See the light: An intro to LED technology

By Karl Rosengarth and Karen Brooks. Photo by Justin Steiner.

You may have noticed that the world of bicycle lights has quickly become dominated by LED technology. These little wonders are different from regular incandescent light bulbs, and superior in many ways, but there were some challenges to making them work for bike use. Read on to get educated.

What is a light-emitting diode?

A light-emitting diode, or LED, is a semiconductor device that produces light when an electrical current passes through it. While the light-emitting properties of semiconductor materials were noted as early as 1907, the phenomenon remained a curiosity until the solid-state electronics revolution led to the creation of the modern LED in 1962.

The LED is a variety of the ubiquitous semiconductor diode—the basic building block of modern electronic components and integrated circuits. In simplest terms, a semiconductor diode consists of a P-type (positive) semiconductor region joined to a N-type (negative) region. (Follow along with the diagram below.) The N-type region contains negatively charged free charge carriers (electrons) and the

P-type region has positively charged free charge carriers (electron holes). When current flows through the diode, the electrons and holes travel in opposite directions.

The magic of the LED happens at the P-N junction. When an electron meets a hole, the electron falls into a lower energy level (from conduction band into valence band) and releases energy in the form of a photon. Viola, light!

The wavelength of light emitted, which determines its color, depends on the difference in energy level between the conduction band and the valence band (i.e., the band gap). Suffice it to say that over the years a huge amount of research has been devoted to developing P-N junctions with the desired band gaps to produce light at useful wavelengths.

LED advantages

LEDs have a number of properties that make them well-suited for bicycle lights. They are compact, efficient, rugged, long lasting, and controllable.

LEDs have lifespans up to 50,000 hours or more, and their solid- state nature gives them inherent shock resistance that’s much appreciated for bicycling applications. Unlike incandescent bulbs, they have no filament to break.

The light output of LEDs may be modulated via relatively simple control circuitry, resulting in lights that offer user-selectable output levels. Fast switching times, on the order of a microsecond, make LEDs ideal for use in flashing mode.

LEDs boast efficiencies around 70 lumens of light per watt of energy, compared to 12 lumens/watt for incandescent light bulbs. Buoyed by continuous progress within the semiconductor industry, the efficiency and light output of LEDs has doubled every three years since the 1960s. This observed trend has been called Haitz’s Law, after Dr. Roland Haitz (similar to Moore’s Law, which states that the number of transistors on integrated circuits doubles approximately every two years, thus rapidly boosting computer processing speed and memory).

Visible-light LEDs generate very little infrared radiation in their light beam; in practical terms they do not exhibit a significant “heat lamp” effect, unlike incandescent bulbs, which convert around 80 percent of electrical energy into infrared radiation (and thus heat). LEDs do generate heat due to inherent inefficiencies, just like other lights, but the heat is dispersed through the base of the LED.

LEDs in bicycle lights

Since early LEDs emitted red light, naturally they found a home in rearward-facing “blinky” lights for bikes. It wasn’t until 1995, however, that an LED that produced white light was developed. After that, it took a few years for lighting manufacturers to figure out how to use these white LEDs to light the way for bikes. There were several issues to solve: heat buildup, optics, and power management.

First of all, heat. Although LEDs don’t produce a “heat lamp” effect, they do build up a significant amount of heat at their bases. Mike Ely, Vice President of Sales and Marketing for NiteRider, explains: “As you apply more power to the LED the brighter it gets, but it also generates more heat, and too much heat will destroy an LED very quickly. The way we manage this heat buildup is through heat sinking. The goal is to pull as much heat away from the LED as possible, brighter light output.”

A heat sink is a structure that dissipates heat, such as the metal fins or grooves you often find on a light’s housing. “The problem with this,” adds Ely, “is that as you add heat sink, you are increasing the weight and size of your light head. So in the early days of LEDs, due to the relative inefficiency, we couldn’t get the light output that we are seeing today. The key is to find the ‘sweet spot’ between light output and heat buildup.”

Second, the optics of an LED is very different from those of a light bulb. Daniel Emerson, CEO of Light & Motion, explains: “Bulbs suspend the [light] source out in space where it is easy to wrap a highly engineered reflector around it and direct the light. Look closely at a halogen bulb used in your home. Note the faceted, parabolic reflector around it with the bulb jutting out into the center. Now compare that with an LED, which is hidden behind a spherical dome and lays flat on a circuit board. It is much harder to build a good optic to support the LED than the bulb.”

Third, power management. “LEDs require very accurate voltage to run efficiently,” says Emerson. But “batteries are extremely poor at delivering constant voltage. So we need to design a circuit that makes up for the lousy voltage regulation of batteries.” This highlights one advantage of rechargeable systems, as the output of the batter- ies can be more tightly regulated so that the amount of light output stays the same whether the battery is freshly charged or nearly spent.

What’s next

Despite these challenges, lighting manufacturers have made huge progress with LED lights in a short amount of time—they’re rapidly getting smaller, more powerful, and more efficient, while not increasing much in cost (and in some cases, actually getting cheaper). According to Emerson, LEDs “are now delivering over 150 lumens per watt, or double what they could deliver just a few years ago. In the next few years, they will approach the theoretical max output in the low 200 lumens/watt range.” And the end is not yet in sight.

“As large LED companies such as Cree expand into residential lighting and pump large sums of money into R&D, I believe we’ll continue to see more advancements,” says Ely. The next frontier for huge leaps in light technology may be batteries. “Hopefully, car battery technology will push batteries to four times the capacity for rechargeable cells that we have today,” says Emerson. “There is interesting work occurring in nanotechnology that has the potential to give battery technology this type of big leap forward.”

A note on measurement

The relative power of bike lights, when they used regular bulbs, used to be measured in terms of wattage. However, wattage only tells you how much energy the bulb is consuming, not how much light it is actually putting out. The lumen is a more accurate unit, as it is a measurement of the visible light emitted from a defined source (like an LED) falling on a surface.

Candlepower is another unit of light measurement, less commonly used, that gives the amount of light from a source, measured at the source itself rather than on a surface.

Less scrupulous companies will list the lumens of their lights based on the theoretical output of the LEDs in it, not the measured output of a production light. But as we’ve seen, the actual output of an LED depends a lot on the way the light is put together. The true way to measure is with an expensive device that encloses the light in a perfect sphere to capture all the light emitted. If you encounter a bike light that claims an incredible amount of lumens for a bargain-basement price, just remember: if it sounds too good to be true, it probably is.

 

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