Random Ramblings of Everyday Life

Introduction to LEDs (light-emitting diodes)

Posted in Guides by ziptrickhead on June 24, 2010

As lighting technology has advanced, LEDs (light-emitting diodes) are slowing overtaking incandescent usage. Over the past few years, LEDs have gotten much brighter and cheaper. LEDs have many benefits over the usual incandescent light bulbs. Generally, LEDs use much less power (several Watts compared to a hundred Watts) and they are more durable (no filament to burn out). Disadvantages are that they are generally more expensive than incandescent lights and require more current and heat management.

LEDs are based on semiconductor technology. Simply put, you have two differently charged materials (P- and N-type material). As electrons recombine with holes, they produce energy in the form of light, hence the term “light-emitting diode”. With an incandescent light bulb, current is passed through the filament and increases to a temperature enough for light to be emitted. This means that LEDs are much more robust, generally lasting from 25,000 to 100,000 hours when operated at low current and low temperatures. With incandescent light bulbs, the filament is very sensitive to spikes in current. With a small electrical surge, the filament can burn out, making the light bulb useless.

Right now, there are LEDs that can run off a single AA battery. This is due to a combination of efficient LEDs and excellent current drivers. Comparing output light to power, LEDs are well ahead. Try powering up a 100W light bulb with a few batteries and you’ll see what I mean. Current LEDs can produce over 150 lumens per watt while the typical 60-100W light bulb produces around 15 lumens per watt and fluorescent light bulb produce around 100 lumens per watt. Lumens are the unit of measure for how much light the human eye perceives.

Technological norm is all relative. Consider the cell phone or the personal computer. Just a few decades ago, both of those things were very expensive and few people could afford or had the use for. Now, practically everyone has both. The point is, LEDs may seem expensive now, but that is only because it is still new compared to incandescent lights. As more and more people use LEDs, they will become less and less expensive. As the green initiative grows stronger, there will be greater demand for LED technology and the technology will improve while prices decrease. Currently high power LEDs can cost from $5-20 each while light bulbs can be purchased for just a few dollars.

Heat is one of the biggest issues with LEDs. With any semiconductor device, as heat increases, efficiency decreases. Since a light bulb works more of material properties (high current across low resistive filament generates heat and light), it can generate a lot of heat without adverse effects. With LEDs however, high heat can greatly shorten its lifetime. So long as the LED has a good heat sink, there are no issues. This basically just involves mounting the LED onto a piece of good heat conductor.

In the past few years alone, there have been leaps and bounds in LED improvements. There are several big companies such as Luxeon, Cree, Seoul, Nichia, and Osram Sylvania that have come out with LEDs that are getting to be as bright as incandescent bulb at the fraction of the power consumption. So far the biggest consumer market for these high power LEDs have been flashlights. Even stubborn flashlight companies, like the Mag Instruments (makers of the famous Maglite) have started to switch to LED in the past year; although they are using what some might consider old technology. Even the NYC MTA is starting to dabble with LED lighting, with one of their buses (Bus 870) from their Jackie Gleason depot and the 8687 subway car having rows of LEDs for internal lighting.

Recently Osram Sylvania came out with a direct replacement for the 40-60W light bulb called the ULTRA Aline (http://www.sylvaniaonlinestore.com/p-101-ultra-aline.aspx). The light temperature is measured at 3000K in a frosted white bulb, making the outputted light very similar to halogen fluorescent light bulbs. Note how only half the bulb is being used for light output while the other half is used solely for heat sinking. I suspect that if the bulbs sell well, manufacturing costs can decrease to the point where the bulbs come out to around $10 each in the next few years. At that point with the energy savings and long lifetime, they will become a more efficient and cost effective means of indoor lighting.

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