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High
Efficiency LED Lighting
Introduction to
LED Lighting
Light-emitting diodes (LEDs) are a relatively old technology (1970s)
that has advanced from use in numeric displays and indicator lights to
a range of new and potential new applications, including exit signs,
accent lights, task lights, traffic lights, signage, cove lighting,
wall sconces, outdoor lighting and downlighting.
LEDs offer benefits such as small size, long lamp life, low heat
output, energy savings and durability. They also allow extraordinary
design flexibility in color changing, dimming and distribution by
combining these small units into desired shapes, colors, sizes and
lumen packages. Currently, relatively low overall light output, poor
color rendering and questions about advertised service life may
indicate that LEDs, while very useful in many applications, are not yet
ready for “prime time” in some architectural applications. Notably
promising current applications include retail display, colored
lighting, tight spaces, areas that require low light levels, exterior
lighting and applications where the integration of light sources and
architectural elements is critical.
LEDs currently dominate the exit sign market and many cities have
adopted them as a replacement for incandescent lamps in traffic
signals. In the architectural market, the development of a
visible/white light LED has awakened lighting designers to new
possibilities with this light source. White light LEDs, however,
currently do not produce enough lumen output to make them competitive
with many general light sources. This restricts their use in
architectural projects to applications where small lumen packages are
needed and where the characteristics of a lower CRI rating and high
color temperature are acceptable.
Characteristics
LEDs are solid state semiconductor devices. LED illumination is
achieved when a semiconductor crystal is excited so that it directly
produces visible light in a desired wavelength range (color). LED units
are small, typically 5mm (T 1-3/4).
Method of
Operation
When an LED unit is activated, a power supply converts AC voltage into
sufficient DC voltage, which is applied across the diode semiconductor
crystal. This results in electrons (negative charge carriers [N]) in
the diode’s electron transport layer and holes (positive charge
carriers [P]) in the diode’s hole transport layer combining at the P-N
junction and converting their excess energy into light. The LED is
sealed in a clear or diffuse plastic lens that can provide a range of
angular distributions of the light.
Color
The color composition of the light being emitted by the LED is based on
the chemical composition of the material being excited. LEDs are
available that can produce colors including white, deep blue, blue,
green, yellow, amber, orange, red, bright red and deep red.
Efficiency
LEDs are low-voltage, low-current devices and efficient light sources.
For red, amber, yellow, green and blue LEDs, new materials have been
developed that are more efficient than traditional materials, producing
efficacies (lumens per watt) greater than incandescent lamps and
rivaling fluorescent lamps. According to Philips Lighting, “In 1993 an
array of 200 LEDs was required whereas only 18 LEDs achieve the same
performance today, with prediction of further reduction to only 10.”
Progress continues. Efficacies as high as 100 LPW have already been
achieved in laboratory conditions. According to Steve Johnson, group
leader of lighting research for the Lawrence Berkeley National
Laboratory, “It is not unrealistic to expect the efficacy of
solid-state sources to achieve 150-200 lumens per watt in the coming
decades.”
White Light LEDs
The utilization of indium gallium nitride (InGaN) as a semiconductor
material resulted in the brightest LEDs and enabled the development of
the white light LED.
White light LEDs feature a phosphor added to a blue LED that converts
some of the light emission into yellow, resulting in a bluish-white
light. White light LEDs are therefore a cool light source with a
spectrum of correlated color temperatures of 4,000-11,000K. Color
rendering is considered poor. White light can also be achieved by color
mixing the light produced by red, blue and green LEDs. The production
of visible white light offers the promise that LEDs can be used in
general lighting applications. As the light output and color rendering
capabilities of LEDs improve, many more architectural applications will
open up for this source.
Several manufacturers currently offer a range of LED fixtures for
replacing MR16 lamps, display lighting, cove lighting, underwater
lighting, architectural details and other applications. Come
manufacturers are using colored LED arrays in these applications,
combining red, blue and green LEDs to produce millions of colors,
including white light. Designers should carefully consider requisite
lumen packages, source brightness, viewing angles and color rendering
when considering use of this technology.
Advantages of
LED Lighting
LEDs offer numerous benefits due to their mode of operation:
Energy
Efficiency
LEDs are highly efficient. In traffic signal lights, a strong market
for LEDs, a red traffic signal head that contains 196 LEDs draws 10W
versus its incandescent counterpart that draws 150W. Various estimates
of potential energy savings range from 82% to 93%. With the red signal
operating about 50% of the day, the complete traffic signal unit is
estimated to save 35-40%. It is estimated that replacing incandescent
lamps in all of America’s some 260,000 traffic signals (red, green and
yellow) could reduce energy consumption by nearly 2.5 billion kWh. At
the end of 1997, more than 150,000 signals were retrofitted, almost all
of them red.
In architectural applications, the greatest penetration of LEDs has
been in exit signs, both new signs and retrofits. LED retrofit
products, which come in various forms including light bars, panels and
screw in LED lamps, typically draw 2-5W per sign, resulting in
significant savings versus incandescent lamps with the bonus benefit of
much longer life, which in turn reduces maintenance requirements. Some
of these products are designed specifically for either on-face or
two-face exit signs. Many new LED exit signs are also available,
including edge-lit designs. LED products currently make up about 50% of
the exit sign market. A study conducted by the Lighting Research Center
in 1998 found that about 80% of new exit signs being sold in the U.S.
utilize LEDs. Note that most retrofits are restricted to use in
stencil-type signs versus panel-type signs.
Long Life
Some LEDs are projected to produce a long service life of about 100,000
hours. For this reason LEDs are ideal for hard-to-reach/maintain
fixtures such as exit sign lighting and, combined with its durability,
pathway lighting. This service life can be affected by the application
and environmental factors, including heat and if being overdriven by
the power supply.
Range of Colors
LEDs are available in a range of colors (see above), including white
light. White light can also be produced through color mixing of red,
blue and green LEDs. In addition, through the innovative combination of
various-colored LEDs, dramatic color-changing effects can be produced
from a single fixture through dynamic activation of various sets of
LEDs. Manufacturers such as Color Kinetics offer fixtures that employ
this principle. Color Kinetics offers track, theatrical, underwater,
outdoor and other fixtures utilizing variable-intensity LEDs that can
provide more than 16.7 million colors, including white light. These
fixtures can be individually controlled via a PC, DMX controller or
proprietary controller to generate effects including fixed color, color
washing, cross fading, random color changing, strobing and variable
strobing.
Dr. Nadarajah Narendran of the Lighting Research Center is doing some
exciting research on the use of colored LEDs in retail display
lighting. Preliminary research suggests that using colored LED
background lighting combined with spot lighting on merchandise may
improve energy efficiency and reduce maintenance costs while catching
the eye of the consumer in a fresh manner.
No UV
Emissions/Little Infrared
LEDs produce no UV radiation and little heat, making them ideal for
illuminating objects, such as works of art, that are sensitive to UV
light.
Durable
LEDs are highly rugged. They feature no filament that can be damaged
due to shock and vibrations. They are subject to heat, however, and
being overdriven by the power supply.
Small
Size/Design Flexibility
A single LED is very small and produces little light overall. However,
this weakness is actually its strength. LEDs can be combined in any
shape to produce desired lumen packages as the design goals and
economics permit. In addition, LEDs can be considered miniature light
fixtures; distribution of light can be controlled by the LEDs’ epoxy
lens, simplifying the construction of architectural fixtures designed
to utilize LEDs. A controller can be connected to an LED fixture to
selectively dim individual LEDs, resulting in the dynamic control of
distribution, light output and color. Finally, DC power enables the
unit to be easily adaptable to different power supplies.
Other
Benefits
The other
benefits of LEDs include:
* Lights instantly
* Can be easily dimmed
* Silent operation
* Low-voltage power supply (increased safety)
Disadvantages
of LED Lighting
There are several major disadvantages to LEDs that restrict their
application and slow their wider adoption in general lighting
applications.
Lack of Choice
There is currently a relative lack of availability of product choices
for white LEDs. LEDs are undergoing continuous development,
particularly in terms of lumen package per unit and lumens per watt.
Progress is continuing rapidly. Philips Lighting and Hewlett-Packard,
for example, recently launched LumiLEDs—a joint venture with a starting
investment of $150 million, 1,000 employees and facilities in
California, The Netherlands and Malaysia—with the goal of rapidly
developing LED technology. Many other manufacturers are also exploring
this emerging technology.
While some products are available now for the architectural market, it
may take a while longer before fixture manufacturers begin to roll out
a large selection of products that take advantage of this light source
for general illumination.
Color Quality
White LEDs currently offer poor color rendering and a high color
temperature. As improvements are made to the technology in the lab,
however, we can expect these problems to be corrected.
Product Standardization
There is currently no standardization for this technology, raising
questions about maintainability. Will the manufacturer offer matching
spare parts when systems begin to fail or will entire systems need to
be replaced?
Article from:
http://lightingdesignlab.com/articles/index.htm
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last updated: 03/20/2008
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