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by Tracy Will
Fri, Mar 27th 2009 05:50 am

The latest buzz in energy efficient lighting has been focused on LEDs, or light emitting diodes.  In the past, LEDs were most frequently seen in clocks and as indicator lights, but lately the technology has been rapidly moving towards general lighting applications.

Traditional light sources are all based on gas-filled glass enclosures, but LEDs are solid state lighting (SSL.)  Simply put, an LED produces light energy as a byproduct of passing an electrical current through semi-conducting material.  Because they use a solid- rather than gas-based process and rarely contain any glass, LEDs are significantly more durable than traditional lamps. They have comparable resistance to direct impact as a cell phone or other electronic device, and offer no risk of shattered glass or hazardous gases if they do break.  There are no filaments or other similar "loose" parts, so they are also resistant to vibration, providing a better alternative for applications like automobiles or ceiling fans. 

Based on figures published by the Department of Energy for late 2007 technology, LED lamps last an average of 5 times longer than compact fluorescent lamps (CFLs) and 43 times longer than incandescent lamps.  This is important when considering the premium purchase price; an LED lamp may cost more up front, but will offer savings over time for replacement.  Another equally important consideration are maintenance costs associated with lamp replacement in various applications, such as street or parking lot lights, that would require special equipment and often require hiring an electrical contractor, making replacement cost significantly higher.  In these instances, the comparable long-life of LED lamps can provide the user with significant savings over the life of the lamp that far outweigh the up-front cost.

Another advantage LEDs have in many applications is their directional nature.  Traditional light sources emit a sphere of light in all directions, resulting in light being wasted in many applications.  LEDs are mounted to circuit boards, which are flat surfaces, resulting in a hemisphere of emitted light.  This provides a significant reduction in the amount of light wasted in directional applications such as task or recessed down lighting.

Unlike fluorescent and high intensity discharge (HID) lamps that can require up to several minutes to reach full brightness, LED lamps reach their full brightness the instant they are turned on.  They also suffer no ill effects from rapid cycling, unlike other lamps, which deteriorate more rapidly when they are frequently switched on and off.  In addition, LED lamps perform well in cold conditions, providing a better alternative to amalgam CFLs for cold temperature applications, such as refrigerated displays.  LED lamps also offer superior controllability over other efficient light sources, fluorescent and HID lamps, in dimming and other lighting control applications.  Finally, they emit virtually no radiated heat or ultraviolet radiation (UV) eliminating many of the associated hazards.

With all of these advantages and the potential to exceed today's most energy efficient lamps, the question remains why aren't LEDs more readily available for general lighting applications?  The answer is there are still some technology issues to be worked out.  One has to do with the quality of light produced and the other addresses the removal of waste heat. 

The big advantage of inefficient, heat-producing incandescent lamps is that they emit light that is most similar to natural light.  This is important because it affects the way that we perceive color; for example, have you ever struggled to distinguish between black and navy socks?  The quality of light is evaluated based on its color rendering index (CRI), which describes how well the light renders color on a scale of 0 to 100, and its correlated color temperature (CCT), which describes the apparent color of a white light source.  A standard incandescent lamp has a CRI of 100 and a CCT of 2700 Kelvin (K), indicating that its light appears yellow/gold, similar to sunlight.  Energy efficient CFLs have a CRI of at least 80 and a CCT of 2700-3000 K. 

Unlike traditional light sources, LEDs do not emit white light, so their light must be converted for use in general lighting applications.  This inherent difference makes it difficult to judge the color rendering capacity of LED lights because CRI scores do not accurately depict their color rendering quality.  In addition, LEDs are most energy efficient with high CCTs, producing a bluish rather than yellow/gold light.  LEDs with the same CCT range as CFLs are often less energy efficient.  The goal is to provide a high-quality light source with comparable or better energy efficiency.

The other problem at hand is effectively dealing with waste heat.  Unlike other light sources that radiate heat, LEDs conduct heat, although minimal in comparison, it can significantly reduce longevity if not removed from the device.  Effectively moving the heat away from the device becomes an issue in applications where space is at a premium, which describes many lighting applications traditionally served by incandescent lamps.  This is further complicated by the fact that traditional incandescent lamp sockets act as insulators rather than helping to conduct heat away from the lamp. 

* All information was obtained from the US Department of Energy, Energy Efficiency and Renewable Energy www.eere.energy.gov.

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