak from a malfunctioning light source and cause harm to human eyes or
skin.
Other white LEDs
Another method used to produce experimental white light LEDs used no phosphors at all and was based on
homoepitaxially grown
zinc selenide (ZnSe) on a ZnSe substrate that simultaneously emitted blue light from its active region and yellow light from the substrate.
[78]
Organic light-emitting diodes (OLEDs)
In an organic light-emitting diode (
OLED), the
electroluminescent material comprising the emissive layer of the diode is an
organic compound. The organic material is electrically conductive due to the
delocalization of pi electrons caused by
conjugation over all or part of the molecule, and the material therefore functions as an
organic semiconductor.
[79] The organic materials can be small organic
molecules in a
crystalline phase, or
polymers.
The potential advantages of OLEDs include thin, low-cost displays
with a low driving voltage, wide viewing angle, and high contrast and
color gamut.
[80] Polymer LEDs have the added benefit of printable
[81][82] and
flexible[83]
displays. OLEDs have been used to make visual displays for portable
electronic devices such as cellphones, digital cameras, and MP3 players
while possible future uses include lighting and televisions.
[80]
Quantum dot LEDs (experimental)
Quantum dots (QD) are semiconductor
nanocrystals that possess unique optical properties.
[84]
Their emission color can be tuned from the visible throughout the
infrared spectrum. This allows quantum dot LEDs to create almost any
color on the
CIE diagram. This provides more color options and better color rendering than white LEDs.
[citation needed] Quantum dot LEDs are available in the same package types as traditional
phosphor-based LEDs.
[citation needed]There
are two types of schemes for QD excitation. One uses photo excitation
with a primary light source LED (typically blue or UV LEDs are used).
The other is direct electrical excitation first demonstrated by
Alivisatos et al.
[85]
One example of the photo-excitation scheme is a method developed by Michael Bowers, at
Vanderbilt University
in Nashville, involving coating a blue LED with quantum dots that glow
white in response to the blue light from the LED. This method emits a
warm, yellowish-white light similar to that made by
incandescent bulbs.
[86] Quantum dots are also being considered for use in white light-emitting diodes in liquid crystal display (LCD) televisions.
[87]
The major difficulty in using quantum dots-based LEDs is the insufficient stability of QDs under prolonged irradiation.
[citation needed]
In February 2011 scientists at PlasmaChem GmbH could synthesize quantum
dots for LED applications and build a light converter on their basis,
which could efficiently convert light from blue to any other color for
many hundred hours.
[88] Such QDs can be used to emit visible or near infrared light of any wavelength being excited by light with a shorter wavelength.
The structure of QD-LEDs used for the electrical-excitation scheme is similar to basic design of
OLED.
A layer of quantum dots is sandwiched between layers of
electron-transporting and hole-transporting materials. An applied
electric field causes electrons and holes to move into the quantum dot
layer and recombine forming an
exciton that excites a QD. This scheme is commonly studied for
quantum dot display.
The tunability of emission wavelengths and narrow bandwidth is also
beneficial as excitation sources for fluorescence imaging. Fluorescence
near-field scanning optical microscopy (
NSOM) utilizing an integrated QD-LED has been demonstrated.
[89]
In February 2008, a
luminous efficacy of 300
lumens of visible light per watt of
radiation (not per electrical watt) and warm-light emission was achieved by using
nanocrystals.
[90]
Types
LEDs are produced in a variety of shapes and sizes. The color of the
plastic lens is often the same as the actual color of light emitted, but
not always. For instance, purple plastic is often used for
infrared
LEDs, and most blue devices have colorless housings. Modern high power
LEDs such as those used for lighting and backlighting are generally
found in
surface-mount technology (SMT) packages, (not shown).
The main types of LEDs are miniature, high power devices and custom designs such as alphanumeric or multi-color.
[91]
Miniature
These are mostly single-die LEDs used as indicators, and they come in various sizes from 2 mm to 8 mm,
through-hole and
surface mount packages. They usually do not use a separate
heat sink.
[92]
Typical current ratings ranges from around 1 mA to above 20 mA. The
small size sets a natural upper boundary on power consumption due to
heat caused by the high current density and need for a heat sink.
Common package shapes include round, with a domed or flat top,
rectangular with a flat top (as used in bar-graph displays), and
triangular or square with a flat top. The encapsulation may also be
clear or tinted to improve contrast and viewing angle.
There are three main categories of miniature single die LEDs:
- Low-current: typically rated for 2 mA at around 2 V (approximately 4 mW consumption).
- Standard: 20 mA LEDs (ranging from approximately 40 mW to 90 mW) at around:
-
- 1.9 to 2.1 V for red, orange and yellow,
- 3.0 to 3.4 V for green and blue,
- 2.9 to 4.2 V for violet, pink, purple and white.
- Ultra-high-output: 20 mA at approximately 2 V or 4–5 V, designed for viewing in direct sunlight.
5 V and 12 V LEDs are ordinary miniature LEDs that incorporate a suitable series
resistor for direct connection to a 5 V or 12 V supply.
-
Different sized LEDs. 8 mm, 5 mm and 3 mm, with a wooden match-stick for scale.
-
LED in its on and off states.
-
Mid-range
Medium-power LEDs are often through-hole-mounted and mostly utilized
when an output of just a few lumen is needed. They sometimes have the
diode mounted to four leads (two cathode leads, two anode leads) for
better heat conduction and carry an integrated lens. An example of this
is the Superflux package, from Philips Lumileds. These LEDs are most
commonly used in light panels, emergency lighting, and automotive
tail-lights. Due to the larger amount of metal in the LED, they are able
to handle higher currents (around 100 mA). The higher current allows
for the higher light output required for tail-lights and emergency
lighting.
High-power
This high powered A19 sized LED light bulb thermal animation, was
created using high resolution CFD analysis, and shows temperature
contoured LED heat sink and flow trajectories, predicted using a
CFD analysis package, courtesy of
NCI.
High-power LEDs (HPLED) can be driven at currents from hundreds of mA
to more than an ampere, compared with the tens of mA for other LEDs.
Some can emit over a thousand lumens.
[93][94] LED power densities up to 300W/cm2 have been achieved.
[42]
Since overheating is destructive, the HPLEDs must be mounted on a heat
sink to allow for heat dissipation. If the heat from a HPLED is not
removed, the device will fail in seconds. One HPLED can often replace an
incandescent bulb in a
flashlight, or be set in an array to form a powerful
LED lamp.
Some well-known HPLEDs in this category are the Nichia 19 series,
Lumileds Rebel Led, Osram Opto Semiconductors Golden Dragon, and Cree
X-lamp. As of September 2009, some HPLEDs manufactured by
Cree Inc. now exceed 105 lm/W
[95]
(e.g. the XLamp XP-G LED chip emitting Cool White light) and are being
sold in lamps intended to replace incandescent, halogen, and even
fluorescent lights, as LEDs grow more cost competitive.
The impact of Haltz's law governing the light output of LEDs over
time can be readily seen in year over year increases in lumen output and
efficiency. For example, the CREE XP-G series LED achieved 105 lm/W in
2009,
[95] while Nichia released the 19 series with a typical efficiency of 140 lm/W in 2010.
[96]
LEDs have been developed by Seoul Semiconductor that can operate on
AC power without the need for a DC converter. For each half-cycle, part
of the LED emits light and part is dark, and this is reversed during the
next half-cycle. The efficacy of this type of HPLED is typically 40
lm/W.
[97]
A large number of LED elements in series may be able to operate
directly from line voltage. In 2009, Seoul Semiconductor released a high
DC voltage LED capable of being driven from AC power with a simple
controlling circuit. The low-power dissipation of these LEDs affords
them more flexibility than the original AC LED design.
[98]
Application-specific variations
- Flashing LEDs are used as attention seeking indicators
without requiring external electronics. Flashing LEDs resemble standard
LEDs but they contain an integrated multivibrator
circuit that causes the LED to flash with a typical period of one
second. In diffused lens LEDs this is visible as a small black dot. Most
flashing LEDs emit light of one color, but more sophisticated devices
can flash between multiple colors and even fade through a color sequence
using RGB color mixing.
- Bi-color LEDs are two different LED emitters in one case.
There are two types of these. One type consists of two dies connected to
the same two leads antiparallel
to each other. Current flow in one direction emits one color, and
current in the opposite direction emits the other color. The other type
consists of two dies with separate leads for both dies and another lead
for common anode or cathode, so that they can be controlled
independently.
- Tri-color LEDs are three different LED emitters in one case.
Each emitter is connected to a separate lead so they can be controlled
independently. A four-lead arrangement is typical with one common lead
(anode or cathode) and an additional lead for each color.
- RGB LEDs are Tri-color LEDs with red, green, and blue
emitters, in general using a four-wire connection with one common lead
(anode or cathode). These LEDs can have either common positive or common
negative leads. Others however, have only two leads (positive and
negative) and have a built in tiny electronic control unit.
- Alphanumeric LED displays are available in seven-segment and starburst
format. Seven-segment displays handle all numbers and a limited set of
letters. Starburst displays can display all letters. Seven-segment LED
displays were in widespread use in the 1970s and 1980s, but rising use
of liquid crystal displays,
with their lower power needs and greater display flexibility, has
reduced the popularity of numeric and alphanumeric LED displays.
Considerations for use
Power sources
The current/voltage characteristic of an LED is similar to other
diodes, in that the current is dependent exponentially on the voltage
(see
Shockley diode equation).
This means that a small change in voltage can cause a large change in
current. If the maximum voltage rating is exceeded by a small amount,
the current rating may be exceeded by a large amount, potentially
damaging or destroying the LED. The typical solution is to use
constant-current
power supplies, or driving the LED at a voltage much below the maximum
rating. Since most common power sources (batteries, mains) are
constant-voltage sources, most LED fixtures must include a power
converter, at least a current-limiting resistor. However, the high
resistance of 3 V
coin cells
combined with the high differential resistance of nitride-based LEDs
makes it possible to power such an LED from such a coin cell without an
external resistor.
[99]
Electrical polarity
As with all diodes, current flows easily from p-type to n-type material.
[100]
However, no current flows and no light is emitted if a small voltage is
applied in the reverse direction. If the reverse voltage grows large
enough to exceed the
breakdown voltage,
a large current flows and the LED may be damaged. If the reverse
current is sufficiently limited to avoid damage, the reverse-conducting
LED is a useful
noise diode.
Safety and health
The vast majority of devices containing LEDs are "safe under all
conditions of normal use", and so are classified as "Class 1 LED
product"/"LED Klasse 1". At present, only a few LEDs—extremely bright
LEDs that also have a tightly focused viewing angle of 8° or less—could,
in theory, cause temporary blindness, and so are classified as "Class
2".
[101]
The Opinion of the French Agency for Food, Environmental and
Occupational Health & Safety (ANSES) of 2010, on the health issues
concerning LEDs, suggested banning public use of lamps which were in the
moderate Risk Group 2, especially those with a high blue component in
places frequented by children.
[102] In general,
laser safety regulations—and the "Class 1", "Class 2", etc. system—also apply to LEDs.
[103]
While LEDs have the advantage over
fluorescent lamps that they do not contain
mercury, they may contain other hazardous metals such as
lead and
arsenic.
A study published in 2011 states: "According to federal standards, LEDs
are not hazardous except for low-intensity red LEDs, which leached Pb
[lead] at levels exceeding regulatory limits (186 mg/L; regulatory
limit: 5). However, according to California regulations, excessive
levels of copper (up to 3892 mg/kg; limit: 2500), lead (up to 8103
mg/kg; limit: 1000),
nickel (up to 4797 mg/kg; limit: 2000), or
silver (up to 721 mg/kg; limit: 500) render all except low-intensity yellow LEDs hazardous."
[104]
Advantages
- Efficiency: LEDs emit more light per watt than incandescent light bulbs.[105] The efficiency of LED lighting fixtures is not affected by shape and size, unlike fluorescent light bulbs or tubes.
- Color: LEDs can emit light of an intended color without using
any color filters as traditional lighting methods need. This is more
efficient and can lower initial costs.
- Size: LEDs can be very small (smaller than 2 mm2[106]) and are easily attached to printed circuit boards.
- On/Off time: LEDs light up very quickly. A typical red indicator LED will achieve full brightness in under a microsecond.[107] LEDs used in communications devices can have even faster response times.
- Cycling: LEDs are ideal for uses subject to frequent on-off cycling, unlike fluorescent lamps that fail faster when cycled often, or HID lamps that require a long time before restarting.
- Dimming: LEDs can very easily be dimmed either by pulse-width modulation or lowering the forward current.[108]
- Cool light: In contrast to most light sources, LEDs radiate very little heat in the form of IR that can cause damage to sensitive objects or fabrics. Wasted energy is dispersed as heat through the base of the LED.
- Slow failure: LEDs mostly fail by dimming over time, rather than the abrupt failure of incandescent bulbs.[109]
- Lifetime: LEDs can have a relatively long useful life. One
report estimates 35,000 to 50,000 hours of useful life, though time to
complete failure may be longer.[110]
Fluorescent tubes typically are rated at about 10,000 to 15,000 hours,
depending partly on the conditions of use, and incandescent light bulbs
at 1,000 to 2,000 hours. Several DOE demonstrations have shown that
reduced maintenance costs from this extended lifetime, rather than
energy savings, is the primary factor in determining the payback period
for an LED product.[111]
- Shock resistance: LEDs, being solid-state components, are
difficult to damage with external shock, unlike fluorescent and
incandescent bulbs, which are fragile.
- Focus: The solid package of the LED can be designed to focus
its light. Incandescent and fluorescent sources often require an
external reflector to collect light and direct it in a usable manner.
For larger LED packages total internal reflection
(TIR) lenses are often used to the same effect. However, when large
quantities of light is needed many light sources are usually deployed,
which are difficult to focus or collimate towards the same target.
Disadvantages
- High initial price: LEDs are currently more expensive, price
per lumen, on an initial capital cost basis, than most conventional
lighting technologies. As of 2010, the cost per thousand lumens
(kilolumen) was about $18. The price is expected to reach $2/kilolumen
by 2015.[112] The additional expense partially stems from the relatively low lumen output and the drive circuitry and power supplies needed.
- Temperature dependence: LED performance largely depends on
the ambient temperature of the operating environment – or "thermal
management" properties. Over-driving an LED in high ambient temperatures
may result in overheating the LED package, eventually leading to device
failure. An adequate heat sink
is needed to maintain long life. This is especially important in
automotive, medical, and military uses where devices must operate over a
wide range of temperatures, which require low failure rates.
- Voltage sensitivity: LEDs must be supplied with the voltage
above the threshold and a current below the rating. This can involve
series resistors or current-regulated power supplies.[113]
- Light quality: Most cool-white LEDs have spectra that differ significantly from a black body radiator like the sun or an incandescent light. The spike at 460 nm and dip at 500 nm can cause the color of objects to be perceived differently under cool-white LED illumination than sunlight or incandescent sources, due to metamerism,[114]
red surfaces being rendered particularly badly by typical
phosphor-based cool-white LEDs. However, the color rendering properties
of common fluorescent lamps are often inferior to what is now available
in state-of-art white LEDs.
- Area light source: Single LEDs do not approximate a point source of light giving a spherical light distribution, but rather a lambertian
distribution. So LEDs are difficult to apply to uses needing a
spherical light field, however different fields of light can be
manipulated by the application of different optics or "lenses". LEDs
cannot provide divergence below a few degrees. In contrast, lasers can
emit beams with divergences of 0.2 degrees or less.[115]
- Electrical polarity: Unlike incandescent light bulbs, which illuminate regardless of the electrical polarity, LEDs will only light with correct electrical polarity. To automatically match source polarity to LED devices, rectifiers can be used.
- Blue hazard: There is a concern that blue LEDs and cool-white LEDs are now capable of exceeding safe limits of the so-called blue-light hazard
as defined in eye safety specifications such as ANSI/IESNA RP-27.1–05:
Recommended Practice for Photobiological Safety for Lamp and Lamp
Systems.[116][117]
- Blue pollution: Because cool-white LEDs with high color temperature emit proportionally more blue light than conventional outdoor light sources such as high-pressure sodium vapor lamps, the strong wavelength dependence of Rayleigh scattering means that cool-white LEDs can cause more light pollution than other light sources. The International Dark-Sky Association discourages using white light sources with correlated color temperature above 3,000 K.[98][not in citation given]
- Droop: The efficiency of conventional InGaN based LEDs decreases as one increases current above a given level.[44][118][45][119]
Common misconceptions
One of the most common misunderstandings about LED lighting is that
energy consumption can be used to measure light output. Unlike
conventional incandescent light bulbs, of which the light output is
relative to energy consumed, the light output of an LED light is
measured in lumens and is given regardless of energy consumption.
LED lights can consume a variable amount of energy, however any given
LED system will have, depending on whether phosphor has been used to
coat the diodes (and the amount) an optimal lumens/watt output that
maximizes the lifespan of that particular LED system. The lifespan of
the LED system is also dependent upon the thermal management properties
employed by the manufacturer.
Other misunderstandings of LED lighting is that there is a limited
color temperature. The application of phosphor coating to a diode will
give a warmer color temperature. The more phosphor applied to a diode,
the warmer the color temperature of the LED. The color temperature
output of LED lighting systems should be given particular consideration
when employing LED lighting in commercial retail display.
One example of this in practice is the increasing use of "warmer"
yellow light to display meat in butchers of supermarkets. A warmer light
will emphasize the red in the meat, while reducing the visibility of
the whiter (fatty) parts, thus making the meat more appealing. The
opposite technique is commonly used in jewelery lighting, whereby a
"colder" color temperature is employed to increase the brightness of
jewels, silver or metallic objects and other precious stones.
The phosphor coating affects the luminous efficacy, heat dissipation and lifespan of the LED system.
Applications
LED uses fall into four major categories:
- Visual signals where light goes more or less directly from the source to the human eye, to convey a message or meaning.
- Illumination where light is reflected from objects to give visual response of these objects.
- Measuring and interacting with processes involving no human vision.[120]
- Narrow band light sensors where LEDs operate in a reverse-bias mode and respond to incident light, instead of emitting light.[121][122][123][124] See LEDs as light sensors.
Indicators and signs
The
low energy consumption,
low maintenance and small size of LEDs has led to uses as status
indicators and displays on a variety of equipment and installations.
Large-area
LED displays
are used as stadium displays and as dynamic decorative displays. Thin,
lightweight message displays are used at airports and railway stations,
and as
destination displays for trains, buses, trams, and ferries.
Red and green traffic signals
One-color light is well suited for
traffic lights and signals,
exit signs,
emergency vehicle lighting, ships' navigation lights or
lanterns
(chromacity and luminance standards being set under the Convention on
the International Regulations for Preventing Collisions at Sea 1972,
Annex I and the CIE) and
LED-based Christmas lights. In cold climates, LED traffic lights may remain snow covered.
[125]
Red or yellow LEDs are used in indicator and alphanumeric displays in
environments where night vision must be retained: aircraft cockpits,
submarine and ship bridges, astronomy observatories, and in the field,
e.g. night time animal watching and military field use.
Automotive applications for LEDs continue to grow
Because of their long life and fast switching times, LEDs have been used in brake lights for cars'
high-mounted brake lights,
trucks, and buses, and in turn signals for some time, but many vehicles
now use LEDs for their rear light clusters. The use in brakes improves
safety, due to a great reduction in the time needed to light fully, or
faster rise time, up to 0.5 second faster than an incandescent bulb.
This gives drivers behind more time to react. It is reported that at
normal highway speeds, this equals one car length equivalent in
increased time to react. In a dual intensity circuit (rear markers and
brakes) if the LEDs are not pulsed at a fast enough frequency, they can
create a
phantom array,
where ghost images of the LED will appear if the eyes quickly scan
across the array. White LED headlamps are starting to be used. Using
LEDs has styling advantages because LEDs can form much thinner lights
than incandescent lamps with
parabolic reflectors.
Due to the relative cheapness of low output LEDs, they are also used in many temporary uses such as
glowsticks,
throwies, and the photonic
textile Lumalive. Artists have also used LEDs for
LED art.
Weather/all-hazards radio receivers with
Specific Area Message Encoding (SAME) have three LEDs: red for warnings, orange for watches, and yellow for advisories & statements whenever issued.
Lighting
With the development of high-efficiency and high-power LEDs, it has
become possible to use LEDs in lighting and illumination. Replacement
light bulbs have been made, as well as dedicated fixtures and
LED lamps. To encourage the shift to very high efficiency lighting, the
US Department of Energy has created the
L Prize competition. The
Philips
Lighting North America LED bulb won the first competition on August 3,
2011 after successfully completing 18 months of intensive field, lab,
and product testing.
[126]
LEDs are used as
street lights and in other
architectural lighting where color changing is used. The mechanical robustness and long lifetime is used in
automotive lighting on cars, motorcycles, and
bicycle lights.
LED street lights are employed on poles and in parking garages. In 2007, the Italian village
Torraca was the first place to convert its entire illumination system to LEDs.
[127]
LEDs are used in aviation lighting.
Airbus has used LED lighting in their
Airbus A320 Enhanced since 2007, and Boeing plans its use in the
787.
LEDs are also being used now in airport and heliport lighting. LED
airport fixtures currently include medium-intensity runway lights,
runway centerline lights, taxiway centerline and edge lights, guidance
signs, and obstruction lighting.
LEDs are also suitable for
backlighting for
LCD televisions and lightweight
laptop displays and light source for
DLP projectors (See
LED TV). RGB LEDs raise the color
gamut by as much as 45%. Screens for TV and computer displays can be made thinner using LEDs for backlighting.
[128]
LEDs are used increasingly in aquarium lights. In particular for reef
aquariums, LED lights provide an efficient light source with less heat
output to help maintain optimal aquarium temperatures. LED-based
aquarium fixtures also have the advantage of being manually adjustable
to emit a specific color-spectrum for ideal coloration of corals, fish,
and invertebrates while optimizing photosynthetically active radiation
(PAR), which raises growth and sustainability of photosynthetic life
such as corals, anemones, clams, and macroalgae. These fixtures can be
electronically programmed to simulate various lighting conditions
throughout the day, reflecting phases of the sun and moon for a dynamic
reef experience. LED fixtures typically cost up to five times as much as
similarly rated fluorescent or high-intensity discharge lighting
designed for reef aquariums and are not as high output to date.
The lack of IR or heat radiation makes LEDs ideal for
stage lights
using banks of RGB LEDs that can easily change color and decrease
heating from traditional stage lighting, as well as medical lighting
where IR-radiation can be harmful. In energy conservation, the lower
heat output of LEDs also means air conditioning (cooling) systems have
less heat to dispose of, reducing carbon dioxide emissions.
LEDs are small, durable and need little power, so they are used in hand held devices such as
flashlights. LED
strobe lights or
camera flashes operate at a safe, low voltage, instead of the 250+ volts commonly found in
xenon flashlamp-based lighting. This is especially useful in cameras on
mobile phones, where space is at a premium and bulky voltage-raising circuitry is undesirable.
LEDs are used for infrared illumination in
night vision uses including
security cameras. A ring of LEDs around a
video camera, aimed forward into a
retroreflective background, allows
chroma keying in
video productions.
LEDs are now used commonly in all market areas from commercial to
home use: standard lighting, AV, stage, theatrical, architectural, and
public installations, and wherever artificial light is used.
LEDs are increasingly finding uses in medical and educational applications, for example as mood enhancement
[citation needed], and new technologies such as
AmBX, exploiting LED versatility.
NASA has even sponsored research for the use of LEDs to promote health for astronauts.
[129]
Smart lighting
Light can be used to transmit
broadband data, which is already implemented in
IrDA standards using infrared LEDs. Because LEDs can
cycle on and off millions of times per second, they can be
wireless transmitters and
access points for
data transport.
[130] Lasers can also be
modulated in this manner.
Sustainable lighting
Efficient lighting is needed for
sustainable architecture. In 2009, a typical 13-watt LED lamp emitted 450 to 650 lumens,
[131]
which is equivalent to a standard 40-watt incandescent bulb. In 2011,
LEDs have become more efficient, so that a 6-watt LED can easily achieve
the same results.
[132]
A standard 40-watt incandescent bulb has an expected lifespan of 1,000
hours, whereas an LED can continue to operate with reduced efficiency
for more than 50,000 hours, 50 times longer than the incandescent bulb.
Energy consumption
In the US, one kilowatt-hour of electricity will cause 1.34 pounds (610 g) of
CO2 emission.
[133] Assuming the average light bulb is on for 10 hours a day, one 40-watt incandescent bulb will cause 196 pounds (89 kg) of
CO2 emission per year. The 6-watt LED equivalent will only cause 30 pounds (14 kg) of
CO2
over the same time span. A building’s carbon footprint from lighting
can be reduced by 85% by exchanging all incandescent bulbs for new LEDs.
Economically sustainable
LED light bulbs could be a cost-effective option for lighting a home
or office space because of their very long lifetimes. Consumer use of
LEDs as a replacement for conventional lighting system is currently
hampered by the high cost and low efficiency of available products. 2009
DOE testing results showed an average efficacy of 35 lm/W, below that
of typical
CFLs, and as low as 9 lm/W, worse than standard incandescents.
[131]
However, as of 2011, there are LED bulbs available as efficient as 150
lm/W and even inexpensive low-end models typically exceed 50 lm/W. The
high initial cost of commercial LED bulbs is due to the expensive
sapphire substrate,
which is key to the production process. The sapphire apparatus must be
coupled with a mirror-like collector to reflect light that would
otherwise be wasted.
Other applications
The light from LEDs can be modulated very quickly so they are used extensively in
optical fiber and
free space optics communications. This includes
remote controls, such as for TVs, VCRs, and LED Computers, where infrared LEDs are often used.
Opto-isolators use an LED combined with a
photodiode or
phototransistor
to provide a signal path with electrical isolation between two
circuits. This is especially useful in medical equipment where the
signals from a low-voltage
sensor
circuit (usually battery-powered) in contact with a living organism
must be electrically isolated from any possible electrical failure in a
recording or monitoring device operating at potentially dangerous
voltages. An optoisolator also allows information to be transferred
between circuits not sharing a common ground potential.
Many sensor systems rely on light as the signal source. LEDs are
often ideal as a light source due to the requirements of the sensors.
LEDs are used as
movement sensors, for example in
optical computer mice. The Nintendo
Wii's sensor bar uses infrared LEDs.
Pulse oximeters use them for measuring
oxygen saturation. Some flatbed scanners use arrays of RGB LEDs rather than the typical
cold-cathode fluorescent lamp
as the light source. Having independent control of three illuminated
colors allows the scanner to calibrate itself for more accurate color
balance, and there is no need for warm-up. Further, its sensors only
need be monochromatic, since at any one time the page being scanned is
only lit by one color of light.
Touch sensing: Since LEDs can also be used as
photodiodes,
they can be used for both photo emission and detection. This could be
used, for example, in a touch-sensing screen that registers reflected
light from a finger or
stylus.
[134]
Many materials and biological systems are sensitive to or dependent on light.
Grow lights use LEDs to increase
photosynthesis in
plants[135] and bacteria and viruses can be removed from water and other substances using
UV LEDs for
sterilization.
[72] Other uses are as
UV curing devices for some ink and coating methods, and in
LED printers.
Plant growers are interested in LEDs because they are more
energy-efficient, emit less heat (can damage plants close to hot lamps),
and can provide the optimum light frequency for plant growth and bloom
periods compared to currently used grow lights:
HPS (high-pressure sodium),
metal-halide (MH) or
CFL/low-energy.
However, LEDs have not replaced these grow lights due to higher price.
As mass production and LED kits develop, the LED products will become
cheaper.
LEDs have also been used as a medium-quality
voltage reference in electronic circuits. The forward voltage drop (e.g., about 1.7 V for a normal red LED) can be used instead of a
Zener diode in low-voltage regulators. Red LEDs have the flattest
I/
V curve above the knee. Nitride-based LEDs have a fairly steep
I/
V
curve and are useless for this purpose. Although LED forward voltage is
far more current-dependent than a good Zener, Zener diodes are not
widely available below voltages of about 3 V.
Light sources for machine vision systems
Machine vision
systems often require bright and homogeneous illumination, so features
of interest are easier to process. LEDs are often used for this purpose,
and this is likely to remain one of their major uses until price drops
low enough to make signaling and illumination uses more widespread.
Barcode scanners
are the most common example of machine vision, and many low cost ones
use red LEDs instead of lasers. Optical computer mice are also another
example of LEDs in machine vision, as it is used to provide an even
light source on the surface for the miniature camera within the mouse.
LEDs constitute a nearly ideal light source for
machine vision systems for several reasons:
The size of the illuminated field is usually comparatively small and
machine vision systems are often quite expensive, so the cost of the
light source is usually a minor concern. However, it might not be easy
to replace a broken light source placed within complex machinery, and
here the long service life of LEDs is a benefit.
LED elements tend to be small and can be placed with high density
over flat or even-shaped substrates (PCBs etc.) so that bright and
homogeneous sources that direct light from tightly controlled directions
on inspected parts can be designed. This can often be obtained with
small, low-cost lenses and diffusers, helping to achieve high light
densities with control over lighting levels and homogeneity. LED sources
can be shaped in several configurations (spot lights for reflective
illumination; ring lights for coaxial illumination; back lights for
contour illumination; linear assemblies; flat, large format panels; dome
sources for diffused, omnidirectional illumination).
LEDs can be easily strobed (in the microsecond range and below) and
synchronized with imaging. High-power LEDs are available allowing
well-lit images even with very short light pulses. This is often used to
obtain crisp and sharp “still” images of quickly moving parts.
LEDs come in several different colors and wavelengths, allowing easy
use of the best color for each need, where different color may provide
better visibility of features of interest. Having a precisely known
spectrum allows tightly matched filters to be used to separate
informative bandwidth or to reduce disturbing effects of ambient light.
LEDs usually operate at comparatively low working temperatures,
simplifying heat management and dissipation. This allows using plastic
lenses, filters, and diffusers. Waterproof units can also easily be
designed, allowing use in harsh or wet environments (food, beverage, oil
industries).
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LEDs used on a train for both overhead lighting and destination signage.
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LED digital display that can display four digits and points
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LED panel light source used in an experiment on
plant growth. The findings of such experiments may be used to grow food in space on long duration missions.
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LED lights reacting dynamically to video feed via
AmBX
From: http://en.wikipedia.org/wiki/Light-emitting_diode
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