Light emitting diode (LED) based exit signs and traffic signals have fulfilled their promise of an energy-efficient, long-lasting, compact, and low-maintenance light source in real-world commercial and industrial applications by saving millions of kilowatt-hours of electricity and thousands of maintenance hours. While a number of technological and economic challenges have combined to limit the availability of LED consumer products beyond these existing niches, this trend is changing: a number of sources have projected that LED-based lighting will gradually replace 25% to 30% of incandescent lighting applications by 2025.
LEDs represent a lighting technology fundamentally different in design from the incandescent light bulb or fluorescent light. In fact, LED technology has more in common with the microchip than any of the current light sources. LEDs are composed of thin layers of semiconducting materials that emit light when a voltage is applied across the LED layers, forming a light producing “chip.” The LED chip must be encased in epoxy with a heat sink, metallic leads, and a light reflector to form a functional light source – collectively referred to as “the package.” Packages are then incorporated into lighting systems. It is important to have a fully optimized and integrated LED package as well as luminaires specifically designed to fully utilize the light output of LEDs.
Another difference of LED lighting is that LEDs produce colored light. The use of different semiconducting materials result in different light colors being produced. This makes the LED technologies especially attractive for applications that require colored light. To get white light, variety of color mixing techniques are used to create “white light” LEDs, including packaging colored LED with a phosphor, or with other complementary colored LEDs that overlap to produce white.
LEDs and Efficiency
LEDs bring many advantages to the lighting marketplace, one of the most notable being energy efficiency, especially in applications requiring colored light. In these situations, colored filters applied to white light sources reject 80 to 90% of the spectrum, wasting the energy expended to create those wavelengths. By contrast, LEDs make only the
wavelengths desired, which is the major reason that monochromatic LEDs to date have been more successful and efficient than white ones. In exit signs and in traffic signals, for example, red LED systems demand only 10% to 15% of the input power that an incandescent light source system would require.
Another admirable quality of the LED technology is that it has a long rated life – up to 100,000 hours. This represents a significant leap when compared to 1000 hours or less for an incandescent, 6,000 to 20,000 hours for fluorescent sources, and about 24,000 hours for a high-pressure sodium lamp. The longer life of a LED system makes LEDs more attractive because it lowers the replacement frequency of the system and increases the time that a customer benefits from the energy savings. Thus, the rated life of LEDs is important to consider when analyzing the overall cost of a LED system as compared to the overall cost of a conventional system.
Measuring LEDs and reporting LED output for use in place of traditional lighting sources actually represents a significant challenge. The introduction of LEDs into lighting
applications has created a demand for photometric descriptions of LEDs. This is due the fact that most lighting designers and energy efficiency program managers are accustomed to comparing the relative efficacy of light sources in terms of lumens per watt (lm/w). This metric assumes that the light source being measured emits light in a fairly broad spatial distribution pattern. However, LED packages emit light in a relatively narrow cone of distribution, so it is more difficult to determine lumens per watt for LED-based systems. Thus, a redefinition of efficacy may be needed in order to effectively compare the performance of these different light sources.
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