White LEDs

White LEDs

Thirty years ago, the most interesting thing I knew of was the digital watch. Never mind that digital watches were harder to read than analog watches, that they went through a set of batteries every few weeks, that they cost a small fortune. These things were not important. What was important was cutting- edge style. You could now wear a computerized device on your wrist that, at the press of a button, would display the time in glowing red LED numerals! How cool was that? When my dad got his first digital watch— a huge, clunky thing— I was deeply envious that he had the best toy in the house.

Just a few years later, though, digital watches had moved into the mainstream. I distinctly remember, as a nine- year- old in1976, saving my allowance to buy my very own$20 digital watch. I was the first kid in my school to have one, the envy of all my friends. ( Yes, my geek roots go way back.) It would be a few years yet before liquid- crystal displays( LCDs) became common; in the meantime, I was thrilled to have a status symbol courtesy of LEDs.

Since their commercial introduction in the1960 s, LEDs( light- emitting diodes) have become common components of every imaginable household product. Your phone, TV, answering machine, clock, and possibly even your kids’ shoes have LEDs in them. LEDs are popular largely because they’re inexpensive and use very little electricity. Compared to ordinary incandescent bulbs, LEDs also produce less heat, last many times longer, and are less sensitive to shock and changes in temperature. These are all wonderful attributes, but until relatively recently, there was one major limitation: color. Your choices were limited to red, amber, and green; anything further along the spectrum just didn’t exist. The invention of the blue LED was groundbreaking, but the white LED that soon followed was cooler by an order of magnitude.

A Little Light Science

To understand why white LEDs are so cool, you need to know a little bit about the technology behind LEDs generally. A diode is a very simple semiconductor device that allows electricity to flow in just one direction. If you apply current in that direction( called forward- biasing), the diode produces light as a side effect. For ordinary diodes, that light is infrared, invisible to the naked eye. By varying the materials used in the diodes, scientists were able to produce diodes that made visible light— red at first, and later, other colors. The physics behind LEDs dictates that whatever light is produced is of a single wavelength( that is, a single color). But for a long time, nobody could figure out how to make colors with shorter wavelengths than green— at least, not at any reasonable level of brightness.

Among the people trying to solve the blue LED problem was Shuji Nakamura, a researcher at Nichia Chemical Industries in Japan. Nakamura thought he had determined just the right type of material( indium gallium nitride, if you care) to use for blue LEDs. The problem was that the existing manufacturing processes did not produce material of sufficient quality. So he had to invent a new process as well. In1993, after years of research, Nakamura finally produced the first commercially viable blue LEDs.

Now with Extra Whiteners

This achievement alone would have earned Nakamura a place in electronics history. But the real genius was in his next step. He applied a special type of phosphorescent coating to a blue LED. The blue light excited the molecules in the coating— much like the way a fluorescent lamp works— and produced bright white light. So white LEDs are actually a clever spin- off of blue LEDs, and without Nakamura’s insights, neither would have been possible.

A white LED is brighter than a comparably sized, conventional incandescent bulb. However, it’s not as bright as a high- output Krypton or Xenon bulb, such as you’ll find in fancy flashlights. In addition, white LEDs cannot be made arbitrarily large, as incandescent bulbs can. So if you want to replace the bulb in your Mag- Lite or kitchen light fixture, you must use a cluster of LEDs. A good LED flashlight will typically have three or more bulbs; for residential lighting, much larger clusters are used— for example, a36- LED fixture will replace a30- watt bulb; to replace a90- watt bulb would require over100 LEDs.

Apart from the issue of sheer luminosity, an important reason for using LED clusters is the pattern of light they produce. With incandescent bulbs, the light is omnidirectional, which is why flashlights and lamps usually have reflectors to concentrate the light in a certain direction. LEDs, because of their built- in lenses, produce much more directional light. This can be either a blessing or a curse, depending your needs. If you want to create a“ Walk” sign at an intersection, an array of white LEDs will give you just what you’re looking for: bright light with a fairly narrow viewing angle. If you want to illuminate a room, however, that directionality is a problem. LED fixtures that need to disperse light more broadly rely on a combination of varying angles for individual LEDs and light- diffusing coverings.

The Price Is White

Although red and amber LEDs can be purchased for just a few cents each in volume, blue LEDs, which are harder to manufacture, typically cost about ten times as much, and white LEDs are more expensive still. You might imagine that a100- LED fixture to replace your hall light would be quite costly, and you’d be right: at current prices, that sort of light will run you well over US$500. Of course, over its lifetime, which will probably be decades, you will save quite a bit of money in both electricity and replacement bulbs. This is precisely why they are attractive to municipal governments, which are rushing to replace conventional bulbs in traffic lights( and in some cases streetlights) with LEDs.

Until economies of scale bring white LED prices down to more reasonable levels, their biggest appeal for ordinary consumers will be in products such as flashlights, bike lights, and headlamps used for caving. It’s in applications like these that the LEDs’ low power consumption makes a profound difference. A Krypton flashlight might run for three hours on a set of alkaline batteries, compared to thirty or more for a flashlight using white LEDs. The extra convenience, not to mention the cost savings, can be enormous.

This Little Light of Mine

A bit of additional circuitry can extend the life of LED- based products even further. For example, I bought a white LED flashlight called an EternaLight. It has a built- in microprocessor that flashes the LEDs very rapidly— so fast that the human eye cannot detect it. Because the LEDs only use power when they’re actually on, this technique stretches the battery life to as much as700 hours— you could leave the flashlight on for a full month. After about three years of frequent intermittent use, I decided to change the batteries simply on principle because I was going on a trip, but for all I know they could have lasted another three years.

There are many reasons to like white LEDs, but for me, the sheer cleverness of the idea tickles my fancy. The novelty has already worn off, but I’ll always be impressed by the creativity and hard work that made them possible. And when some pseudo- retro designer gets the idea to make a white LED digital watch, I’ll be first in line to buy one. — Joe Kissell