Solid state lighting with light-emitting diodes, or LEDs, provides five to 10 times as much illumination per watt as incandescent bulbs -- or even more. LEDs have useful lifetimes in the tens of thousands of hours -- rather than the thousand or so offered by incandescent bulbs. And LEDs offer precise control of light output, as opposed to incandescent bulbs, which spray their light in all directions.
All of these characteristics translate into significant performance advantages for LEDs, but that doesn't mean they're without their problems. They can have issues with color, output can degrade over time and LEDs must be cooled. Failure in any of several LED fixture components can cause the whole LED to fail. Industry, government and academia have been addressing those problems, leading to explosive growth in the availability of LEDs for general lighting.
The character and color of the light that comes out of your incandescent bulb depends upon the temperature of the tiny filament inside. Even if one filament is built differently from another, it will still be heated to just about the same temperature and give out roughly the same color light. Not so with LEDs.
LEDs are built kind of like computer chips, with precisely deposited layers of semiconductor materials. Tiny changes in the thickness of the layers can change the color of the LED's light. In addition, most white-light LEDs have another layer called a phosphor. Tiny changes in the phosphor will also lead to color changes that can make one white LED look bluish while another looks reddish and another yellowish.
Every light source degrades with time. Even your incandescent bulbs will change color and dim as they get older -- but they break completely before those effects get too noticeable. LEDs dim and change color as they get older also. Over their lifetime heat and light exposure cause physical and chemical changes to the LEDs and the phosphors that modify the illumination. Because LEDs last 25 to 50 times as long as incandescents, those effects become apparent.
LEDs are far more efficient than incandescent bulbs. Incandescent bulbs turn 5 percent to 10 percent of their electricity into light, while LEDs convert about half their electricity to light. The rest of that energy -- the wasted part -- goes into heat. Incandescent bulbs get rid of that heat by emitting it as invisible infrared radiation -- which is why your hand feels warm in front of an incandescent bulb. LEDs don't emit much infrared radiation.
LEDs still generate heat, so it needs to be taken away through some other method. LEDs need to be connected to heat sinks to move the heat energy away from the LED, then the heat sinks need to be engineered to get rid of that heat somehow. If LEDs are not cooled, they degrade very quickly, then fail completely.
Integrating Multiple Components
When you buy an incandescent light, you can plug it in a desk lamp, a wall sconce or a recessed ceiling fixture -- it will work anywhere. It's a different story for LEDs. An LED light source consists of far more than the LED itself. It also contains the heat sink and the driver electronics -- a circuit assembly that converts the 120 V from the socket into a DC voltage the LED can use. For an LED to work properly, the LED itself, the phosphor, the heat sink and the electronics all need to be failure-free. If you've ever seen an LED traffic light with a patch of darkness in the middle, you haven't seen a failed LED; you've seen a failed LED electronics package. Because LEDs themselves can last many tens of thousands of hours, the other elements of the package must also be designed with that kind of lifetime -- a technical challenge.
Dealing With the Problems
In the early 2000s, all of these issues -- and a few others -- were technical problems for LED lighting. Coordinated corporate, university and government work has improved the situation dramatically. Now there are sets of design and test standards. Although consumers probably don't want to get buried in the details of test procedures, they can take a simple step by verifying that the LED products they purchase have a "Lighting Facts" label. Only manufacturers that follow the test procedures are allowed to use the label.
About the Author
First published in 1998, Richard Gaughan has contributed to publications such as "Photonics Spectra," "The Scientist" and other magazines. He is the author of "Accidental Genius: The World's Greatest By-Chance Discoveries." Gaughan holds a Bachelor of Science in physics from the University of Chicago.
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