Introduction Financial considerations—namely, purchase price and operating costs—always figure in the selection of lighting products, but many other aspects also come into play, varying in importance depending on the application. LEDs have several unique attributes,
and it is critical to understand how they can be used advantageously.
Some considerations are dependent on product design, Building Technologies Program SOLID-STATE LIGHTING TECHNOLOGY FACT SHEET Image Credit: Cree
but others amount to using LEDs in appropriate situations. Some of the potentially favorable characteristics of LED sources compared
to traditional lamps include:
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Directional light emission
•
Size and form factor
•
Resistance to mechanical failure (i.e., breaking)
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Instant on at full output
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Rapid on-off cycling capability without detrimental effects
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Improved performance at cold temperatures
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Dimming and control capability
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Opportunity for color tuning
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Minimal nonvisible radiation [e.g., ultraviolet (UV), infrared
(IR)]
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Extended lifetime
LEDs are semiconductor devices that emit light through electroluminescence.
1 This basic fact is the foundation for many of the
1 LEDs rely on injection luminescence, a specific type of electroluminescence. In this case, light is generated directly when electrons recombine with holes, in the process emitting photons. For more on the physics of LED light generation, see the IES Lighting Handbook or other reference sources.
advantages of LEDs, since it is different from traditional light sources. For example, incandescent lamps rely on a heated filament
to emit light, fluorescent lamps create light using a gas discharge
to excite phosphors, and high-intensity discharge (HID) lamps utilize an electric arc discharge. All of these traditional technologies require a glass bulb to contain essential gases and/or coatings.
In contrast to the large form factors of traditional lamps, LED lighting starts with a tiny chip (also called a die; most commonly about 1 mm2) comprised of layers of semiconducting material— the exact material determines the wavelength (color) of radiation that is emitted. At the next level are LED packages, which may contain one or more chips mounted on heat-conducting material and usually enclosed in a lens or encapsulant. The resulting device, typically less than 1 cm2, can then be used individually or in an array. Finally, LEDs are mounted on a circuit board and incorporated into a lighting fixture, attached to an architectural structure, or made to fit the form factor of a traditional lamp (or as it is colloquially known, a light bulb).

LED Package Design
Although not all LED packages are built the same way, the basic components are often similar.
Besides the chip that is responsible for emitting
light, the various components are needed for thermal regulation, producing the desired spectrum, regulating electrical characteristics, or creating the appropriate distribution of light. All these components must work in harmony to produce a high-performance product. Many of the advantages of LEDs are derived from their unique physical attributes.

Directional Light Emission
Traditional light sources emit radiant energy in all directions. As such, an optical system—a lamp housing or a luminaire, with elements
such as a reflector or lens—is typically necessary to direct output in the desired direction. Because no optical system is perfectly
efficient, losses in efficacy result. Further, if the optical system
is not well designed (or is not present), light can be wasted, going in undesired directions.
Due to their physical characteristics and because they are mounted on a flat surface, LEDs emit light hemispherically, rather than spherically. For task lighting and other applications requiring directional lighting, this may increase the application effi cacy2 of the source. In contrast, with LEDs it is more diffi cult to obtain an omnidirectional distribution when it is desired, although innovative system designs now provide this capability.
Size and Form Factor
The small size, scalability of arrays, and directional light emission
of LEDs offer the potential for innovative, low profi le, or compact lighting products. This advantage can be aesthetic, but may also be functional. For example, reducing the depth of a luminaire may allow more room for ducts, conduit, or other building
systems in a ceiling cavity. It is even possible that the size of the ceiling plenum could be reduced. In contrast, the unique form factor of LEDs can be a disadvantage when competing with high-wattage HID sources. To match the lumen output, a very large array of LEDs is necessary.
2 Application effi cacy is defined as the lumens delivered to the target plane divided by the input watts to the lamp (or the ballast or driver, if applicable).
The physical characteristics of LEDs allow for the design of luminaires
that are different shapes and sizes compared to those made for conventional lamps. In this example, the depth of the LED parking garage luminaire is significantly less than a more traditional luminaire with a metal halide lamp.
Achieving small form factors requires careful design, specifi cally with regard to thermal management. Although LEDs used for general lighting do not emit infrared radiation (i.e., heat), they do generate thermal energy that must be moved away from the chip by a mass of material, which is called a heat sink. In order to produce
more light output, LEDs are often grouped into arrays, which dictate the use of additional heat-sinking material. Thus, although LED packages are small, matching the performance of small traditional lamps, such as MR16s, can be challenging.
Breakage Resistance
LEDs are largely impervious to vibration because they do not have filaments or glass enclosures. The life of standard incandescent
and discharge lamps may be reduced by vibration when operated in vehicular or industrial applications, although specialized
vibration-resistant lamps can help alleviate this problem. The inherent vibration resistance of LEDs may be beneficial in applications
such as transportation lighting (planes, trains, or automobiles),
lighting on and near industrial equipment, or exterior area and roadway lighting.
In addition to benefits during operation, LEDs offer increased resistance to breaking during transport, storage, handling, and installation. LED devices mounted on a circuit board are connected
with soldered leads that may be vulnerable to direct impact, but no more so than cell phones and other electronic devices. Because they do not contain any glass, LED fi xtures may be especially appropriate in applications with a high likelihood of lamp breakage, such as sports facilities or vandalism-prone areas, although they are not indestructible. LED durability may also be beneficial in applications where broken lamps present a hazard to occupants, such as children’s rooms, assisted living facilities, or food preparation areas.
Instant On
Most fluorescent lamps do not provide full brightness immediately
after being turned on. This is particularly relevant to amalgam
compact fluorescent lamps (CFLs), which can take three minutes or more to reach full light output. HID lamps have even longer warm up times, ranging from several minutes for metal halide to ten minutes or more for high-pressure sodium (HPS). HID lamps also have a restrike time delay; if turned off, they must be allowed to cool before turning on again, usually for 2 to 20 minutes, depending on the ballast. In contrast to traditional technologies, LEDs turn on at full brightness almost instantly, with no restrike delay. This advantage can be simply aesthetic or a user preference, but can also be beneficial for emergency egress or high-security situations. It is also especially important for vehicle brake lights—LED versions illuminate 170 to 200 milliseconds
faster than standard incandescent lamps, providing an estimated 19 feet of additional stopping distance at highway speeds (65 mph).3
Rapid Cycling
LEDs are impervious to the deleterious effects of on-off cycling. In fact, one method for dimming LEDs is to switch them on and