All About LEDs

Despite being about the size of your pinkie's fingernail, light-emitting diodes (LEDs) aren't far behind transistors in terms of ubiquity. Over the past few decades, they've found a home in everything from TV remote controls to Jumbotrons. Chances are if you haven't encountered LEDs in an AV installation, you will soon.

All diodes — including LEDs — have an anode (positive) terminal at one end and a cathode (negative) terminal at the other, but they allow current to flow in only one direction. (In an LED, the anode and cathode are the two wires poking out the bottom.) Diodes are unbalanced in the sense that they don't have equal amounts of negatively and positively charged particles. The more extra electrons that are floating around, looking for a home, the more conductive the diode is. A byproduct of their movement is light — a phenomenon LEDs leverage.

Here's how the process works: Multiple electrons orbit around an atom, and the ones with more energy orbit farther away. When an electron jumps to a closer orbit, energy is released as photons, which are a rudimentary form of light. As the homeless electrons move across a diode, they change orbits, producing light. However, sometimes that light isn't visible — as is the case with the infrared produced by the LEDs poking out of the end of your TV's remote control, for example. By adjusting the size of the jump electrons have to make when changing orbits, the light can be made visible and its color adjusted. The bulbous shape of an LED throws that light in one direction.

Although they look and act like miniature incandescent light bulbs, LEDs don't have a filament, so they don't burn out after just a year or two of heavy use. They're also more efficient because most of the energy goes toward producing light, whereas in an incandescent bulb, a lot of energy is wasted in the form of heat.

Although the underlying principles of LED technology were discovered in 1907, LEDs as we know them today originated in the early 1960s. First-gen LEDs produced only invisible, infrared light, but by the end of the 1960s, red LEDs debuted. Other colors appeared over the next few decades, and the technology also advanced in terms of the light's brightness and purity. “They were pretty mundane devices until the early '90s,” says Jagdish Rebello, principal analyst for communication systems and optical components at iSuppli, an analyst firm based in El Segundo, CA.

That's when high-brightness red, yellow, and amber LEDs made their commercial debut, followed by high-brightness blue and green LEDs around 1994. “That basically revolutionized the market from the signage perspective, because by mixing the output of red, green, and blue LEDs, you could produce colors across the entire spectral range,” Rebello says. “In 1996, when white LEDs came along, that was even better because now you could do white or color.”

“White LED” is somewhat of a misnomer because no material emits white light. The technology typically gets around that limitation by putting a fluorescent phosphor coating on a blue LED. What's the catch? The phosphor's quality goes a long way toward determining the white light's purity. Different phosphor coatings are used to produce other colors, such as pink.

LEDs also have improved in terms of brightness, which has doubled roughly every two years over the past decade. “Now you have LEDs with intensities on the order of about 50 lumens per watt,” Rebello says.

An LED's brightness — often referred to as its “intensity” — also is measured in candelas. “The brightness of an LED depends on the properties of the LED chip itself, on how the LED is packaged (through-hole versus surface-mount), and on how much power is used to turn an LED on,” says Reece Kurtenbach, vice president of video systems at display manufacturer Daktronics, based in Brookings, SD. “When LEDs are clustered together and arranged in a display, they're measured by area in candelas per meter squared, or ‘nits.' The formula for the sum of LED intensity is the number of LEDs in a square meter multiplied by each LED's intensity.”

One obvious way to increase the brightness of an LED display is to use brighter components, with an equally obvious downside. Because the LEDs cost more, so does the display — a reality that proves to be a drawback for price-sensitive applications.