Catalyst Fall 2012

[HOW IT WORKS]

LED Lightbulbs



© Amanda Wait/NonprofitDesign.com

LEDs (light-emitting diodes) have come a long way from their early days forming the red numbers in 1960s-era scientific calculators; today they produce nearly every color of the rainbow and are found in seemingly every gadget we encounter. But what has recently put LEDs in the spotlight is their potential to overtake incandescent and compact fluorescent lightbulbs (CFLs) as the lightbulb of choice.

Big Things in a Small Package

LEDs are tiny semiconductors that emit light. Each semiconductor, typically built on a base of sapphire crystal, consists of two layers of material on which chemicals have been applied (a process called doping) to facilitate an electric current. One layer has “holes” where electrons would normally go, and thus has an overall positive charge (P); the other layer has extra electrons in it and thus has a negative charge (N)

The surface between the “p-type” and “n-type” layers is called the P-N junction (see the diagram); electron movement at this surface produces an electric field that allows electrons to flow only from the p-type layer to the n-type layer. But when current is applied to the LED, the electrons move in the other direction and fill in the holes in the p-type layer. In the process, they release energy in the form of light (photons). Different doping chemicals are used to produce different colors.

To prevent most of these photons from being reflected back into the LED or traveling parallel to its surface (thereby diminishing their light), most LEDs are enclosed in a plastic shell, typically with a domed top and reflective base. The shell not only helps concentrate and direct the light outward, like a lens, but also protects the LED’s fragile circuitry.

Tricks of the Trade

LEDs differ from incandescent bulbs and CFLs in a number of ways. Their bright, narrowly focused light makes them well suited for indicator lights (e.g., “new message” lights on phones) and task lighting (e.g., under-cabinet kitchen lights). For omnidirectional, diffuse light—the kind emitted by incandescents and CFLs—which is preferable in residential and office lighting, LEDs are arranged to shine in multiple directions and covered with filters to scatter and soften the light.

To recreate the “soft white” light of incandescent bulbs (which current LEDs cannot create on their own), blue LEDs are coated with a yellow phosphorescent coating. The exact composition of the coating determines the bulb’s color “temperature” (i.e., “warm” white versus “cool” white).  

A Bright Idea in Energy Savings

LED lightbulbs are more durable and much longer-lived than both incandescent bulbs and CFLs: LEDs last 25,000 to 50,000 hours—equivalent to at least 17 years if used four hours per day—compared with about 2,000 for incandescents and 8,000 for CFLs. And LEDs are much more energy-efficient than incandescents: a 12-watt LED generates the same amount of light as a 60-watt incandescent bulb. Each such incandescent bulb replaced with an LED bulb eliminates 70 kilowatt-hours (kWh) of electricity consumption each year, assuming the lights stay on four hours per day; in turn, this prevents an average of nearly 87 pounds of heat-trapping carbon dioxide¾the main contributor to global warming—from being released into the atmosphere by power plants.

This efficiency translates into lower energy costs. Left on for four hours per day, a 12-watt LED lightbulb uses about $1.75 of electricity per year (assuming an average electricity rate of 10 cents per kWh), versus $8.76 for an incandescent bulb. Of course, consumers can only benefit from these savings if they can afford the high up-front cost of the bulbs themselves; at $20 or more per bulb, it can be expensive to outfit a whole house with LEDs at once. However, scientists at the University of Cambridge are attempting to create LED semiconductors using a silicon base instead of the more expensive sapphire; if successful, LED prices could be significantly lower in the near future.

While not significantly more energy-efficient than CFLs, LEDs offer several other advantages. They do not contain mercury (which may appeal to consumers who worry about mercury exposure if CFLs break or are improperly discarded), reach their full brightness without any noticeable delay, are often dimmable, and can be turned on and off frequently with no adverse impact.

Manufacturers See the Light
LEDs are making their way into many products.

Televisions. Most liquid-crystal display (LCD) televisions are backlit with white fluorescent tubes, but a growing number of models use LEDs instead. “Full-array” LED TVs (in which LEDs are arranged behind the entire display panel) can brighten or dim specific portions of the screen to make darker colors deeper and more realistic. “Edge-lit” models (which place LEDs around the edge of the screen and direct their light inward) weigh less and are much thinner.

Brake lights. In addition to their slimmer profile and longer life, LEDs can enhance vehicle safety compared with halogen and xenon bulbs: they reach full brightness up to a half-second faster, giving drivers at highway speeds as much as an extra car length of space to react.

Grow lights. Home and commercial gardeners often get a head start on the planting season by starting their seedlings under bright “grow lights.” LEDs offer the same brightness but with much less heat, preventing damage to tender young leaves.

 

Room for Improvement

Like any technology, LEDs are not free from drawbacks. For example, as wattage increases, LEDs generate more heat but not more light, reducing their overall efficiency. And since they do not dissipate this heat in the way other lightbulbs do (incandescent bulbs get hot because they emit infrared radiation), LEDs need a heat “sink” to dissipate this heat and keep the LEDs from overheating; the heat sink often takes the form of aluminum “fins” around the bulb.

These factors slowed manufacturers’ efforts to create LED bulbs that exceed the light output of 60-watt incandescents, but several that match the brightness of 75-watt incandescents are now available, and manufacturers recently unveiled the first prototype LED bulbs with light output rivaling that of a 100-watt incandescent. Continuing research promises to make LED lighting even better, cheaper, and more energy-efficient. Given how far lightbulbs have come from their humble origins to their current role in helping reduce the threat of global warming, Thomas Edison would no doubt be impressed.

Heather Tuttle is assistant editor at UCS.

 

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