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Ledinfo
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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.
Efficacy
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.
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