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Introduction to LED Lighting

Time:2012/7/26 Click: 7536

Pictured left is a color-changing LED system from Color Kinetics located in the ballroom of the Sheraton Hotel in Seattle, WA..

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.

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.

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.

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

Advantages and disadvantages 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.

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?

The Future? - OLED Lighting

Organic light-emitting diodes (OLEDs) represent another emerging technology that is still in the laboratory. If it can be made practical, it may make even more of a dramatic impact on how spaces are lighted than LEDs. In fact, it may one day replacing LEDs as an energy-efficient alternative for general lighting.

OLEDs are similar to electroluminescent lighting, in which a sheet of material is excited so that it emits light. An OLED light source is a thin, flexible sheet of material consisting of three layers, a polymer or sublimed molecular film sandwiched between two layers of electrodes, one of them transparent. Current passes through the material until it emits light through its transparent layer.

According to Uniax Corporation, in laboratory conditions, low-voltage OLED light sources can reach efficacies of 3-4 LPW. Unfortunately, such efforts produce too much heat and reduce the life of the light source. Manufacturers including Uniax, Philips Electronics NV, Photonics Spectra, Seiko Epson, Hoechst Innovative Display Technologies Inc., DuPont and Intel are all currently working on developing commercial OLED products.

What’s interesting about this light source still in development is that it may challenge our very perception of lighting and architecture in the future. Lighting designers often try to integrate lighting hardware and architecture in a cohesive manner; with OLEDs, the architecture may be the lighting hardware. Sheets of material can be cut and placed like “lighting wallpaper” or integrated with building materials such as wood, glass and other materials, converting them into luminous surfaces.

The Next Step
Based on current lumen packages and the great potential for this light source to be adopted for more general lighting applications, the next step is for the fixture community to begin building more products designed to utilize LEDs as the primary or supplementary light source. Writes Andrew Bierman of the Lighting Research Center in the LRC white paper, “LED: From Indicators to Illuminators?”:

There are … many lighting applications that require only a few lumens, or tens of thousands of lumens, for which LEDs are ideal. In the past, most of the talk about LEDs has focused on efficiency. Now that the efficiencies have exceeded other light sources, future work should focus on packaging LEDs into useful products.